Loading...
[PDF] World Oceans Day - British Council Learn English
This is divided into five geographical regions: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean and the Southern Ocean Introduction
[PDF] Layers of the Ocean - Westdale Junior School
There are five main oceans which all flow into each other: the Arctic Ocean, the Atlantic Ocean, the Indian Ocean, the Pacific Ocean and the Southern or
[PDF] WOR1_enpdf - World Ocean Review
small English town of Happisburgh The old coastal defences are largely ineffective Here, a Second World War bunker has fallen from the eroded cliffs,
[PDF] Oceans and Coasts - Wedocsuneporg
The oceans SDG target 14 2 “by 2020, sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by
[PDF] Understanding the State of the Ocean: - Wedocsuneporg
Complexities of ocean monitoring and marine indicators Colombian National Register of Protected Areas (in Spanish: Registro Único de
[PDF] Continents and Oceans - St Mary's CE High Crompton
Page 1 Continents and Oceans
[PDF] Identifying Continents and Oceans - American Geosciences Institute
identify the four major oceans (Pacific, Atlantic, Indian, and Arctic) and the seven continents (Asia, Europe, Africa, Antarctica, Australia, North
[PDF] Basic Act on Ocean Policy (Act No 33 of April 27, 2007)
This English translation of the Basic Act on Ocean Policy (Effective July 20, The purpose of this Act is, with regard to the oceans, to stipulate the
![[PDF] WOR1_enpdf - World Ocean Review](https://pdfprof.com/EN_PDFV2/Docs/PDF_8/1060_8WOR1_en.pdf.jpg "[PDF] WOR1_enpdf - World Ocean Review")
1060_8WOR1_en.pdf 1WorldOceanReview <
mareworld ocean reviewLivingwiththeoceans.20101Publishedbymaribusincooperationwith > Inhalt2Livingwiththeoceans.world ocean review2010
Authors:Moritz Bollmann,Thomas Bosch,Franciscus Colijn,Ralf Ebinghaus,Rainer Froese,Kerstin Güssow,Setareh Khalilian,Sebastian Krastel,Arne Körtzinger,Martina Langenbuch,Mojib Latif,Birte Matthiessen,Frank Melzner,Andreas Oschlies,Sven Petersen,Alexander Proelß,Martin Quaas,Johanna Reichenbach,Till Requate,Thorsten Reusch,Philip Rosenstiel,Jörn O.Schmidt, KerstinSchrottke, HenningSichelschmidt,Ursula Siebert,Rüdiger Soltwedel,Ulrich Sommer,Karl Stattegger,Horst Sterr,Renate Sturm,Tina Treude,Athanasios Vafeidis,Carlo vanBernem,Justus vanBeusekom, RüdigerVoss, MartinVisbeck,Martin Wahl,Klaus Wallmann,Florian Weinberger
4Ourenvironmentalawarenessissteadilyincreasing,albeitveryslowly.Thisprocessbeganwhenwestartedtoaddressobviousandvisibleproblems.Asaresult,ourstreets,beaches,fieldsandforestsbecamecleaner,industrialemissionsdecreased,andourchimneyspro-duced lessand lessair pollution.When wesee thatthere isa problemand thereis scopeforadvocacy,wetakeaction.Theoceans,however,arevastandlargelyinaccessible,andourawarenessandunder-standingofthemarecorrespondinglysmall.What"smore,theyhavehardlyanyadvocatesorlobbytorepresenttheirinterests.Thisisespeciallyremarkablewhenweconsiderthattheseascruciallyinfluenceourclimateandareanincreasinglyimportantsourceoffood.The FourthAssessment Reportby theIntergovernmental Panelon ClimateChange (IPCC)in 2007and theStern Review,published in2006, createdan unprecedentedlevel ofaware-nessworldwideoftheproblemsandimpactsofclimatechange.Thissparkedtheideaofproducingasimilartypeofreportfortheoceans,whichcoverthree-quartersoftheEarth"ssurface,thusfocusingattentiononsomeofthemosturgentissuesfacingustoday.Tothatend,thepublishinghousemareverlaginHamburgsetupthenon-profitcompanymaribustwoyearsago.Thiswasmotivatednotbycommercialinterestbutbythedesiretofocusmaximumpossibleattentiononthestateoftheworld"soceans.Partnersweresoughttosupportthepursuitofthisobjective,andtheInternationalOceanInstitute(IOI)andthenon-profitOceanScienceandResearchFoundation(OSRF)-bothfoundedbyElisabethMannBorgese-joinedtheproject.TheIOIprovideslogisticalsupport,itscloseassociationwiththeworkoftheUnitedNationsplayinganimportantroleinthiscontext.TheOSRFprovidesfinancialbackingfortheproject.ThekeyscientificpartneristheClusterofExcellence"TheFutureOcean"-aresearchgroupmadeupofmorethan250scientistsinvestigatingclimateandoceanchangeatanumber ofresearch institutionsin Kiel.Drawing ontheir outstandingexpertise andapplyingan interdisciplinaryapproach, morethan 40scientists withinthe Clusterhave authoredthisfirstWorldOceanReview(WOR).Theprimarypurposeofthisfirstreviewisnottofocusonspectacularnewfindingsorlaunchhigh-profileappeals.Rather,withitsjudiciouscombinationofwell-researchedandsubstantivecontentpresentedinaclearandaccessiblestyle-thankstothecooperationwithmareverlag-theWorldOceanReviewaimstopaintaclearandcompellingpictureofthecomplexstateoftheworld"soceansandunderlinetheurgentneedforaction.Itisuptoustoactonthisknowledge.Wehopethataswelooktothefuture,theWorldOceanReviewwillinspireadvocacytoprotectandpreserveourblueplanet.Nikolaus GelpkeManagingDirectorofmaribusgGmbHandmareverlagpublisherPreface>worldoceanreview
5The scientistsin theCluster ofExcellence "TheFuture Ocean"undertake researchin arangeofdisciplines,evaluatingthecomplexinteractionsbetweentheoceansandglobalchangeandassessingopportunitiesandrisks.Buthowmuchdowereallyknowaboutthestateofthe oceanstoday? Whatdo weknow aboutthe manyinfluences thatcome intoplay inrela-tiontotheincreasingoverexploitationoftheseasorclimatechange,bothofwhichhaveadirectandindirectimpactonthemarineenvironment?Whatarethelimitstoourunder-standing?Andcansustainablesolutionsbedevisedforthefutureuseoftheseas?Journalists,teachersandinterestedmembersofthepublicoftenaskus,thescientists,thesequestions,buttheyaredifficultforindividualmarineresearcherstoanswercom-prehensively.TheKiel-basedClusterofExcellence"TheFutureOcean"thereforebringstogetherresearchersfrommanydifferentdisciplines-marinescientists,earthscientists,bio-logistsandchemists,aswellasmathematicians,economists,lawyersandmedicalscien-tists-toengageinjointinterdisciplinaryresearchonthemarineenvironment.Howcanunresolvedquestionsbeinvestigatedandreliableanswersprovided?Thediversityofdisci-plinesandresearchinstitutionsinvolvedintheClusterofExcellencecomesintoplayhereandhelpstoprovideacomprehensiveoverviewofthestateoftheworld"soceans.TheWorldOceanReviewisourattempttopresentasrealisticapictureaspossibleofthecurrent stateof theoceans. Theaim isto drawtogether findingsfrom thevarious disciplinesand shareour knowledgewith thegeneral public.We beganby identifyingthe majorissuesofrelevancetothestateoftheoceans.WeaskedscientistswithintheClusterofExcellenceto writeabout thecurrent situationand topicalissues froma varietyof perspectives.Profes-sionaljournalistswereonhandtoguidetheexpertswithhelpfuladviceonstyleandthechoiceofphotographsandillustrations.Readerswillobservethatmanyofthetopicscoveredinthevariouschaptersfocusonhumankind"suseandoverexploitationofthemarineenvironment.Thisapparentlyinfiniteresource hasbecome finite.As theWorld OceanReview makesclear, thestate ofthe oceans,asdepictedhere,givesfrequentcauseforconcern.Theoutlookonpossibledevelopmentsandconsequencesoffurtheroverexploitationandpollutionofthemarineenvironmentsadly onlyreinforces ourconcern andhighlights theimportant rolethat preventiveresearchinthefieldofmarinescienceshastoplayforthefutureofhumankind.ThescientistsinKielwishtomakeacontributionhere.WewishalltheusersofthisfirstWorldOceanReviewaninterestingandinformativeread.Prof. Dr.Martin VisbeckChairman,ClusterofExcellence"TheFutureOcean"Preface <
6Preface4Theworldoceans,globalclimatedriverschapter 1Earth"s climatesystem -a complexframework10The greatocean currents- theclimate engine16Time toact25Howclimatechangealtersoceanchemistrychapter 2The oceans- thelargest CO2-reservoir28The consequencesof oceanacidification36Oxygen inthe ocean44Climate changeimpacts onmethane hydrates48Materialfluxes - gettingthe full picture53Theuncertainfutureofthecoastschapter 3Sea-level rise- anunavoidable threat56How natureand humankindalter thecoasts60The battlefor thecoast68Thefutureof the coast - defence or orderly retreat?73Laststop:Theocean-pollutingtheseaschapter 4Over-fertilization ofthe seas76Organic pollutantsin themarine environment82Litter -pervading theocean86Oil pollutionof marinehabitats92Muchto bedone...99Climatechangeimpactsonmarineecosystemschapter 5Biological systemsunder stress102Disruption tothe planktoncycle106Species encroachingon alienterritories110Marine biodiversity- avital resource114Impactsand repercussions117>worldoceanreview
7Contents Theworldoceans,globalclimatedrivers1
>Theoceanscoveraround70percentoftheEarth"ssurface.TheythusplayanimportantroleintheEarth"sclimateandinglobalwarming.Oneimportantfunctionoftheoceansistotransportheatfromthetropicstohigherlatitudes.Theyrespondveryslowlytochangesintheatmos-phere.Besideheat,theytakeuplargeamountsofthecarbondioxideemittedbyhumankind.
>Chapter0110TheinertiaofclimateAswealllearnedinschool,theworld"soceansareoneof themost importantelements inthe globalclimate sys-tem.Butwhatdoes"climate"actuallymean?Thediffer-encebetweenweatherandclimatecanbeexpressedinasinglesentence:"Climateiswhatyouexpect;weatheriswhatyouget."Thisrevealsafundamentaldifferencebetweenweatherandclimate.Weatherresearchiscon-cernedwiththeformation,movement,andpredictionoftheindividualelementsofweather,suchasaparticularlow-pressure systemor ahurricane. Climateresearch, ontheotherhand,dealswiththemorecomprehensivetotalityoflowpressuresystemsandhurricanes,andisdedicatedtoaddressingquestionssuchashowmanymidlatitudinalstormsorhurricaneswilloccurnextyear,or whetherthey willbecome morefrequent orintense inthecomingyearsasaresultofglobalwarming.Sotheterm "weather"refers toshort-term eventsin theatmos-phere, while"climate" relatesto longertime periods.Fordescribingclimate,asarule,atimespanof30yearsisgenerally usedas aframe ofreference.Peoplemainlyperceiveclimatechangeaschangesinatmosphericvariables,forexample,variationsintem-peratureorprecipitation.Inprinciple,duetoitschaoticdynamics, theatmosphere itselfcan generatemany natu-ralclimaticchanges.OneexampleofthisistheNorthAtlanticoscillation(NAO),whichsignificantlyinfluencesthe climateover partsof Europeand NorthAmerica. Itisa kindof pressurefluctuation betweentheIcelandic Lowand theAzores Highthat determinesthe strengthof win-ter westerlywinds acrossthe NorthAtlantic. Ifthese arestrong,theresultismildandrainyweatherinWesternEurope; ifthey areweak itis dryand cold.These kindsofnaturaloscillationsmakeitdifficulttorecognizeanthro-pogenic climatechanges dueto anenhanced greenhouseeffect.Theatmosphereisnotanisolatedsystem.Itinteractswith othercomponents ofthe Earthsystem -the oceans,for example.But itis alsoin contactwith thecryosphere(iceandsnow),thebiosphere(animalsandplants),thepedosphere (soil)and thelithosphere (rocks).All oftheseelementstogethercomposetheclimatesystem,whoseindividualcomponentsandprocessesareconnectedandinfluence eachother indiverse ways.These componentsall reactto changeat differentrates.TheatmosphereadjuststotheconditionsattheEarth"ssurfacesuchasoceantemperatureoricecoverwithinafewhourstodays.Furthermore,weatherisvariableandcanonlybepredictedafewdaysinadvance.Infact,ithas beenshown thatthe theoreticallimit ofweather pre-Earth"sclimatesystem-acomplexframework>TheEarth"sclimateisinfluencedbymanyfactors,includingsolarradia-tion,wind,andoceancurrents.Researcherstrytointegratealloftheseinfluencingvariablesintotheirmodels.Manyoftheprocessesinvolvedarenowwellunderstood.Butinteractionamongthevariousfactorsisverycomplexandnumerousquestionsremainunresolved.DaysMinutesYear100yearsThousandyearsMillionyearsAtmosphereOceansurfacelayerDeepoceanIcesheetsSeaiceBiosphere1.1>Differentcomponentsoftheclimatesystemreacttoperturbationsatdifferentrates.Thedeepocean,forexample,isanimportantcauseoftheslowresponseofclimate.Thecolouredareaonthetopscalerepresentstheshorttimespanofahumanlife.
11Theworldoceans,globalclimatedriversTheclimatesystem,itssub-systemsandrelevantprocessesandinteractions.
SeaicePrecipitationWindAbsorptionReflectionEmissionAir-oceaninteractionLand-airinteractionVolcanicgasesandparticlesBiogeochemicalcyclesTerrestrialoutgoingradiation(long-wave)CRYOSPHER ERiversInfluxHumaninterventionLakesCloudsIncomingsolarradiation(short-wave)Ice-oceaninteractionCurrentAir-iceinteractionLI T HOSPHER EPE D OSPH E R EBIOSPHER E
>Chapter0112Climatefluctuationsarenotunusual.IntheNorthAtlanticSector,forexample,itiswellknownthattheaveragetemperaturesandwindscanfluctuateondecadaltimescales.Climatechangescausedbyhumans(anthropogenic)alsoevolveoverthecourseofseveraldecades.Thenaturaldecadalchangesandthosecausedbyhumansarethereforesuperimposedupononeanother.Thismakesitdifficulttoassesstheimpactofhumansonclimatewithcertainty.IncontrasttothedynamicNorthAtlanticregion,theeffectsofclimatechangeareeasiertodetectinmorestableregionssuchasthetropicalIndi-anOcean.Thereisnodoubtthattheoceansdriveinterannualordecadalclimatefluctuations.DecadalfluctuationsofAtlantichurricaneactivityorprecipitationintheSahelcorrelateremarkablywellwithoscillationsofoceantemperatureintheNorthAtlantic.Althoughtheprecisemechanismsbehindthesedecadalchangesarenotyetfullyunderstood,thereisgeneralagreementthatvariationsintheAtlanticoverturningcirculationplayanimportantrole.Thishypoth-esisisalsosupportedbythefactthatAtlanticseasurfacetempera-tureanomaliesoccurincyclesofseveraldecades,withapatternwhichischaracterizedbyaninterhemisphericdipole.Whentherateofnorthwardwarmwatertransportincreases,thesurfaceairtem-peraturerisesintheNorthAtlanticandfallsintheSouthAtlantic.Ifitbecomescoolerinthenorthandwarmerinthesouth,itisanindicationofweakoceancurrents.Theair-temperaturedifferencebetweentheNorthandSouthAtlanticisthereforeameasureoftheoverturningcirculationstrength.Modernclimatemodelscansimulatethepresent-dayclimateandsomehistoricalclimatefluctuationsreasonablywell.Thesemodelsdescribetheclimatewithsatisfactoryreliability,especiallyonaglobalscale.ButforsmallergeographicalareasthemodelsarelessThedifficultyofdetectinganthropogenicclimatechange-5 -4 -2 -1 -0.6 -0.2 0.2 0.6 1245.6
20051.3>Europeexperiencedanunusuallycoldbeginningoftheyear2010.Butfromaglobalperspective,thewinterof2010wasthethirdwarmestinthepast131years.Ifthefirstfivemonthsoftheyearareconsidered,then2010isactuallythewarmest,anditevenreachedtheprevioustem-peraturerecordhighsforthemonthsofAprilandMay(top).Theyears1998and2005havebeensofarthetwowarmestyearsintheannualmean(relativetotheaverageof1951to1980).
1998JFMAMJJASOND0.30.40.50.60.70.8199820052010RecordhighTemperatureanomaly(°C)
13Theworldoceans,globalclimatedrivers -5-4-2-1-0.6-0.20.20.61245.6Temperatureanomaly(°C)2010 >Chapter0114HowhumansarechangingtheclimateThe humanimpact onclimate hasgreatly increasedoverthepasthundredyears.Wereleasevastamountsofclimate-relevanttracegasesintotheatmosphere.Thischangestheradiationbalanceoftheatmosphereandleads toglobal warming.In additionto carbondioxide, thesetrace gasesincludemethane,nitrousoxide(laughinggas),halogenatedfluoro-carbons, perfluorinatedhydrocarbons, andsulphur hexa-fluoride. Butcarbon dioxide(CO2) isespecially importantfortheEarth"sclimatesystembecausetheworldwideoutputissoenormous.Itisreleasedprimarilythroughtheburningoffossilfuels(oil,naturalgas,andcoal)inpowerplants,vehicleenginesorinhouseholdheatingsystems.Itsatmosphericlevelshaverisentoalmost390 partsper million(ppm) todayas comparedto thepre-industrialvalueof280ppm.Withthisincreasethetem-perature hasalso risenduring thetwentieth century.TheinternallydrivenchangesintheoceanssuchaschangesintheGulfStreamalsooccurwithinatimeframeofdecadesorafewcenturies.Thesehaveadecisiveinflu-enceonclimateandontheconcentrationofgreenhousegasesintheatmospherebecausetheyarestronglyinvolvedinglobalmasscyclessuchasthecarboncycle.For example,CO2dissolves easilyin water.However, theoceans havetaken upabout halfof allthe carbondioxideproducedbytheburningoffossilfuelssincethebegin-ning ofthe industrialrevolution, whichhas clearlydomi-natedthenaturalvariations.Whethertheclimatewillchange inthe future,and byhow much,can thereforebealso deducedfrom theoceans.Climatewillchangeveryslowlyinthefuturebecausetheoceanswiththeirimmensevolumesofwaterreactverygraduallytochange.Therefore,manybutnotalloftheconsequencesofclimatechangetriggeredbyhumanactivitywillonlygraduallybecomenoticeable.Someoftheseconsequencescouldactuallybeirreversiblewhencertainthresholdsarecrossed.Atsomepointitwillnolongerbepossible,forinstance,tostopthecompletemeltingoftheGreenlandicesheetandtheresultingseven-meterriseofsealevel.Thepositionofthethresh-old,however,isnotpreciselyknown.Butonethingiscertain:Eveniftheemissionofcarbondioxidewerestabilized attoday"s levelsit wouldnot leadto astabiliza-tionofthecarbondioxideconcentrationintheatmos-phere,becausecarbondioxideisextremelylong-livedandthecarbondioxidesinks,mainlytheoceans,donotabsorb itas quicklyas weproduce it.The situationis differentfor short-livedtrace gaseslikemethane(CH4).Ifmethaneemissionswerestabilizedatthepresentlevel,themethaneconcentrationintheatmospherewouldalsostabilize,becausemethanediminishes inthe atmosphereat aboutthe samerate asitisemitted.Inordertomaintainthecarbondioxidecon-centrationatagivenlevel,theemissionshavetobereduced toa fractionof thepresent amounts.1.4>Evenifitispossibletosignificantlyreducetheemissionofgreenhousegases,andCO2inparticular,bytheendofthiscentury,theimpactwillstillbeextensive.CO2hasalonglifeandremainsintheatmosphereformanycenturies.Becauseofthis,thetempera-tureontheEarthwillcontinuetorisebyafewtenthsofadegreeforacenturyorlonger.Becauseheatpenetratesveryslowlyintotheoceandepths,thewateralsoexpandsslowlyandsealevelwillcontinuetorisegraduallyoveralongperiodoftime.MeltingofthelargecontinentalicesheetsintheAntarcticandGreenlandisalsoaverygradualprocess.Meltwaterfromthesewillflowintotheoceanforcenturiesorevenmillennia,causingsealeveltocontinuetorise.ThefigureillustratestheprincipleofstabilizationatarbitrarylevelsofCO2between450and1000partspermillion(ppm),andthereforedoesnotshowanyunitsontheresponseaxis.CO2emissionpeakToday100years1000yearsMagnitudeofresponseCO2emissionsCO2stabilization:100to300yearsTemperaturestabilization:afewcenturiesSea-levelriseduetothermalexpansion:centuriestomillenniaSea-levelriseduetoicemelting:severalmillenniaTimetakentoreachequilibrium
15Theworldoceans,globalclimatedriversTobringatten-tiontothethreatofglobalwarming,thegovernmentoftheRepublicofMaldivesheldameetingontheseafloorinautumn2009justbeforetheCopenhagensummit.CarbondioxideandthegreenhouseeffectTheatmosphereisbecomingmoreenrichedincarbondioxide(CO2),ortobemoreprecise,car-bondioxideandotherclimate-relevanttracegases.Initiallytheyallowtheincomingshort-waveradiationofthesuntopassthrough.ThisenergyistransformedtoheatattheEarth"ssurfaceandisthenemittedbackaslong-waveradiation.Thegasesintheatmosphere,liketheglasspanesofagreenhouse,preventthislong-waveradia-tionfromescapingintospace,andtheEarth'ssurfacewarmsup.
>Chapter0116WhatdrivesthewatermassesWaterplaysacentralroleintheclimatesystem.Itsdensityvariesdependingonsalinityandtemperature.Cold, saltywater isheavy andsinks togreat depths.Thiscauses thecirculation ofmillions ofcubic metresof waterintheocean.Thispowerfulphenomenon,whichprima-rilyoccursinafewpolarregionsoftheocean,iscalledconvection.The surfacewater inthe NorthAtlantic regionsinks toa depthof around2000 metresdue toconvection. Thereitsettlesonanevendenserdeep-waterlayerfromtheAntarctic thatextends downto thesea floor.As thecoldandsaltysurfacewatersinksbyconvection,saltywaterflows infrom nearbywarmer regions,from thedirectionof theequator. Thiswater isthen cooledin theArctic airand alsobegins tosink, sothat theconvection iscontinu-ous.Beforesinking,thewaterabsorbsenormousamountsofgasessuchascarbondioxideattheseasurface,andthentransportsthemrapidlytomuchgreaterdepths.That iswhy thehighest concentrationsof carbondioxidein theocean arefound inthe convectionareas.Thehighcarbondioxideconcentrationspumpedintothewaterbyconvectionhavebeenshowntoreachdepthstodayofaround3000metres.Carbondioxideistransportedrelativelyrapidlybyconvectiontoadepthof2000metres.IntheNorthAtlanticthetransporttogreaterdepthstakessignificantlylongerbecausecarbondioxideandothergasescanonlypenetratethedeepwater byslow mixingprocesses.Lowtemperatureandhighsalinityaretheprimarydrivingforcesofconvection.Theypullthedensewaterofthepolarregionsdownward,whichdrivesaworld-wideconvectionenginecalledthermohalinecirculation(thermo-drivenbytemperaturedifferences;haline-driven bysalinity differences).The cold,salty watersub-mergesprimarilyintheLabradorandGreenlandSeas,andthenflowssouthwardtowardtheequatorandbeyond.Althoughconvectiononlyoccurslocallyinthepolarregions,itpropelsthermohalinecirculation,whichspans theglobe likea giantconveyor belt.Even theGulfStreamanditsbranchesaredrivenbyconvectionandthermohalinecirculation.Althoughwindalsoinfluencesthetransportofwatermasses,itscontributionissignifi-cantly less.Buthowdothewatermassesofdifferentdensitiesthatdriveoceanconvectionactuallyoriginate?Airtem-perature,evaporationandprecipitationareamongtheThegreatoceancurrents-theclimateengine>Oceancurrentstransportenormousamountsofheataroundtheworld.Thismakesthemoneofthemostimportantdrivingforcesofclimate.Becausetheyrespondextremelyslowlytochanges,theeffectsofglobalwarmingwillgraduallybecomenoticeablebutoveraperiodofcenturies.Climatechangesassociatedwithwindandseaicecouldbecomerecognizablemorequickly.SeafloorDensecoldwatersinksDeepwater2000mSouth
WarmwaterColdwaterIceSealevel1.6>TheconvectionprocessintheNorthAtlantic:Cold,saltywatersinksintheLabradorandintheGreenlandSea.ThiswaterformsalayerabovethedenserdeepwaterfromtheAntarcticatadepthofaround2000metresandflowstowardtheequator.Warmerwatersfromtheupperoceanlayersmoveintotheconvectionareatoreplacethesinkingwater.
17Theworldoceans,globalclimatedrivers WarmwaterIceColdwater
1.7>Thewatermoleculeisasymmetricalandisthereforeoppositelychargedatitstwoends(left).Thisiscalledadipole.Itthusbehavesdifferentlyfromothersubstancesinmanyways.Iceislessdense(top)andfloatsonthesurface.Freshwaterhasitsgreatestdensityatfourdegrees(bottom),andsinkstothebottom.Thisisthenoverlainbywarmwater(middle).Saltywaterhasdifferentcharacteristics.-++
>Chapter0118mostimportantfactorsintheanswertothisquestion.The freezingof waterin thepolar convectionregions alsoplaysacentralrole.Becauseiceonlycontainsaboutfivetenthsofapercentsalt,itleavesbehindaconsiderableamountofsaltinthewaterwhenitfreezes,whichincreases thesalinity ofthe surroundingocean waterandthusincreasesitsdensity.Thewatermassproducedbyconvection inthe Arcticis calledthe NorthAtlantic DeepWater (NADW).TheglobalconveyorbeltConvectionalsooccursintheAntarcticregions.Becauseoftheirevenhighersalinity,thewatermassesproducedheresinkallthewaytotheseafloor.ThisiscalledtheAntarcticBottomWater(AABW),anditflowsacrosstheoceanfloorhalfwayaroundtheglobeintotheNorthAtlantic. TheAABW isalso thedeep waterlayer thatthethick intermediateNADW overlieswhen itsinks bycon-vection.TheNADWformsintheGreenlandandLabra-dor Seas.Figure 1.8schematically illustratesits flowpathandthereturnflowofwarmwaterinthenear-surfacelayers, inthe globalconveyor beltof thermohalinecircu-lation.TheNADW,andespeciallytheAABW,remaininthedeepoceanforanamazinglylongtime.Radioactivecarbon-isotopedatingofthedeepwatersindicatesthatfromthetimeofsinkingintothedeepuntilitsreturntothesurface,aperiodofseveralhundredorevenupto1000 yearswill pass.Formostofthistimethewaterremainsinthecolderdeepregionsofthethermohalineconveyorbeltbecausetheretheflowrateisslow,ataroundonetothreekilo-metresperday,duetoitshighdensity.Theamountofwater involvedin thiscycle istruly immense.Its volumeis around400,000 cubickilometres, whichis equivalenttoaboutonethirdofthetotalwaterintheocean.Thisisenoughwatertofillaswimmingpool400kilometreslong,100kilometreswide,andtenkilometresdeep.Theoceanicconveyorbelttransportsabout20millioncubicmetresofwaterpersecond,whichisalmost5000timestheamountthatflowsoverNiagaraFallsinNorthAmerica.ConcernsaboutthebreakdownoftheGulfStreamTherehasbeenagreatdealofdiscussionabouttheextenttowhichclimatechangecouldinfluencethermohalinecirculationanditsturnoverprocessesintheAtlantic.After all,convection athigh latitudescould beweakenedbyanthropogenic(causedbyhumans)warmingoftheatmosphereandtheaccompanyingdecreaseinsurface-Thereisnootherareaintheoceanwherethesurfacewaterfindsitswaysoquicklyintothedeepasintheconvectionareas,andatnootherplacedochangesattheseasurfaceorintheatmospherebecomesorapidlyapparentintheocean"sinterior,forexample,intheincreasedcarbondioxidelevelsinthewaterasaresultofhighercarbondioxideconcentrationsintheatmos-phere.Convectionconnectstwodistinctlydifferentcomponentsoftheocean:thenear-surfacelayersthatareincontactwiththevariableatmos-phericfieldsofwind,radiationandprecipitation,andthedeepregionsoftheocean.Atthesurface,currents,temperatureandsalinityfluctuateonascaleofweekstomonths.Butatgreaterdepthstheenvironmentalcondi-tionschangeattimescalesofdecadesorcenturies.Intheconsistentlywarmoceanicregionsofthetropics(thewarmregionsoftheEarthbetween23.5degreesnorthand23.5degreessouthlatitude)andthesubtropics(theregionsbetween23.5and40degreesinthenorth-ernandsouthernhemispheres),thereisnoexchangebetweenthesurfaceanddeepwatersthatiscomparabletopolarconvection.Thisisbecause,averagedovertheyear,thereisanetradiationexcessofthesurface-layerwaters.Thewarmwater,withaminimumtemperatureoftendegreesCel-sius,hasarelativelylowdensityandfloatsasawarmlayerontopofthedeeper,colderwatermasses.Thetwolayersaredistinctlyseparatedwithnogradualtransitionbetweenthem.Attheboundarywheretheymeetthereisasharptemperaturejump,andthereforealsoanabruptdensitydifferencethatinhibitspenetrationoftheheattogreaterdepths.Thewarmsurfacelayerhasanaveragethicknessofseveralhundredmetres,whichisrela-tivelythincomparedtothetotaldepthoftheoceans.InverywarmoceanregionssuchasthewesternequatorialPacific,thereishardlyanyverticalmixingatall.Nearertothepoles,however,thereismoreverticalmixingoftheoceansandlayeringislesswell-defined.Becausethereisnoabrupttem-peratureanddensitychangethere,changesintheseasurfacecanbetrans-mittedtotheinteriordepthsoftheocean.Buttheconvectionareasarestilltheexpresselevatortothedeep.Thepathofwaterintothedeepocean
19Theworldoceans,globalclimatedriversTheworldwideoceancurrentsofthethermohalinecircu-lationsystemareextremelycomplex.Theflowofcold,salinesurfacewater(blue)downwardandtowardtheequatorcanonlybeclearlyrecognizedintheAtlantic.Warmsurfacewater(red)flowsintheoppositedirection,towardthepole.Inotherareasthecurrentrelationshipsarenotasclear-cutastheyareintheGulfStreamsystem(betweenNorthAmericaandEurope).TheCircumpolarCurrentflowsaroundAntarctica,anddoessothroughoutthetotaldepthofthewatercolumn.Thesmallyellowcirclesinthepolarregionsindicateconvectionareas.Thedarkareasarecharacterizedbyhighsalinityandthewhiteareasbylowsalinity.Saltyareasaremostlylocatedinthewarmsubtropicsbecauseofthehighevaporationrateshere.
>Chapter0120atmosphericlow-pressuresystems,whichcanoftenbeseveral hundredkilometres wide.These mesoscaleeddiesformwhenwaterflowsbetweenregionswithlargedensityortemperaturedifferences.Theycanbeclearlyrecognizedonsatellitephotographs.Investigationshaveshownthattheynotonlyoccurattheoceansurfaceas,forexample,intheNorthAtlanticarea,butcanalsobelocated atgreat depthsof thousandsof metres,e.g. offthecoastofBrazil.Becauseoftheirstronginfluenceonthelarge-scale heattransport, thesedeep-sea eddiesalso playan importantrole inlong-term climateprocesses.Variableanddynamic-theinfluenceofwindAlongwithconvection,windsalsoprovideanimportantcontributionindrivingtheoceancurrents.Incombina-tionwiththedivertingforcecausedbytheEarth"srota-tion(Coriolisforce)andtheshapeoftheoceanbasins,winds determinethe characteristicpatterns ofthe world-widesystemofsurfacecurrents.Especiallystrikingarethe largegyres thatextend acrossentire oceanbasins, forexamplebetweenAmericaandEurope.ThesesurfacecurrentsincludetheGulfStreamintheAtlanticOcean,whichisdrivenbothbywindandthethermohalineforce, aswell asthe Kuroshioin thePacific Ocean,whoseintensity justdecreases withdepth.TheGulfStreamisarelativelyfastcurrent.AlongthecoastofNorthAmericaitreachesaspeedofaround3.6kilometresperhourattheseasurface,whichisacasualwalkingspeed.Itextendsdowntoadepthofaround 2000metres, wherethe speedis aroundten timesslowerbecausetheinfluenceofthewindislessandthedensityofthewaterisgreater.Nevertheless,thewindcaninfacthaveadirectinfluencedowntogreatdepths.Typicalwindconditionscanchangeforextendedtimeperiods.Forexample,thenormallysteadytradewindscanblowfromadifferentdirectionformonthsatatime,causingchangesintheupwellingofwatermasses,andcreatingwavesandcurrentsintheocean"sinteriorthatresonateatdepthfordecades.Thesewavescanalsochangetheoceantemperatureandthusalsotheregionalclimate.Fromsatellitesthesewavesareperceivedasslowly movingups anddowns ofthe oceansurface.Furthermore,incertainregionstheprevailingwindscausepersistentupwellinganddownwellingmotion.Insomeareasthewindsdrivesurfacewatersawayfromthe landmasses, allowingcold waterfrom greaterdepthstoriseinitsplace.Thesurface-watertemperaturesintheseareasarethereforeespeciallylow.Importantup-wellingregionsareoftenfoundonthewesternmarginsof continentswhere thewinds blowparallel tothe coast(Chile, California,Namibia). Inthe southernhemisphere,forexample,becauseoftheCoriolisforce,thewaterispushed tothe leftaway fromthe coastwhen thewind isblowingfromthesouth.Thisproducesarollingmotioninthewater,wherebythewateronthesurfacemovesawayfromthecoastandwaterrisestoreplaceitfrombelow.Thisupwellingwaterisusuallyrichinnutrients,which iswhy manyupwelling regionsare alsoabundantwith fish.Theocean-aglobalstorehouseforheatIn additionto hugemasses ofwater, largeocean currentsalsotransportenormousamountsofheataroundtheglobe. Similarto theway thewater tankin aheating sys-1.9>SatellitephotographoftheGulfStreamanditseddies.Warmareasarered,coldareasareblue.TheCoriolisforceTheEarth"srotationcausesallfreelinearmotionontheEarth,suchasairorwatercurrents,tobedivertedtooneside.ThedivertingforceiscalledtheCoriolisforceorCoriolisaccel-eration.Itworksinoppositedirectionsinthenorthernandsouthernhemispheres.TheCoriolisforceisnamedaftertheFrenchnaturalscientistGaspareGustavedeCoriolis(1792to1843),whoderiveditmathe-matically.
21Theworldoceans,globalclimatedriversTheworld"slargeoceancurrentsarealsoinfluencedbytheprevailingwinds.Warmoceancurrentsarered,andcoldcurrentsareshowninblue.
>Chapter0122The Atlanticand PacificOceans eachcarry aroundonepetawattofheatnorthwardfromthetropicsandsub-tropics.Bycomparison,thesharemovedbytheIndianOcean isnegligible.InthissystemtheAtlantichasauniquefunctionamong theworld"s oceans.It isthe onlyocean basinthattransports heatnorthward throughoutits length,even inthe southernhemisphere. Europeansall benefitfrom thenorthwardtrend,thankstotheGulfStreamandtheNorthAtlanticCurrent.TheclimateintheregionoftheNorth Atlanticis comparativelymild, especiallyin north-westEurope,includingGermany.Thewintersinotherregionsatthesamelatitudearenotablycolder.InCana-da,forexample,thewintertemperaturesarearoundtendegreesCelsiuslowerthaninWesternEurope.Butitisnottheoceancirculationalonethatcausesthemildclimate.Aircurrentsalsocontributesignificantlytothisphenomenon.Thedistributionofmountainranges,par-ticularlytheRockyMountains,whichrunfromnorthtosouthalongthewestcoastofNorthAmerica,togetherwith theinfluence ofthe Coriolisforce, causesthe forma-tion ofvery stable,large-scale vorticesin theatmospherecalled standingplanetary waves.Such avortex liesabovethe USAbecause theRocky Mountainsact asan obstacletodivertlargeairmasses.AsaconsequencethewindsarepredominantlywesterlyovertheAtlanticcarryingrelativelymildairtonorthwestEurope,andfendoffthecold fromthe east.TheuncertainfutureofseaiceSeaiceintheArcticregionshasasignificantimpactonheatexchangebetweentheatmosphereandocean,because itacts asan insulatinglayer toprevent heatfrom0-40-804080120Wattspersquaremetre-1201.11>Heatexchangebetweentheatmosphereandtheseasur-face(inwattspersquaremetre)isveryvariabledependingontheoceanregion.Positivevaluesindicateabsorptionofheatbytheocean,whichischaracteristicofthetropics,andnegativevaluesindicateaheatloss,whichistypicalforthenorthernlatitudes.Inthehigharcticregions,however,heatlossisrela-tivelylowbecausetheseaiceactsasaninsulatinglayerandpreventsheatescapingfromthewater.
23Theworldoceans,globalclimatedriversOceanscontributetotheglobaltransportofheatwithdifferentintensities.Inthesouthernhemisphere,onlytheAt-lantictransportsheattothenorth(positivevalues).Theequa-torliesatzerodegrees.TheAtlanticandPacificeachcarryaroundonepetawattofheatasfaras20degreesnorthlatitude.Furthertothenorth,theAtlanticcarriesmorethanthePacific.TheIndianOcean,ontheotherhand,makesanegligiblecon-tributiontonorthwardheattransport.
>Chapter0124incomingsunlight.Thiseffectisenhancedwhentheiceiscoveredwithsnow.TheseaicethereforeinfluencestheradiationbalanceoftheEarthandthusplaysanimportant rolein theclimate system.Theimpactofseaiceonclimateisfurtheramplifiedbyitsinsulatingeffectbetweentheatmosphereandocean.Itinhibitstheexchangeofheatandwindenergybetweentheatmosphereandoceanconsiderably.Theatmosphereisthereforemuchcolderabovethesea-icesurface thanabove theopen ocean.This hasthe effectofincreasingtheair-temperaturedifferencebetweenthetropics,subtropics,andthepolarregions.Inwarmerregions theair hasa greatertendency torise, whichlow-ers theair pressuresignificantly. Bycontrast, invery coldregionstheairisheavier,andhighpressurezonesarecreated. Accordingly,the compensatingair flowbetweenhighandlowpressureareasisstrongand,inconcertwiththeCoriolisforce,createsstrongerwestlywindsinthemiddlelatitudes.Ofcourse,seaicealsoinfluencesconvectionprocessesintheocean,andthustheforma-tionofdeepandbottomwater.Seaicethereforeplaysanimportantroleinthelarge-scaleoceancirculation,especially withregard tothermohaline circulation.Itisnotyetknownhowglobalwarmingaffectstheformationofseaiceandtherelatedprocesses.Icemeltswhenitbecomeswarmer.Butitisdifficulttopredictwhateffectthishasonthecurrents.Inanycase,allclimatemodelspredictanaccelerationofwarmingintheArcticwithacontinuingriseintrace-gasconcentra-tions.Inaddition,observationsindicateacleardecreaseinArcticsea-icecoverinrecentdecades.Thisispartlyrelatedtoapositivefeedbackmechanismcalledtheice-albedofeedback.Lightsurfaceshaveaveryhighalbedo.Whentheseaiceretreatsasaresultofglobalwarming,albedodecreasesandmoresolarenergyisavailable,whichleadstoadditionalwarming,andmeltsmoreice.Thisprocessprimarilyoccursatthemarginsofthesea1.13>Asarule,icebergsconsistoffreshwaterorcontainonlysmallamountsofsalt.Becauseoftheirslightlylowerdensitycomparedtoseawater,asmallfractionextendsabovethewater.Thelargestpartisbelowthesurface.
25Theworldoceans,globalclimatedrivers >Chapter0226 Howclimatechangealtersoceanchemistry2
27Howclimatechangealtersoceanchemistry<
>Massiveemissionsofcarbondioxideintotheatmospherehaveanimpactonthechemicalandbiologicalprocessesintheocean.Thewarmingofoceanwatercouldleadtoadestabilizationofsolidmethanedepositsontheseafloor.BecauseoftheexcessCO2,theoceansarebecomingmoreacidic.ScientistsaremakingextensivemeasurementstodeterminehowmuchofthehumanmadeCO2isbeingabsorbedbytheoceans.Importantcluesareprovidedbylookingatoxygen.
>Chapter0228ThemutabilityofcarbonCarbonistheelementoflife.Thehumanbodystructureisbasedonit,andotheranimalandplantbiomasssuchasleavesandwoodconsistpredominantlyofcarbon(C).Plantsonlandandalgaeintheoceanassimilateitintheformofcarbondioxide(CO2)fromtheatmosphereorwater,andtransformitthroughphotosynthesisintoenergy-richmoleculessuchassugarsandstarches.Car-bonconstantlychangesitsstatethroughthemetabolismoforganismsandbynaturalchemicalprocesses.CarboncanbestoredinandexchangesbetweenparticulateanddissolvedinorganicandorganicformsandexchangedwiththetheatmosphereasCO2.Theoceansstoremuchmorecarbonthantheatmosphereandtheterrestrialbiosphere(plantsandanimals).Evenmorecarbon,how-ever,isstoredinthelithosphere,i.e.therocksontheplanet, includinglimestones (calciumcarbonate, CaCO3).Thethreemostimportantrepositorieswithinthecontextofanthropogenicclimatechange-atmosphere,terrestrial biosphereand ocean- areconstantly exchang-ingcarbon.Thisprocesscanoccurovertimespansofupto centuries,which atfirst glanceappears quiteslow. Butconsidering thatcarbon remainsbound upin therocks ofthe Earth"scrust formillions ofyears, thenthe exchangebetween theatmosphere, terrestrialbiosphere andoceancarbonreservoirscouldactuallybedescribedasrelativelyrapid. Todayscientists canestimate fairlyaccurately howmuchcarbonisstoredintheindividualreservoirs.Theocean,witharound38,000gigatons(Gt)ofcarbon(1gigaton=1billiontons),contains16timesasmuchcarbonastheterrestrialbiosphere,thatisallplantandtheunderlyingsoilsonourplanet,andaround60timesasmuchasthepre-industrialatmosphere,i.e.,atatimebeforepeoplebegantodrasticallyaltertheatmosphericCO2contentby theincreased burningof coal,oil andgas.Atthattimethecarboncontentoftheatmospherewasonlyaround600gigatonsofcarbon.Theoceanisthere-fore thegreatest ofthe carbonreservoirs, andessentiallydeterminestheatmosphericCO2content.Thecarbon,however,requirescenturiestopenetrateintothedeepocean,becausethemixingoftheoceansisaratherslow(Chapter1).Consequently,changesinatmosphericcar-boncontentthatareinducedbytheoceansalsooccuroveratimeframeofcenturies.Ingeologicaltimethatisquite fast,but froma humanperspective itis tooslow toextensively bufferclimate change.Withrespecttoclimatechange,thegreenhousegasCO2isofprimaryinterestintheglobalcarboncycle.Today,weknowthattheCO2concentrationintheatmos-pherechangedonlyslightlyduringthe12,000yearsbe-tweenthelasticeageandtheonsetoftheindustrialrevolutionatthebeginningofthe19thcentury.Thisrela-tivelystableCO2concentrationsuggeststhatthepre-industrialcarboncyclewaslargelyinequilibriumwiththeatmosphere.Itisassumedthat,inthispre-industrialequilibrium state,the oceanreleased around0.6 gigatonsofcarbonperyeartotheatmosphere.Thisisaresultoftheinputofcarbonfromlandplantscarriedbyriverstotheoceanand,afterdecompositionbybacteria,releasedintotheatmosphereasCO2,aswellasfrominorganiccarbonfromtheweatheringofcontinentalrockssuchaslimestones.Thistransportpresumablystilloccurstodayatratesessentiallyunchanged.SincethebeginningofTheoceans-thelargestCO2-reservoir>Theoceansabsorbsubstantialamountsofcarbondioxide,andtherebyconsumealargeportionofthisgreenhousegas,whichisreleasedbyhumanactivity.Thisdoesnotmean,however,thattheproblemcanbeignored,becausethisprocesstakescenturiesandcannotpreventtheconsequencesofclimatechange.Furthermore,itcannotbepredictedhowthemarinebiospherewillreacttotheuptakeofadditionalCO2.
29HowclimatechangealtersoceanchemistryThecarboncycleinthe1990swiththesizesofthevariousreservoirs(ingigatonsofcarbon,GtC),aswellastheannualfluxesbetweenthese.Pre-industrialnaturalfluxesareshowninblack,anthropogenicchangesinred.Thelossof140GtCintheterrestrialbiospherereflectsthecumulativeCO2emis-sionsfromland-usechange(primarilyslashandburnagricul-tureinthetropicalrainforests),andisaddedtothe244GtCemittedbytheburningoffossilfuels.TheterrestrialsinkforanthropogenicCO2of101GtCisnotdirectlyverifiable,butisderivedfromthedifferencebetweencumulativeemissions(244+140=384GtC)andthecombinationofatmosphericincrease(165GtC)andoceanicsinks(100+18=118GtC).
Rivers0.8Weathering0.2Weathering0.2Surfaceocean900+18Intermediateanddeepocean37100+100Surfacesediment1500.2507070.60.4391122.2206.4Marinebiota3Fossilfuels3700-244Atmosphere597+165ReservoirsizesinGtCFluxesandratesinGtCperyear90.21.6101Landsink2.6Landusechange1.6Respiration119.6GPP120
Vegetation,soilanddetritus2300+101-140
>Chapter0230IronisacrucialnutrientforplantsandthesecondmostabundantchemicalelementonEarth,althoughthegreatestportionbyfarislockedintheEarth"score.Manyregionshavesufficientironforplants.Inlargeregionsoftheocean,however,ironissoscarcethatthegrowthofsingle-celledalgaeislimitedbyitsabsence.Iron-limitationregionsincludethetropicaleasternPacificandpartsoftheNorthPacific,aswellastheentireSouthernOcean.Theseoceanregionsarerichintheprimarynutrients(macronutrients)nitrateandphosphate.Theiron,however,whichplantsrequireonlyinverysmallamounts(micronutrients),ismissing.ScientistsrefertothesemarineregionsasHNLCregions(highnutrient,lowchlorophyll)becausealgalgrowthhereisrestrictedandtheamountoftheplantpigmentchlorophyllisreducedaccordingly.Researchusingfertiliza-tionexperimentshasshownthatplantgrowthinalloftheseregionscanbestimulatedbyfertilizingthewaterwithiron.Becauseplantsassimilatecarbon,carbondioxidefromtheatmosphereisthuscon-vertedtobiomass,atleastfortheshortterm.Ironfertilizationisacompletelynaturalphenomenon.Forexam-ple,iron-richdustfromdesertsisblowntotheseabythewind.Ironalsoenterstheoceanswiththemeltwateroficebergsorbycontactofthewaterwithiron-richsedimentsontheseafloor.ItispresumedthatdifferentwindpatternsandadryeratmosphereduringthelasticeageledtoasignificantlyhigherinputofironintotheSouthernOcean.Thiscould,atleastinpart,explaintheconsiderablyloweratmosphericCO2levelsduringthelasticeage.Accordingly,modernmodellingsimulationsindicatethatlarge-scaleironfertilizationoftheoceanscoulddecreasethepresentatmosphericCO2levelsbyaround30ppm(partspermillion).Bycomparison,humanactivitieshaveincreasedtheatmosphericCO2levelsfromaround280ppmtoapresent-dayvalueof390ppm.Marinealgaeassimilatebetweenathousandandamilliontimeslessironthancarbon.Thusevenverylowquantitiesofironaresufficienttostimulatetheuptakeoflargeamountsofcarbondioxideinplants.UnderfavourableconditionslargeamountsofCO2canbeconvertedwithrelativelylittleiron.ThisraisestheobviousideaoffertilizingtheoceansonalargescaleandreducingtheCO2concen-trationsintheatmospherebystorageinmarineorganisms(seques-tration).Whenthealgaedie,however,andsinktothebottomandaredigestedbyanimalsorbrokendownbymicroorganisms,thecar-bondioxideisreleasedagain.InordertoevaluatewhetherthefixedFertilizingtheoceanwithiron2.2>Ironisacrucialnutrientforalgae,anditisscarceinmanyoceanregions,whichinhibitsalgalgrowth.Ifthewaterisfertilizedwithironthereisarapidincreaseinalgae.Microscopicinvestigationsofwatersamplestakenbytheresearchvessel"Polarstern"clearlyshowthatalgaeinthisiron-poorregionproliferatequicklyafterironfertilization.Aroundthreeweeksafterfertilizationthemarinealgalcommunitywasdominatedbyelongate,hard-shelleddiatoms.
31Howclimatechangealtersoceanchemistry >Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.
33Howclimatechangealtersoceanchemistry
Theworldoceans,globalclimatedrivers1
>Theoceanscoveraround70percentoftheEarth"ssurface.TheythusplayanimportantroleintheEarth"sclimateandinglobalwarming.Oneimportantfunctionoftheoceansistotransportheatfromthetropicstohigherlatitudes.Theyrespondveryslowlytochangesintheatmos-phere.Besideheat,theytakeuplargeamountsofthecarbondioxideemittedbyhumankind.
>Chapter0110TheinertiaofclimateAswealllearnedinschool,theworld"soceansareoneof themost importantelements inthe globalclimate sys-tem.Butwhatdoes"climate"actuallymean?Thediffer-encebetweenweatherandclimatecanbeexpressedinasinglesentence:"Climateiswhatyouexpect;weatheriswhatyouget."Thisrevealsafundamentaldifferencebetweenweatherandclimate.Weatherresearchiscon-cernedwiththeformation,movement,andpredictionoftheindividualelementsofweather,suchasaparticularlow-pressure systemor ahurricane. Climateresearch, ontheotherhand,dealswiththemorecomprehensivetotalityoflowpressuresystemsandhurricanes,andisdedicatedtoaddressingquestionssuchashowmanymidlatitudinalstormsorhurricaneswilloccurnextyear,or whetherthey willbecome morefrequent orintense inthecomingyearsasaresultofglobalwarming.Sotheterm "weather"refers toshort-term eventsin theatmos-phere, while"climate" relatesto longertime periods.Fordescribingclimate,asarule,atimespanof30yearsisgenerally usedas aframe ofreference.Peoplemainlyperceiveclimatechangeaschangesinatmosphericvariables,forexample,variationsintem-peratureorprecipitation.Inprinciple,duetoitschaoticdynamics, theatmosphere itselfcan generatemany natu-ralclimaticchanges.OneexampleofthisistheNorthAtlanticoscillation(NAO),whichsignificantlyinfluencesthe climateover partsof Europeand NorthAmerica. Itisa kindof pressurefluctuation betweentheIcelandic Lowand theAzores Highthat determinesthe strengthof win-ter westerlywinds acrossthe NorthAtlantic. Ifthese arestrong,theresultismildandrainyweatherinWesternEurope; ifthey areweak itis dryand cold.These kindsofnaturaloscillationsmakeitdifficulttorecognizeanthro-pogenic climatechanges dueto anenhanced greenhouseeffect.Theatmosphereisnotanisolatedsystem.Itinteractswith othercomponents ofthe Earthsystem -the oceans,for example.But itis alsoin contactwith thecryosphere(iceandsnow),thebiosphere(animalsandplants),thepedosphere (soil)and thelithosphere (rocks).All oftheseelementstogethercomposetheclimatesystem,whoseindividualcomponentsandprocessesareconnectedandinfluence eachother indiverse ways.These componentsall reactto changeat differentrates.TheatmosphereadjuststotheconditionsattheEarth"ssurfacesuchasoceantemperatureoricecoverwithinafewhourstodays.Furthermore,weatherisvariableandcanonlybepredictedafewdaysinadvance.Infact,ithas beenshown thatthe theoreticallimit ofweather pre-Earth"sclimatesystem-acomplexframework>TheEarth"sclimateisinfluencedbymanyfactors,includingsolarradia-tion,wind,andoceancurrents.Researcherstrytointegratealloftheseinfluencingvariablesintotheirmodels.Manyoftheprocessesinvolvedarenowwellunderstood.Butinteractionamongthevariousfactorsisverycomplexandnumerousquestionsremainunresolved.DaysMinutesYear100yearsThousandyearsMillionyearsAtmosphereOceansurfacelayerDeepoceanIcesheetsSeaiceBiosphere1.1>Differentcomponentsoftheclimatesystemreacttoperturbationsatdifferentrates.Thedeepocean,forexample,isanimportantcauseoftheslowresponseofclimate.Thecolouredareaonthetopscalerepresentstheshorttimespanofahumanlife.
11Theworldoceans,globalclimatedriversTheclimatesystem,itssub-systemsandrelevantprocessesandinteractions.
SeaicePrecipitationWindAbsorptionReflectionEmissionAir-oceaninteractionLand-airinteractionVolcanicgasesandparticlesBiogeochemicalcyclesTerrestrialoutgoingradiation(long-wave)CRYOSPHER ERiversInfluxHumaninterventionLakesCloudsIncomingsolarradiation(short-wave)Ice-oceaninteractionCurrentAir-iceinteractionLI T HOSPHER EPE D OSPH E R EBIOSPHER E
>Chapter0112Climatefluctuationsarenotunusual.IntheNorthAtlanticSector,forexample,itiswellknownthattheaveragetemperaturesandwindscanfluctuateondecadaltimescales.Climatechangescausedbyhumans(anthropogenic)alsoevolveoverthecourseofseveraldecades.Thenaturaldecadalchangesandthosecausedbyhumansarethereforesuperimposedupononeanother.Thismakesitdifficulttoassesstheimpactofhumansonclimatewithcertainty.IncontrasttothedynamicNorthAtlanticregion,theeffectsofclimatechangeareeasiertodetectinmorestableregionssuchasthetropicalIndi-anOcean.Thereisnodoubtthattheoceansdriveinterannualordecadalclimatefluctuations.DecadalfluctuationsofAtlantichurricaneactivityorprecipitationintheSahelcorrelateremarkablywellwithoscillationsofoceantemperatureintheNorthAtlantic.Althoughtheprecisemechanismsbehindthesedecadalchangesarenotyetfullyunderstood,thereisgeneralagreementthatvariationsintheAtlanticoverturningcirculationplayanimportantrole.Thishypoth-esisisalsosupportedbythefactthatAtlanticseasurfacetempera-tureanomaliesoccurincyclesofseveraldecades,withapatternwhichischaracterizedbyaninterhemisphericdipole.Whentherateofnorthwardwarmwatertransportincreases,thesurfaceairtem-peraturerisesintheNorthAtlanticandfallsintheSouthAtlantic.Ifitbecomescoolerinthenorthandwarmerinthesouth,itisanindicationofweakoceancurrents.Theair-temperaturedifferencebetweentheNorthandSouthAtlanticisthereforeameasureoftheoverturningcirculationstrength.Modernclimatemodelscansimulatethepresent-dayclimateandsomehistoricalclimatefluctuationsreasonablywell.Thesemodelsdescribetheclimatewithsatisfactoryreliability,especiallyonaglobalscale.ButforsmallergeographicalareasthemodelsarelessThedifficultyofdetectinganthropogenicclimatechange-5 -4 -2 -1 -0.6 -0.2 0.2 0.6 1245.6
20051.3>Europeexperiencedanunusuallycoldbeginningoftheyear2010.Butfromaglobalperspective,thewinterof2010wasthethirdwarmestinthepast131years.Ifthefirstfivemonthsoftheyearareconsidered,then2010isactuallythewarmest,anditevenreachedtheprevioustem-peraturerecordhighsforthemonthsofAprilandMay(top).Theyears1998and2005havebeensofarthetwowarmestyearsintheannualmean(relativetotheaverageof1951to1980).
1998JFMAMJJASOND0.30.40.50.60.70.8199820052010RecordhighTemperatureanomaly(°C)
13Theworldoceans,globalclimatedrivers -5-4-2-1-0.6-0.20.20.61245.6Temperatureanomaly(°C)2010 >Chapter0114HowhumansarechangingtheclimateThe humanimpact onclimate hasgreatly increasedoverthepasthundredyears.Wereleasevastamountsofclimate-relevanttracegasesintotheatmosphere.Thischangestheradiationbalanceoftheatmosphereandleads toglobal warming.In additionto carbondioxide, thesetrace gasesincludemethane,nitrousoxide(laughinggas),halogenatedfluoro-carbons, perfluorinatedhydrocarbons, andsulphur hexa-fluoride. Butcarbon dioxide(CO2) isespecially importantfortheEarth"sclimatesystembecausetheworldwideoutputissoenormous.Itisreleasedprimarilythroughtheburningoffossilfuels(oil,naturalgas,andcoal)inpowerplants,vehicleenginesorinhouseholdheatingsystems.Itsatmosphericlevelshaverisentoalmost390 partsper million(ppm) todayas comparedto thepre-industrialvalueof280ppm.Withthisincreasethetem-perature hasalso risenduring thetwentieth century.TheinternallydrivenchangesintheoceanssuchaschangesintheGulfStreamalsooccurwithinatimeframeofdecadesorafewcenturies.Thesehaveadecisiveinflu-enceonclimateandontheconcentrationofgreenhousegasesintheatmospherebecausetheyarestronglyinvolvedinglobalmasscyclessuchasthecarboncycle.For example,CO2dissolves easilyin water.However, theoceans havetaken upabout halfof allthe carbondioxideproducedbytheburningoffossilfuelssincethebegin-ning ofthe industrialrevolution, whichhas clearlydomi-natedthenaturalvariations.Whethertheclimatewillchange inthe future,and byhow much,can thereforebealso deducedfrom theoceans.Climatewillchangeveryslowlyinthefuturebecausetheoceanswiththeirimmensevolumesofwaterreactverygraduallytochange.Therefore,manybutnotalloftheconsequencesofclimatechangetriggeredbyhumanactivitywillonlygraduallybecomenoticeable.Someoftheseconsequencescouldactuallybeirreversiblewhencertainthresholdsarecrossed.Atsomepointitwillnolongerbepossible,forinstance,tostopthecompletemeltingoftheGreenlandicesheetandtheresultingseven-meterriseofsealevel.Thepositionofthethresh-old,however,isnotpreciselyknown.Butonethingiscertain:Eveniftheemissionofcarbondioxidewerestabilized attoday"s levelsit wouldnot leadto astabiliza-tionofthecarbondioxideconcentrationintheatmos-phere,becausecarbondioxideisextremelylong-livedandthecarbondioxidesinks,mainlytheoceans,donotabsorb itas quicklyas weproduce it.The situationis differentfor short-livedtrace gaseslikemethane(CH4).Ifmethaneemissionswerestabilizedatthepresentlevel,themethaneconcentrationintheatmospherewouldalsostabilize,becausemethanediminishes inthe atmosphereat aboutthe samerate asitisemitted.Inordertomaintainthecarbondioxidecon-centrationatagivenlevel,theemissionshavetobereduced toa fractionof thepresent amounts.1.4>Evenifitispossibletosignificantlyreducetheemissionofgreenhousegases,andCO2inparticular,bytheendofthiscentury,theimpactwillstillbeextensive.CO2hasalonglifeandremainsintheatmosphereformanycenturies.Becauseofthis,thetempera-tureontheEarthwillcontinuetorisebyafewtenthsofadegreeforacenturyorlonger.Becauseheatpenetratesveryslowlyintotheoceandepths,thewateralsoexpandsslowlyandsealevelwillcontinuetorisegraduallyoveralongperiodoftime.MeltingofthelargecontinentalicesheetsintheAntarcticandGreenlandisalsoaverygradualprocess.Meltwaterfromthesewillflowintotheoceanforcenturiesorevenmillennia,causingsealeveltocontinuetorise.ThefigureillustratestheprincipleofstabilizationatarbitrarylevelsofCO2between450and1000partspermillion(ppm),andthereforedoesnotshowanyunitsontheresponseaxis.CO2emissionpeakToday100years1000yearsMagnitudeofresponseCO2emissionsCO2stabilization:100to300yearsTemperaturestabilization:afewcenturiesSea-levelriseduetothermalexpansion:centuriestomillenniaSea-levelriseduetoicemelting:severalmillenniaTimetakentoreachequilibrium
15Theworldoceans,globalclimatedriversTobringatten-tiontothethreatofglobalwarming,thegovernmentoftheRepublicofMaldivesheldameetingontheseafloorinautumn2009justbeforetheCopenhagensummit.CarbondioxideandthegreenhouseeffectTheatmosphereisbecomingmoreenrichedincarbondioxide(CO2),ortobemoreprecise,car-bondioxideandotherclimate-relevanttracegases.Initiallytheyallowtheincomingshort-waveradiationofthesuntopassthrough.ThisenergyistransformedtoheatattheEarth"ssurfaceandisthenemittedbackaslong-waveradiation.Thegasesintheatmosphere,liketheglasspanesofagreenhouse,preventthislong-waveradia-tionfromescapingintospace,andtheEarth'ssurfacewarmsup.
>Chapter0116WhatdrivesthewatermassesWaterplaysacentralroleintheclimatesystem.Itsdensityvariesdependingonsalinityandtemperature.Cold, saltywater isheavy andsinks togreat depths.Thiscauses thecirculation ofmillions ofcubic metresof waterintheocean.Thispowerfulphenomenon,whichprima-rilyoccursinafewpolarregionsoftheocean,iscalledconvection.The surfacewater inthe NorthAtlantic regionsinks toa depthof around2000 metresdue toconvection. Thereitsettlesonanevendenserdeep-waterlayerfromtheAntarctic thatextends downto thesea floor.As thecoldandsaltysurfacewatersinksbyconvection,saltywaterflows infrom nearbywarmer regions,from thedirectionof theequator. Thiswater isthen cooledin theArctic airand alsobegins tosink, sothat theconvection iscontinu-ous.Beforesinking,thewaterabsorbsenormousamountsofgasessuchascarbondioxideattheseasurface,andthentransportsthemrapidlytomuchgreaterdepths.That iswhy thehighest concentrationsof carbondioxidein theocean arefound inthe convectionareas.Thehighcarbondioxideconcentrationspumpedintothewaterbyconvectionhavebeenshowntoreachdepthstodayofaround3000metres.Carbondioxideistransportedrelativelyrapidlybyconvectiontoadepthof2000metres.IntheNorthAtlanticthetransporttogreaterdepthstakessignificantlylongerbecausecarbondioxideandothergasescanonlypenetratethedeepwater byslow mixingprocesses.Lowtemperatureandhighsalinityaretheprimarydrivingforcesofconvection.Theypullthedensewaterofthepolarregionsdownward,whichdrivesaworld-wideconvectionenginecalledthermohalinecirculation(thermo-drivenbytemperaturedifferences;haline-driven bysalinity differences).The cold,salty watersub-mergesprimarilyintheLabradorandGreenlandSeas,andthenflowssouthwardtowardtheequatorandbeyond.Althoughconvectiononlyoccurslocallyinthepolarregions,itpropelsthermohalinecirculation,whichspans theglobe likea giantconveyor belt.Even theGulfStreamanditsbranchesaredrivenbyconvectionandthermohalinecirculation.Althoughwindalsoinfluencesthetransportofwatermasses,itscontributionissignifi-cantly less.Buthowdothewatermassesofdifferentdensitiesthatdriveoceanconvectionactuallyoriginate?Airtem-perature,evaporationandprecipitationareamongtheThegreatoceancurrents-theclimateengine>Oceancurrentstransportenormousamountsofheataroundtheworld.Thismakesthemoneofthemostimportantdrivingforcesofclimate.Becausetheyrespondextremelyslowlytochanges,theeffectsofglobalwarmingwillgraduallybecomenoticeablebutoveraperiodofcenturies.Climatechangesassociatedwithwindandseaicecouldbecomerecognizablemorequickly.SeafloorDensecoldwatersinksDeepwater2000mSouth
WarmwaterColdwaterIceSealevel1.6>TheconvectionprocessintheNorthAtlantic:Cold,saltywatersinksintheLabradorandintheGreenlandSea.ThiswaterformsalayerabovethedenserdeepwaterfromtheAntarcticatadepthofaround2000metresandflowstowardtheequator.Warmerwatersfromtheupperoceanlayersmoveintotheconvectionareatoreplacethesinkingwater.
17Theworldoceans,globalclimatedrivers WarmwaterIceColdwater
1.7>Thewatermoleculeisasymmetricalandisthereforeoppositelychargedatitstwoends(left).Thisiscalledadipole.Itthusbehavesdifferentlyfromothersubstancesinmanyways.Iceislessdense(top)andfloatsonthesurface.Freshwaterhasitsgreatestdensityatfourdegrees(bottom),andsinkstothebottom.Thisisthenoverlainbywarmwater(middle).Saltywaterhasdifferentcharacteristics.-++
>Chapter0118mostimportantfactorsintheanswertothisquestion.The freezingof waterin thepolar convectionregions alsoplaysacentralrole.Becauseiceonlycontainsaboutfivetenthsofapercentsalt,itleavesbehindaconsiderableamountofsaltinthewaterwhenitfreezes,whichincreases thesalinity ofthe surroundingocean waterandthusincreasesitsdensity.Thewatermassproducedbyconvection inthe Arcticis calledthe NorthAtlantic DeepWater (NADW).TheglobalconveyorbeltConvectionalsooccursintheAntarcticregions.Becauseoftheirevenhighersalinity,thewatermassesproducedheresinkallthewaytotheseafloor.ThisiscalledtheAntarcticBottomWater(AABW),anditflowsacrosstheoceanfloorhalfwayaroundtheglobeintotheNorthAtlantic. TheAABW isalso thedeep waterlayer thatthethick intermediateNADW overlieswhen itsinks bycon-vection.TheNADWformsintheGreenlandandLabra-dor Seas.Figure 1.8schematically illustratesits flowpathandthereturnflowofwarmwaterinthenear-surfacelayers, inthe globalconveyor beltof thermohalinecircu-lation.TheNADW,andespeciallytheAABW,remaininthedeepoceanforanamazinglylongtime.Radioactivecarbon-isotopedatingofthedeepwatersindicatesthatfromthetimeofsinkingintothedeepuntilitsreturntothesurface,aperiodofseveralhundredorevenupto1000 yearswill pass.Formostofthistimethewaterremainsinthecolderdeepregionsofthethermohalineconveyorbeltbecausetheretheflowrateisslow,ataroundonetothreekilo-metresperday,duetoitshighdensity.Theamountofwater involvedin thiscycle istruly immense.Its volumeis around400,000 cubickilometres, whichis equivalenttoaboutonethirdofthetotalwaterintheocean.Thisisenoughwatertofillaswimmingpool400kilometreslong,100kilometreswide,andtenkilometresdeep.Theoceanicconveyorbelttransportsabout20millioncubicmetresofwaterpersecond,whichisalmost5000timestheamountthatflowsoverNiagaraFallsinNorthAmerica.ConcernsaboutthebreakdownoftheGulfStreamTherehasbeenagreatdealofdiscussionabouttheextenttowhichclimatechangecouldinfluencethermohalinecirculationanditsturnoverprocessesintheAtlantic.After all,convection athigh latitudescould beweakenedbyanthropogenic(causedbyhumans)warmingoftheatmosphereandtheaccompanyingdecreaseinsurface-Thereisnootherareaintheoceanwherethesurfacewaterfindsitswaysoquicklyintothedeepasintheconvectionareas,andatnootherplacedochangesattheseasurfaceorintheatmospherebecomesorapidlyapparentintheocean"sinterior,forexample,intheincreasedcarbondioxidelevelsinthewaterasaresultofhighercarbondioxideconcentrationsintheatmos-phere.Convectionconnectstwodistinctlydifferentcomponentsoftheocean:thenear-surfacelayersthatareincontactwiththevariableatmos-phericfieldsofwind,radiationandprecipitation,andthedeepregionsoftheocean.Atthesurface,currents,temperatureandsalinityfluctuateonascaleofweekstomonths.Butatgreaterdepthstheenvironmentalcondi-tionschangeattimescalesofdecadesorcenturies.Intheconsistentlywarmoceanicregionsofthetropics(thewarmregionsoftheEarthbetween23.5degreesnorthand23.5degreessouthlatitude)andthesubtropics(theregionsbetween23.5and40degreesinthenorth-ernandsouthernhemispheres),thereisnoexchangebetweenthesurfaceanddeepwatersthatiscomparabletopolarconvection.Thisisbecause,averagedovertheyear,thereisanetradiationexcessofthesurface-layerwaters.Thewarmwater,withaminimumtemperatureoftendegreesCel-sius,hasarelativelylowdensityandfloatsasawarmlayerontopofthedeeper,colderwatermasses.Thetwolayersaredistinctlyseparatedwithnogradualtransitionbetweenthem.Attheboundarywheretheymeetthereisasharptemperaturejump,andthereforealsoanabruptdensitydifferencethatinhibitspenetrationoftheheattogreaterdepths.Thewarmsurfacelayerhasanaveragethicknessofseveralhundredmetres,whichisrela-tivelythincomparedtothetotaldepthoftheoceans.InverywarmoceanregionssuchasthewesternequatorialPacific,thereishardlyanyverticalmixingatall.Nearertothepoles,however,thereismoreverticalmixingoftheoceansandlayeringislesswell-defined.Becausethereisnoabrupttem-peratureanddensitychangethere,changesintheseasurfacecanbetrans-mittedtotheinteriordepthsoftheocean.Buttheconvectionareasarestilltheexpresselevatortothedeep.Thepathofwaterintothedeepocean
19Theworldoceans,globalclimatedriversTheworldwideoceancurrentsofthethermohalinecircu-lationsystemareextremelycomplex.Theflowofcold,salinesurfacewater(blue)downwardandtowardtheequatorcanonlybeclearlyrecognizedintheAtlantic.Warmsurfacewater(red)flowsintheoppositedirection,towardthepole.Inotherareasthecurrentrelationshipsarenotasclear-cutastheyareintheGulfStreamsystem(betweenNorthAmericaandEurope).TheCircumpolarCurrentflowsaroundAntarctica,anddoessothroughoutthetotaldepthofthewatercolumn.Thesmallyellowcirclesinthepolarregionsindicateconvectionareas.Thedarkareasarecharacterizedbyhighsalinityandthewhiteareasbylowsalinity.Saltyareasaremostlylocatedinthewarmsubtropicsbecauseofthehighevaporationrateshere.
>Chapter0120atmosphericlow-pressuresystems,whichcanoftenbeseveral hundredkilometres wide.These mesoscaleeddiesformwhenwaterflowsbetweenregionswithlargedensityortemperaturedifferences.Theycanbeclearlyrecognizedonsatellitephotographs.Investigationshaveshownthattheynotonlyoccurattheoceansurfaceas,forexample,intheNorthAtlanticarea,butcanalsobelocated atgreat depthsof thousandsof metres,e.g. offthecoastofBrazil.Becauseoftheirstronginfluenceonthelarge-scale heattransport, thesedeep-sea eddiesalso playan importantrole inlong-term climateprocesses.Variableanddynamic-theinfluenceofwindAlongwithconvection,windsalsoprovideanimportantcontributionindrivingtheoceancurrents.Incombina-tionwiththedivertingforcecausedbytheEarth"srota-tion(Coriolisforce)andtheshapeoftheoceanbasins,winds determinethe characteristicpatterns ofthe world-widesystemofsurfacecurrents.Especiallystrikingarethe largegyres thatextend acrossentire oceanbasins, forexamplebetweenAmericaandEurope.ThesesurfacecurrentsincludetheGulfStreamintheAtlanticOcean,whichisdrivenbothbywindandthethermohalineforce, aswell asthe Kuroshioin thePacific Ocean,whoseintensity justdecreases withdepth.TheGulfStreamisarelativelyfastcurrent.AlongthecoastofNorthAmericaitreachesaspeedofaround3.6kilometresperhourattheseasurface,whichisacasualwalkingspeed.Itextendsdowntoadepthofaround 2000metres, wherethe speedis aroundten timesslowerbecausetheinfluenceofthewindislessandthedensityofthewaterisgreater.Nevertheless,thewindcaninfacthaveadirectinfluencedowntogreatdepths.Typicalwindconditionscanchangeforextendedtimeperiods.Forexample,thenormallysteadytradewindscanblowfromadifferentdirectionformonthsatatime,causingchangesintheupwellingofwatermasses,andcreatingwavesandcurrentsintheocean"sinteriorthatresonateatdepthfordecades.Thesewavescanalsochangetheoceantemperatureandthusalsotheregionalclimate.Fromsatellitesthesewavesareperceivedasslowly movingups anddowns ofthe oceansurface.Furthermore,incertainregionstheprevailingwindscausepersistentupwellinganddownwellingmotion.Insomeareasthewindsdrivesurfacewatersawayfromthe landmasses, allowingcold waterfrom greaterdepthstoriseinitsplace.Thesurface-watertemperaturesintheseareasarethereforeespeciallylow.Importantup-wellingregionsareoftenfoundonthewesternmarginsof continentswhere thewinds blowparallel tothe coast(Chile, California,Namibia). Inthe southernhemisphere,forexample,becauseoftheCoriolisforce,thewaterispushed tothe leftaway fromthe coastwhen thewind isblowingfromthesouth.Thisproducesarollingmotioninthewater,wherebythewateronthesurfacemovesawayfromthecoastandwaterrisestoreplaceitfrombelow.Thisupwellingwaterisusuallyrichinnutrients,which iswhy manyupwelling regionsare alsoabundantwith fish.Theocean-aglobalstorehouseforheatIn additionto hugemasses ofwater, largeocean currentsalsotransportenormousamountsofheataroundtheglobe. Similarto theway thewater tankin aheating sys-1.9>SatellitephotographoftheGulfStreamanditseddies.Warmareasarered,coldareasareblue.TheCoriolisforceTheEarth"srotationcausesallfreelinearmotionontheEarth,suchasairorwatercurrents,tobedivertedtooneside.ThedivertingforceiscalledtheCoriolisforceorCoriolisaccel-eration.Itworksinoppositedirectionsinthenorthernandsouthernhemispheres.TheCoriolisforceisnamedaftertheFrenchnaturalscientistGaspareGustavedeCoriolis(1792to1843),whoderiveditmathe-matically.
21Theworldoceans,globalclimatedriversTheworld"slargeoceancurrentsarealsoinfluencedbytheprevailingwinds.Warmoceancurrentsarered,andcoldcurrentsareshowninblue.
>Chapter0122The Atlanticand PacificOceans eachcarry aroundonepetawattofheatnorthwardfromthetropicsandsub-tropics.Bycomparison,thesharemovedbytheIndianOcean isnegligible.InthissystemtheAtlantichasauniquefunctionamong theworld"s oceans.It isthe onlyocean basinthattransports heatnorthward throughoutits length,even inthe southernhemisphere. Europeansall benefitfrom thenorthwardtrend,thankstotheGulfStreamandtheNorthAtlanticCurrent.TheclimateintheregionoftheNorth Atlanticis comparativelymild, especiallyin north-westEurope,includingGermany.Thewintersinotherregionsatthesamelatitudearenotablycolder.InCana-da,forexample,thewintertemperaturesarearoundtendegreesCelsiuslowerthaninWesternEurope.Butitisnottheoceancirculationalonethatcausesthemildclimate.Aircurrentsalsocontributesignificantlytothisphenomenon.Thedistributionofmountainranges,par-ticularlytheRockyMountains,whichrunfromnorthtosouthalongthewestcoastofNorthAmerica,togetherwith theinfluence ofthe Coriolisforce, causesthe forma-tion ofvery stable,large-scale vorticesin theatmospherecalled standingplanetary waves.Such avortex liesabovethe USAbecause theRocky Mountainsact asan obstacletodivertlargeairmasses.AsaconsequencethewindsarepredominantlywesterlyovertheAtlanticcarryingrelativelymildairtonorthwestEurope,andfendoffthecold fromthe east.TheuncertainfutureofseaiceSeaiceintheArcticregionshasasignificantimpactonheatexchangebetweentheatmosphereandocean,because itacts asan insulatinglayer toprevent heatfrom0-40-804080120Wattspersquaremetre-1201.11>Heatexchangebetweentheatmosphereandtheseasur-face(inwattspersquaremetre)isveryvariabledependingontheoceanregion.Positivevaluesindicateabsorptionofheatbytheocean,whichischaracteristicofthetropics,andnegativevaluesindicateaheatloss,whichistypicalforthenorthernlatitudes.Inthehigharcticregions,however,heatlossisrela-tivelylowbecausetheseaiceactsasaninsulatinglayerandpreventsheatescapingfromthewater.
23Theworldoceans,globalclimatedriversOceanscontributetotheglobaltransportofheatwithdifferentintensities.Inthesouthernhemisphere,onlytheAt-lantictransportsheattothenorth(positivevalues).Theequa-torliesatzerodegrees.TheAtlanticandPacificeachcarryaroundonepetawattofheatasfaras20degreesnorthlatitude.Furthertothenorth,theAtlanticcarriesmorethanthePacific.TheIndianOcean,ontheotherhand,makesanegligiblecon-tributiontonorthwardheattransport.
>Chapter0124incomingsunlight.Thiseffectisenhancedwhentheiceiscoveredwithsnow.TheseaicethereforeinfluencestheradiationbalanceoftheEarthandthusplaysanimportant rolein theclimate system.Theimpactofseaiceonclimateisfurtheramplifiedbyitsinsulatingeffectbetweentheatmosphereandocean.Itinhibitstheexchangeofheatandwindenergybetweentheatmosphereandoceanconsiderably.Theatmosphereisthereforemuchcolderabovethesea-icesurface thanabove theopen ocean.This hasthe effectofincreasingtheair-temperaturedifferencebetweenthetropics,subtropics,andthepolarregions.Inwarmerregions theair hasa greatertendency torise, whichlow-ers theair pressuresignificantly. Bycontrast, invery coldregionstheairisheavier,andhighpressurezonesarecreated. Accordingly,the compensatingair flowbetweenhighandlowpressureareasisstrongand,inconcertwiththeCoriolisforce,createsstrongerwestlywindsinthemiddlelatitudes.Ofcourse,seaicealsoinfluencesconvectionprocessesintheocean,andthustheforma-tionofdeepandbottomwater.Seaicethereforeplaysanimportantroleinthelarge-scaleoceancirculation,especially withregard tothermohaline circulation.Itisnotyetknownhowglobalwarmingaffectstheformationofseaiceandtherelatedprocesses.Icemeltswhenitbecomeswarmer.Butitisdifficulttopredictwhateffectthishasonthecurrents.Inanycase,allclimatemodelspredictanaccelerationofwarmingintheArcticwithacontinuingriseintrace-gasconcentra-tions.Inaddition,observationsindicateacleardecreaseinArcticsea-icecoverinrecentdecades.Thisispartlyrelatedtoapositivefeedbackmechanismcalledtheice-albedofeedback.Lightsurfaceshaveaveryhighalbedo.Whentheseaiceretreatsasaresultofglobalwarming,albedodecreasesandmoresolarenergyisavailable,whichleadstoadditionalwarming,andmeltsmoreice.Thisprocessprimarilyoccursatthemarginsofthesea1.13>Asarule,icebergsconsistoffreshwaterorcontainonlysmallamountsofsalt.Becauseoftheirslightlylowerdensitycomparedtoseawater,asmallfractionextendsabovethewater.Thelargestpartisbelowthesurface.
25Theworldoceans,globalclimatedrivers >Chapter0226 Howclimatechangealtersoceanchemistry2
27Howclimatechangealtersoceanchemistry<
>Massiveemissionsofcarbondioxideintotheatmospherehaveanimpactonthechemicalandbiologicalprocessesintheocean.Thewarmingofoceanwatercouldleadtoadestabilizationofsolidmethanedepositsontheseafloor.BecauseoftheexcessCO2,theoceansarebecomingmoreacidic.ScientistsaremakingextensivemeasurementstodeterminehowmuchofthehumanmadeCO2isbeingabsorbedbytheoceans.Importantcluesareprovidedbylookingatoxygen.
>Chapter0228ThemutabilityofcarbonCarbonistheelementoflife.Thehumanbodystructureisbasedonit,andotheranimalandplantbiomasssuchasleavesandwoodconsistpredominantlyofcarbon(C).Plantsonlandandalgaeintheoceanassimilateitintheformofcarbondioxide(CO2)fromtheatmosphereorwater,andtransformitthroughphotosynthesisintoenergy-richmoleculessuchassugarsandstarches.Car-bonconstantlychangesitsstatethroughthemetabolismoforganismsandbynaturalchemicalprocesses.CarboncanbestoredinandexchangesbetweenparticulateanddissolvedinorganicandorganicformsandexchangedwiththetheatmosphereasCO2.Theoceansstoremuchmorecarbonthantheatmosphereandtheterrestrialbiosphere(plantsandanimals).Evenmorecarbon,how-ever,isstoredinthelithosphere,i.e.therocksontheplanet, includinglimestones (calciumcarbonate, CaCO3).Thethreemostimportantrepositorieswithinthecontextofanthropogenicclimatechange-atmosphere,terrestrial biosphereand ocean- areconstantly exchang-ingcarbon.Thisprocesscanoccurovertimespansofupto centuries,which atfirst glanceappears quiteslow. Butconsidering thatcarbon remainsbound upin therocks ofthe Earth"scrust formillions ofyears, thenthe exchangebetween theatmosphere, terrestrialbiosphere andoceancarbonreservoirscouldactuallybedescribedasrelativelyrapid. Todayscientists canestimate fairlyaccurately howmuchcarbonisstoredintheindividualreservoirs.Theocean,witharound38,000gigatons(Gt)ofcarbon(1gigaton=1billiontons),contains16timesasmuchcarbonastheterrestrialbiosphere,thatisallplantandtheunderlyingsoilsonourplanet,andaround60timesasmuchasthepre-industrialatmosphere,i.e.,atatimebeforepeoplebegantodrasticallyaltertheatmosphericCO2contentby theincreased burningof coal,oil andgas.Atthattimethecarboncontentoftheatmospherewasonlyaround600gigatonsofcarbon.Theoceanisthere-fore thegreatest ofthe carbonreservoirs, andessentiallydeterminestheatmosphericCO2content.Thecarbon,however,requirescenturiestopenetrateintothedeepocean,becausethemixingoftheoceansisaratherslow(Chapter1).Consequently,changesinatmosphericcar-boncontentthatareinducedbytheoceansalsooccuroveratimeframeofcenturies.Ingeologicaltimethatisquite fast,but froma humanperspective itis tooslow toextensively bufferclimate change.Withrespecttoclimatechange,thegreenhousegasCO2isofprimaryinterestintheglobalcarboncycle.Today,weknowthattheCO2concentrationintheatmos-pherechangedonlyslightlyduringthe12,000yearsbe-tweenthelasticeageandtheonsetoftheindustrialrevolutionatthebeginningofthe19thcentury.Thisrela-tivelystableCO2concentrationsuggeststhatthepre-industrialcarboncyclewaslargelyinequilibriumwiththeatmosphere.Itisassumedthat,inthispre-industrialequilibrium state,the oceanreleased around0.6 gigatonsofcarbonperyeartotheatmosphere.Thisisaresultoftheinputofcarbonfromlandplantscarriedbyriverstotheoceanand,afterdecompositionbybacteria,releasedintotheatmosphereasCO2,aswellasfrominorganiccarbonfromtheweatheringofcontinentalrockssuchaslimestones.Thistransportpresumablystilloccurstodayatratesessentiallyunchanged.SincethebeginningofTheoceans-thelargestCO2-reservoir>Theoceansabsorbsubstantialamountsofcarbondioxide,andtherebyconsumealargeportionofthisgreenhousegas,whichisreleasedbyhumanactivity.Thisdoesnotmean,however,thattheproblemcanbeignored,becausethisprocesstakescenturiesandcannotpreventtheconsequencesofclimatechange.Furthermore,itcannotbepredictedhowthemarinebiospherewillreacttotheuptakeofadditionalCO2.
29HowclimatechangealtersoceanchemistryThecarboncycleinthe1990swiththesizesofthevariousreservoirs(ingigatonsofcarbon,GtC),aswellastheannualfluxesbetweenthese.Pre-industrialnaturalfluxesareshowninblack,anthropogenicchangesinred.Thelossof140GtCintheterrestrialbiospherereflectsthecumulativeCO2emis-sionsfromland-usechange(primarilyslashandburnagricul-tureinthetropicalrainforests),andisaddedtothe244GtCemittedbytheburningoffossilfuels.TheterrestrialsinkforanthropogenicCO2of101GtCisnotdirectlyverifiable,butisderivedfromthedifferencebetweencumulativeemissions(244+140=384GtC)andthecombinationofatmosphericincrease(165GtC)andoceanicsinks(100+18=118GtC).
Rivers0.8Weathering0.2Weathering0.2Surfaceocean900+18Intermediateanddeepocean37100+100Surfacesediment1500.2507070.60.4391122.2206.4Marinebiota3Fossilfuels3700-244Atmosphere597+165ReservoirsizesinGtCFluxesandratesinGtCperyear90.21.6101Landsink2.6Landusechange1.6Respiration119.6GPP120
Vegetation,soilanddetritus2300+101-140
>Chapter0230IronisacrucialnutrientforplantsandthesecondmostabundantchemicalelementonEarth,althoughthegreatestportionbyfarislockedintheEarth"score.Manyregionshavesufficientironforplants.Inlargeregionsoftheocean,however,ironissoscarcethatthegrowthofsingle-celledalgaeislimitedbyitsabsence.Iron-limitationregionsincludethetropicaleasternPacificandpartsoftheNorthPacific,aswellastheentireSouthernOcean.Theseoceanregionsarerichintheprimarynutrients(macronutrients)nitrateandphosphate.Theiron,however,whichplantsrequireonlyinverysmallamounts(micronutrients),ismissing.ScientistsrefertothesemarineregionsasHNLCregions(highnutrient,lowchlorophyll)becausealgalgrowthhereisrestrictedandtheamountoftheplantpigmentchlorophyllisreducedaccordingly.Researchusingfertiliza-tionexperimentshasshownthatplantgrowthinalloftheseregionscanbestimulatedbyfertilizingthewaterwithiron.Becauseplantsassimilatecarbon,carbondioxidefromtheatmosphereisthuscon-vertedtobiomass,atleastfortheshortterm.Ironfertilizationisacompletelynaturalphenomenon.Forexam-ple,iron-richdustfromdesertsisblowntotheseabythewind.Ironalsoenterstheoceanswiththemeltwateroficebergsorbycontactofthewaterwithiron-richsedimentsontheseafloor.ItispresumedthatdifferentwindpatternsandadryeratmosphereduringthelasticeageledtoasignificantlyhigherinputofironintotheSouthernOcean.Thiscould,atleastinpart,explaintheconsiderablyloweratmosphericCO2levelsduringthelasticeage.Accordingly,modernmodellingsimulationsindicatethatlarge-scaleironfertilizationoftheoceanscoulddecreasethepresentatmosphericCO2levelsbyaround30ppm(partspermillion).Bycomparison,humanactivitieshaveincreasedtheatmosphericCO2levelsfromaround280ppmtoapresent-dayvalueof390ppm.Marinealgaeassimilatebetweenathousandandamilliontimeslessironthancarbon.Thusevenverylowquantitiesofironaresufficienttostimulatetheuptakeoflargeamountsofcarbondioxideinplants.UnderfavourableconditionslargeamountsofCO2canbeconvertedwithrelativelylittleiron.ThisraisestheobviousideaoffertilizingtheoceansonalargescaleandreducingtheCO2concen-trationsintheatmospherebystorageinmarineorganisms(seques-tration).Whenthealgaedie,however,andsinktothebottomandaredigestedbyanimalsorbrokendownbymicroorganisms,thecar-bondioxideisreleasedagain.InordertoevaluatewhetherthefixedFertilizingtheoceanwithiron2.2>Ironisacrucialnutrientforalgae,anditisscarceinmanyoceanregions,whichinhibitsalgalgrowth.Ifthewaterisfertilizedwithironthereisarapidincreaseinalgae.Microscopicinvestigationsofwatersamplestakenbytheresearchvessel"Polarstern"clearlyshowthatalgaeinthisiron-poorregionproliferatequicklyafterironfertilization.Aroundthreeweeksafterfertilizationthemarinealgalcommunitywasdominatedbyelongate,hard-shelleddiatoms.
31Howclimatechangealtersoceanchemistry >Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.
33Howclimatechangealtersoceanchemistry
>Chapter0114HowhumansarechangingtheclimateThe humanimpact onclimate hasgreatly increasedoverthepasthundredyears.Wereleasevastamountsofclimate-relevanttracegasesintotheatmosphere.Thischangestheradiationbalanceoftheatmosphereandleads toglobal warming.In additionto carbondioxide, thesetrace gasesincludemethane,nitrousoxide(laughinggas),halogenatedfluoro-carbons, perfluorinatedhydrocarbons, andsulphur hexa-fluoride. Butcarbon dioxide(CO2) isespecially importantfortheEarth"sclimatesystembecausetheworldwideoutputissoenormous.Itisreleasedprimarilythroughtheburningoffossilfuels(oil,naturalgas,andcoal)inpowerplants,vehicleenginesorinhouseholdheatingsystems.Itsatmosphericlevelshaverisentoalmost390 partsper million(ppm) todayas comparedto thepre-industrialvalueof280ppm.Withthisincreasethetem-perature hasalso risenduring thetwentieth century.TheinternallydrivenchangesintheoceanssuchaschangesintheGulfStreamalsooccurwithinatimeframeofdecadesorafewcenturies.Thesehaveadecisiveinflu-enceonclimateandontheconcentrationofgreenhousegasesintheatmospherebecausetheyarestronglyinvolvedinglobalmasscyclessuchasthecarboncycle.For example,CO2dissolves easilyin water.However, theoceans havetaken upabout halfof allthe carbondioxideproducedbytheburningoffossilfuelssincethebegin-ning ofthe industrialrevolution, whichhas clearlydomi-natedthenaturalvariations.Whethertheclimatewillchange inthe future,and byhow much,can thereforebealso deducedfrom theoceans.Climatewillchangeveryslowlyinthefuturebecausetheoceanswiththeirimmensevolumesofwaterreactverygraduallytochange.Therefore,manybutnotalloftheconsequencesofclimatechangetriggeredbyhumanactivitywillonlygraduallybecomenoticeable.Someoftheseconsequencescouldactuallybeirreversiblewhencertainthresholdsarecrossed.Atsomepointitwillnolongerbepossible,forinstance,tostopthecompletemeltingoftheGreenlandicesheetandtheresultingseven-meterriseofsealevel.Thepositionofthethresh-old,however,isnotpreciselyknown.Butonethingiscertain:Eveniftheemissionofcarbondioxidewerestabilized attoday"s levelsit wouldnot leadto astabiliza-tionofthecarbondioxideconcentrationintheatmos-phere,becausecarbondioxideisextremelylong-livedandthecarbondioxidesinks,mainlytheoceans,donotabsorb itas quicklyas weproduce it.The situationis differentfor short-livedtrace gaseslikemethane(CH4).Ifmethaneemissionswerestabilizedatthepresentlevel,themethaneconcentrationintheatmospherewouldalsostabilize,becausemethanediminishes inthe atmosphereat aboutthe samerate asitisemitted.Inordertomaintainthecarbondioxidecon-centrationatagivenlevel,theemissionshavetobereduced toa fractionof thepresent amounts.1.4>Evenifitispossibletosignificantlyreducetheemissionofgreenhousegases,andCO2inparticular,bytheendofthiscentury,theimpactwillstillbeextensive.CO2hasalonglifeandremainsintheatmosphereformanycenturies.Becauseofthis,thetempera-tureontheEarthwillcontinuetorisebyafewtenthsofadegreeforacenturyorlonger.Becauseheatpenetratesveryslowlyintotheoceandepths,thewateralsoexpandsslowlyandsealevelwillcontinuetorisegraduallyoveralongperiodoftime.MeltingofthelargecontinentalicesheetsintheAntarcticandGreenlandisalsoaverygradualprocess.Meltwaterfromthesewillflowintotheoceanforcenturiesorevenmillennia,causingsealeveltocontinuetorise.ThefigureillustratestheprincipleofstabilizationatarbitrarylevelsofCO2between450and1000partspermillion(ppm),andthereforedoesnotshowanyunitsontheresponseaxis.CO2emissionpeakToday100years1000yearsMagnitudeofresponseCO2emissionsCO2stabilization:100to300yearsTemperaturestabilization:afewcenturiesSea-levelriseduetothermalexpansion:centuriestomillenniaSea-levelriseduetoicemelting:severalmillenniaTimetakentoreachequilibrium
15Theworldoceans,globalclimatedriversTobringatten-tiontothethreatofglobalwarming,thegovernmentoftheRepublicofMaldivesheldameetingontheseafloorinautumn2009justbeforetheCopenhagensummit.CarbondioxideandthegreenhouseeffectTheatmosphereisbecomingmoreenrichedincarbondioxide(CO2),ortobemoreprecise,car-bondioxideandotherclimate-relevanttracegases.Initiallytheyallowtheincomingshort-waveradiationofthesuntopassthrough.ThisenergyistransformedtoheatattheEarth"ssurfaceandisthenemittedbackaslong-waveradiation.Thegasesintheatmosphere,liketheglasspanesofagreenhouse,preventthislong-waveradia-tionfromescapingintospace,andtheEarth'ssurfacewarmsup.
>Chapter0116WhatdrivesthewatermassesWaterplaysacentralroleintheclimatesystem.Itsdensityvariesdependingonsalinityandtemperature.Cold, saltywater isheavy andsinks togreat depths.Thiscauses thecirculation ofmillions ofcubic metresof waterintheocean.Thispowerfulphenomenon,whichprima-rilyoccursinafewpolarregionsoftheocean,iscalledconvection.The surfacewater inthe NorthAtlantic regionsinks toa depthof around2000 metresdue toconvection. Thereitsettlesonanevendenserdeep-waterlayerfromtheAntarctic thatextends downto thesea floor.As thecoldandsaltysurfacewatersinksbyconvection,saltywaterflows infrom nearbywarmer regions,from thedirectionof theequator. Thiswater isthen cooledin theArctic airand alsobegins tosink, sothat theconvection iscontinu-ous.Beforesinking,thewaterabsorbsenormousamountsofgasessuchascarbondioxideattheseasurface,andthentransportsthemrapidlytomuchgreaterdepths.That iswhy thehighest concentrationsof carbondioxidein theocean arefound inthe convectionareas.Thehighcarbondioxideconcentrationspumpedintothewaterbyconvectionhavebeenshowntoreachdepthstodayofaround3000metres.Carbondioxideistransportedrelativelyrapidlybyconvectiontoadepthof2000metres.IntheNorthAtlanticthetransporttogreaterdepthstakessignificantlylongerbecausecarbondioxideandothergasescanonlypenetratethedeepwater byslow mixingprocesses.Lowtemperatureandhighsalinityaretheprimarydrivingforcesofconvection.Theypullthedensewaterofthepolarregionsdownward,whichdrivesaworld-wideconvectionenginecalledthermohalinecirculation(thermo-drivenbytemperaturedifferences;haline-driven bysalinity differences).The cold,salty watersub-mergesprimarilyintheLabradorandGreenlandSeas,andthenflowssouthwardtowardtheequatorandbeyond.Althoughconvectiononlyoccurslocallyinthepolarregions,itpropelsthermohalinecirculation,whichspans theglobe likea giantconveyor belt.Even theGulfStreamanditsbranchesaredrivenbyconvectionandthermohalinecirculation.Althoughwindalsoinfluencesthetransportofwatermasses,itscontributionissignifi-cantly less.Buthowdothewatermassesofdifferentdensitiesthatdriveoceanconvectionactuallyoriginate?Airtem-perature,evaporationandprecipitationareamongtheThegreatoceancurrents-theclimateengine>Oceancurrentstransportenormousamountsofheataroundtheworld.Thismakesthemoneofthemostimportantdrivingforcesofclimate.Becausetheyrespondextremelyslowlytochanges,theeffectsofglobalwarmingwillgraduallybecomenoticeablebutoveraperiodofcenturies.Climatechangesassociatedwithwindandseaicecouldbecomerecognizablemorequickly.SeafloorDensecoldwatersinksDeepwater2000mSouth
WarmwaterColdwaterIceSealevel1.6>TheconvectionprocessintheNorthAtlantic:Cold,saltywatersinksintheLabradorandintheGreenlandSea.ThiswaterformsalayerabovethedenserdeepwaterfromtheAntarcticatadepthofaround2000metresandflowstowardtheequator.Warmerwatersfromtheupperoceanlayersmoveintotheconvectionareatoreplacethesinkingwater.
17Theworldoceans,globalclimatedrivers WarmwaterIceColdwater
1.7>Thewatermoleculeisasymmetricalandisthereforeoppositelychargedatitstwoends(left).Thisiscalledadipole.Itthusbehavesdifferentlyfromothersubstancesinmanyways.Iceislessdense(top)andfloatsonthesurface.Freshwaterhasitsgreatestdensityatfourdegrees(bottom),andsinkstothebottom.Thisisthenoverlainbywarmwater(middle).Saltywaterhasdifferentcharacteristics.-++
>Chapter0118mostimportantfactorsintheanswertothisquestion.The freezingof waterin thepolar convectionregions alsoplaysacentralrole.Becauseiceonlycontainsaboutfivetenthsofapercentsalt,itleavesbehindaconsiderableamountofsaltinthewaterwhenitfreezes,whichincreases thesalinity ofthe surroundingocean waterandthusincreasesitsdensity.Thewatermassproducedbyconvection inthe Arcticis calledthe NorthAtlantic DeepWater (NADW).TheglobalconveyorbeltConvectionalsooccursintheAntarcticregions.Becauseoftheirevenhighersalinity,thewatermassesproducedheresinkallthewaytotheseafloor.ThisiscalledtheAntarcticBottomWater(AABW),anditflowsacrosstheoceanfloorhalfwayaroundtheglobeintotheNorthAtlantic. TheAABW isalso thedeep waterlayer thatthethick intermediateNADW overlieswhen itsinks bycon-vection.TheNADWformsintheGreenlandandLabra-dor Seas.Figure 1.8schematically illustratesits flowpathandthereturnflowofwarmwaterinthenear-surfacelayers, inthe globalconveyor beltof thermohalinecircu-lation.TheNADW,andespeciallytheAABW,remaininthedeepoceanforanamazinglylongtime.Radioactivecarbon-isotopedatingofthedeepwatersindicatesthatfromthetimeofsinkingintothedeepuntilitsreturntothesurface,aperiodofseveralhundredorevenupto1000 yearswill pass.Formostofthistimethewaterremainsinthecolderdeepregionsofthethermohalineconveyorbeltbecausetheretheflowrateisslow,ataroundonetothreekilo-metresperday,duetoitshighdensity.Theamountofwater involvedin thiscycle istruly immense.Its volumeis around400,000 cubickilometres, whichis equivalenttoaboutonethirdofthetotalwaterintheocean.Thisisenoughwatertofillaswimmingpool400kilometreslong,100kilometreswide,andtenkilometresdeep.Theoceanicconveyorbelttransportsabout20millioncubicmetresofwaterpersecond,whichisalmost5000timestheamountthatflowsoverNiagaraFallsinNorthAmerica.ConcernsaboutthebreakdownoftheGulfStreamTherehasbeenagreatdealofdiscussionabouttheextenttowhichclimatechangecouldinfluencethermohalinecirculationanditsturnoverprocessesintheAtlantic.After all,convection athigh latitudescould beweakenedbyanthropogenic(causedbyhumans)warmingoftheatmosphereandtheaccompanyingdecreaseinsurface-Thereisnootherareaintheoceanwherethesurfacewaterfindsitswaysoquicklyintothedeepasintheconvectionareas,andatnootherplacedochangesattheseasurfaceorintheatmospherebecomesorapidlyapparentintheocean"sinterior,forexample,intheincreasedcarbondioxidelevelsinthewaterasaresultofhighercarbondioxideconcentrationsintheatmos-phere.Convectionconnectstwodistinctlydifferentcomponentsoftheocean:thenear-surfacelayersthatareincontactwiththevariableatmos-phericfieldsofwind,radiationandprecipitation,andthedeepregionsoftheocean.Atthesurface,currents,temperatureandsalinityfluctuateonascaleofweekstomonths.Butatgreaterdepthstheenvironmentalcondi-tionschangeattimescalesofdecadesorcenturies.Intheconsistentlywarmoceanicregionsofthetropics(thewarmregionsoftheEarthbetween23.5degreesnorthand23.5degreessouthlatitude)andthesubtropics(theregionsbetween23.5and40degreesinthenorth-ernandsouthernhemispheres),thereisnoexchangebetweenthesurfaceanddeepwatersthatiscomparabletopolarconvection.Thisisbecause,averagedovertheyear,thereisanetradiationexcessofthesurface-layerwaters.Thewarmwater,withaminimumtemperatureoftendegreesCel-sius,hasarelativelylowdensityandfloatsasawarmlayerontopofthedeeper,colderwatermasses.Thetwolayersaredistinctlyseparatedwithnogradualtransitionbetweenthem.Attheboundarywheretheymeetthereisasharptemperaturejump,andthereforealsoanabruptdensitydifferencethatinhibitspenetrationoftheheattogreaterdepths.Thewarmsurfacelayerhasanaveragethicknessofseveralhundredmetres,whichisrela-tivelythincomparedtothetotaldepthoftheoceans.InverywarmoceanregionssuchasthewesternequatorialPacific,thereishardlyanyverticalmixingatall.Nearertothepoles,however,thereismoreverticalmixingoftheoceansandlayeringislesswell-defined.Becausethereisnoabrupttem-peratureanddensitychangethere,changesintheseasurfacecanbetrans-mittedtotheinteriordepthsoftheocean.Buttheconvectionareasarestilltheexpresselevatortothedeep.Thepathofwaterintothedeepocean
19Theworldoceans,globalclimatedriversTheworldwideoceancurrentsofthethermohalinecircu-lationsystemareextremelycomplex.Theflowofcold,salinesurfacewater(blue)downwardandtowardtheequatorcanonlybeclearlyrecognizedintheAtlantic.Warmsurfacewater(red)flowsintheoppositedirection,towardthepole.Inotherareasthecurrentrelationshipsarenotasclear-cutastheyareintheGulfStreamsystem(betweenNorthAmericaandEurope).TheCircumpolarCurrentflowsaroundAntarctica,anddoessothroughoutthetotaldepthofthewatercolumn.Thesmallyellowcirclesinthepolarregionsindicateconvectionareas.Thedarkareasarecharacterizedbyhighsalinityandthewhiteareasbylowsalinity.Saltyareasaremostlylocatedinthewarmsubtropicsbecauseofthehighevaporationrateshere.
>Chapter0120atmosphericlow-pressuresystems,whichcanoftenbeseveral hundredkilometres wide.These mesoscaleeddiesformwhenwaterflowsbetweenregionswithlargedensityortemperaturedifferences.Theycanbeclearlyrecognizedonsatellitephotographs.Investigationshaveshownthattheynotonlyoccurattheoceansurfaceas,forexample,intheNorthAtlanticarea,butcanalsobelocated atgreat depthsof thousandsof metres,e.g. offthecoastofBrazil.Becauseoftheirstronginfluenceonthelarge-scale heattransport, thesedeep-sea eddiesalso playan importantrole inlong-term climateprocesses.Variableanddynamic-theinfluenceofwindAlongwithconvection,windsalsoprovideanimportantcontributionindrivingtheoceancurrents.Incombina-tionwiththedivertingforcecausedbytheEarth"srota-tion(Coriolisforce)andtheshapeoftheoceanbasins,winds determinethe characteristicpatterns ofthe world-widesystemofsurfacecurrents.Especiallystrikingarethe largegyres thatextend acrossentire oceanbasins, forexamplebetweenAmericaandEurope.ThesesurfacecurrentsincludetheGulfStreamintheAtlanticOcean,whichisdrivenbothbywindandthethermohalineforce, aswell asthe Kuroshioin thePacific Ocean,whoseintensity justdecreases withdepth.TheGulfStreamisarelativelyfastcurrent.AlongthecoastofNorthAmericaitreachesaspeedofaround3.6kilometresperhourattheseasurface,whichisacasualwalkingspeed.Itextendsdowntoadepthofaround 2000metres, wherethe speedis aroundten timesslowerbecausetheinfluenceofthewindislessandthedensityofthewaterisgreater.Nevertheless,thewindcaninfacthaveadirectinfluencedowntogreatdepths.Typicalwindconditionscanchangeforextendedtimeperiods.Forexample,thenormallysteadytradewindscanblowfromadifferentdirectionformonthsatatime,causingchangesintheupwellingofwatermasses,andcreatingwavesandcurrentsintheocean"sinteriorthatresonateatdepthfordecades.Thesewavescanalsochangetheoceantemperatureandthusalsotheregionalclimate.Fromsatellitesthesewavesareperceivedasslowly movingups anddowns ofthe oceansurface.Furthermore,incertainregionstheprevailingwindscausepersistentupwellinganddownwellingmotion.Insomeareasthewindsdrivesurfacewatersawayfromthe landmasses, allowingcold waterfrom greaterdepthstoriseinitsplace.Thesurface-watertemperaturesintheseareasarethereforeespeciallylow.Importantup-wellingregionsareoftenfoundonthewesternmarginsof continentswhere thewinds blowparallel tothe coast(Chile, California,Namibia). Inthe southernhemisphere,forexample,becauseoftheCoriolisforce,thewaterispushed tothe leftaway fromthe coastwhen thewind isblowingfromthesouth.Thisproducesarollingmotioninthewater,wherebythewateronthesurfacemovesawayfromthecoastandwaterrisestoreplaceitfrombelow.Thisupwellingwaterisusuallyrichinnutrients,which iswhy manyupwelling regionsare alsoabundantwith fish.Theocean-aglobalstorehouseforheatIn additionto hugemasses ofwater, largeocean currentsalsotransportenormousamountsofheataroundtheglobe. Similarto theway thewater tankin aheating sys-1.9>SatellitephotographoftheGulfStreamanditseddies.Warmareasarered,coldareasareblue.TheCoriolisforceTheEarth"srotationcausesallfreelinearmotionontheEarth,suchasairorwatercurrents,tobedivertedtooneside.ThedivertingforceiscalledtheCoriolisforceorCoriolisaccel-eration.Itworksinoppositedirectionsinthenorthernandsouthernhemispheres.TheCoriolisforceisnamedaftertheFrenchnaturalscientistGaspareGustavedeCoriolis(1792to1843),whoderiveditmathe-matically.
21Theworldoceans,globalclimatedriversTheworld"slargeoceancurrentsarealsoinfluencedbytheprevailingwinds.Warmoceancurrentsarered,andcoldcurrentsareshowninblue.
>Chapter0122The Atlanticand PacificOceans eachcarry aroundonepetawattofheatnorthwardfromthetropicsandsub-tropics.Bycomparison,thesharemovedbytheIndianOcean isnegligible.InthissystemtheAtlantichasauniquefunctionamong theworld"s oceans.It isthe onlyocean basinthattransports heatnorthward throughoutits length,even inthe southernhemisphere. Europeansall benefitfrom thenorthwardtrend,thankstotheGulfStreamandtheNorthAtlanticCurrent.TheclimateintheregionoftheNorth Atlanticis comparativelymild, especiallyin north-westEurope,includingGermany.Thewintersinotherregionsatthesamelatitudearenotablycolder.InCana-da,forexample,thewintertemperaturesarearoundtendegreesCelsiuslowerthaninWesternEurope.Butitisnottheoceancirculationalonethatcausesthemildclimate.Aircurrentsalsocontributesignificantlytothisphenomenon.Thedistributionofmountainranges,par-ticularlytheRockyMountains,whichrunfromnorthtosouthalongthewestcoastofNorthAmerica,togetherwith theinfluence ofthe Coriolisforce, causesthe forma-tion ofvery stable,large-scale vorticesin theatmospherecalled standingplanetary waves.Such avortex liesabovethe USAbecause theRocky Mountainsact asan obstacletodivertlargeairmasses.AsaconsequencethewindsarepredominantlywesterlyovertheAtlanticcarryingrelativelymildairtonorthwestEurope,andfendoffthecold fromthe east.TheuncertainfutureofseaiceSeaiceintheArcticregionshasasignificantimpactonheatexchangebetweentheatmosphereandocean,because itacts asan insulatinglayer toprevent heatfrom0-40-804080120Wattspersquaremetre-1201.11>Heatexchangebetweentheatmosphereandtheseasur-face(inwattspersquaremetre)isveryvariabledependingontheoceanregion.Positivevaluesindicateabsorptionofheatbytheocean,whichischaracteristicofthetropics,andnegativevaluesindicateaheatloss,whichistypicalforthenorthernlatitudes.Inthehigharcticregions,however,heatlossisrela-tivelylowbecausetheseaiceactsasaninsulatinglayerandpreventsheatescapingfromthewater.
23Theworldoceans,globalclimatedriversOceanscontributetotheglobaltransportofheatwithdifferentintensities.Inthesouthernhemisphere,onlytheAt-lantictransportsheattothenorth(positivevalues).Theequa-torliesatzerodegrees.TheAtlanticandPacificeachcarryaroundonepetawattofheatasfaras20degreesnorthlatitude.Furthertothenorth,theAtlanticcarriesmorethanthePacific.TheIndianOcean,ontheotherhand,makesanegligiblecon-tributiontonorthwardheattransport.
>Chapter0124incomingsunlight.Thiseffectisenhancedwhentheiceiscoveredwithsnow.TheseaicethereforeinfluencestheradiationbalanceoftheEarthandthusplaysanimportant rolein theclimate system.Theimpactofseaiceonclimateisfurtheramplifiedbyitsinsulatingeffectbetweentheatmosphereandocean.Itinhibitstheexchangeofheatandwindenergybetweentheatmosphereandoceanconsiderably.Theatmosphereisthereforemuchcolderabovethesea-icesurface thanabove theopen ocean.This hasthe effectofincreasingtheair-temperaturedifferencebetweenthetropics,subtropics,andthepolarregions.Inwarmerregions theair hasa greatertendency torise, whichlow-ers theair pressuresignificantly. Bycontrast, invery coldregionstheairisheavier,andhighpressurezonesarecreated. Accordingly,the compensatingair flowbetweenhighandlowpressureareasisstrongand,inconcertwiththeCoriolisforce,createsstrongerwestlywindsinthemiddlelatitudes.Ofcourse,seaicealsoinfluencesconvectionprocessesintheocean,andthustheforma-tionofdeepandbottomwater.Seaicethereforeplaysanimportantroleinthelarge-scaleoceancirculation,especially withregard tothermohaline circulation.Itisnotyetknownhowglobalwarmingaffectstheformationofseaiceandtherelatedprocesses.Icemeltswhenitbecomeswarmer.Butitisdifficulttopredictwhateffectthishasonthecurrents.Inanycase,allclimatemodelspredictanaccelerationofwarmingintheArcticwithacontinuingriseintrace-gasconcentra-tions.Inaddition,observationsindicateacleardecreaseinArcticsea-icecoverinrecentdecades.Thisispartlyrelatedtoapositivefeedbackmechanismcalledtheice-albedofeedback.Lightsurfaceshaveaveryhighalbedo.Whentheseaiceretreatsasaresultofglobalwarming,albedodecreasesandmoresolarenergyisavailable,whichleadstoadditionalwarming,andmeltsmoreice.Thisprocessprimarilyoccursatthemarginsofthesea1.13>Asarule,icebergsconsistoffreshwaterorcontainonlysmallamountsofsalt.Becauseoftheirslightlylowerdensitycomparedtoseawater,asmallfractionextendsabovethewater.Thelargestpartisbelowthesurface.
25Theworldoceans,globalclimatedrivers >Chapter0226 Howclimatechangealtersoceanchemistry2
27Howclimatechangealtersoceanchemistry<
>Massiveemissionsofcarbondioxideintotheatmospherehaveanimpactonthechemicalandbiologicalprocessesintheocean.Thewarmingofoceanwatercouldleadtoadestabilizationofsolidmethanedepositsontheseafloor.BecauseoftheexcessCO2,theoceansarebecomingmoreacidic.ScientistsaremakingextensivemeasurementstodeterminehowmuchofthehumanmadeCO2isbeingabsorbedbytheoceans.Importantcluesareprovidedbylookingatoxygen.
>Chapter0228ThemutabilityofcarbonCarbonistheelementoflife.Thehumanbodystructureisbasedonit,andotheranimalandplantbiomasssuchasleavesandwoodconsistpredominantlyofcarbon(C).Plantsonlandandalgaeintheoceanassimilateitintheformofcarbondioxide(CO2)fromtheatmosphereorwater,andtransformitthroughphotosynthesisintoenergy-richmoleculessuchassugarsandstarches.Car-bonconstantlychangesitsstatethroughthemetabolismoforganismsandbynaturalchemicalprocesses.CarboncanbestoredinandexchangesbetweenparticulateanddissolvedinorganicandorganicformsandexchangedwiththetheatmosphereasCO2.Theoceansstoremuchmorecarbonthantheatmosphereandtheterrestrialbiosphere(plantsandanimals).Evenmorecarbon,how-ever,isstoredinthelithosphere,i.e.therocksontheplanet, includinglimestones (calciumcarbonate, CaCO3).Thethreemostimportantrepositorieswithinthecontextofanthropogenicclimatechange-atmosphere,terrestrial biosphereand ocean- areconstantly exchang-ingcarbon.Thisprocesscanoccurovertimespansofupto centuries,which atfirst glanceappears quiteslow. Butconsidering thatcarbon remainsbound upin therocks ofthe Earth"scrust formillions ofyears, thenthe exchangebetween theatmosphere, terrestrialbiosphere andoceancarbonreservoirscouldactuallybedescribedasrelativelyrapid. Todayscientists canestimate fairlyaccurately howmuchcarbonisstoredintheindividualreservoirs.Theocean,witharound38,000gigatons(Gt)ofcarbon(1gigaton=1billiontons),contains16timesasmuchcarbonastheterrestrialbiosphere,thatisallplantandtheunderlyingsoilsonourplanet,andaround60timesasmuchasthepre-industrialatmosphere,i.e.,atatimebeforepeoplebegantodrasticallyaltertheatmosphericCO2contentby theincreased burningof coal,oil andgas.Atthattimethecarboncontentoftheatmospherewasonlyaround600gigatonsofcarbon.Theoceanisthere-fore thegreatest ofthe carbonreservoirs, andessentiallydeterminestheatmosphericCO2content.Thecarbon,however,requirescenturiestopenetrateintothedeepocean,becausethemixingoftheoceansisaratherslow(Chapter1).Consequently,changesinatmosphericcar-boncontentthatareinducedbytheoceansalsooccuroveratimeframeofcenturies.Ingeologicaltimethatisquite fast,but froma humanperspective itis tooslow toextensively bufferclimate change.Withrespecttoclimatechange,thegreenhousegasCO2isofprimaryinterestintheglobalcarboncycle.Today,weknowthattheCO2concentrationintheatmos-pherechangedonlyslightlyduringthe12,000yearsbe-tweenthelasticeageandtheonsetoftheindustrialrevolutionatthebeginningofthe19thcentury.Thisrela-tivelystableCO2concentrationsuggeststhatthepre-industrialcarboncyclewaslargelyinequilibriumwiththeatmosphere.Itisassumedthat,inthispre-industrialequilibrium state,the oceanreleased around0.6 gigatonsofcarbonperyeartotheatmosphere.Thisisaresultoftheinputofcarbonfromlandplantscarriedbyriverstotheoceanand,afterdecompositionbybacteria,releasedintotheatmosphereasCO2,aswellasfrominorganiccarbonfromtheweatheringofcontinentalrockssuchaslimestones.Thistransportpresumablystilloccurstodayatratesessentiallyunchanged.SincethebeginningofTheoceans-thelargestCO2-reservoir>Theoceansabsorbsubstantialamountsofcarbondioxide,andtherebyconsumealargeportionofthisgreenhousegas,whichisreleasedbyhumanactivity.Thisdoesnotmean,however,thattheproblemcanbeignored,becausethisprocesstakescenturiesandcannotpreventtheconsequencesofclimatechange.Furthermore,itcannotbepredictedhowthemarinebiospherewillreacttotheuptakeofadditionalCO2.
29HowclimatechangealtersoceanchemistryThecarboncycleinthe1990swiththesizesofthevariousreservoirs(ingigatonsofcarbon,GtC),aswellastheannualfluxesbetweenthese.Pre-industrialnaturalfluxesareshowninblack,anthropogenicchangesinred.Thelossof140GtCintheterrestrialbiospherereflectsthecumulativeCO2emis-sionsfromland-usechange(primarilyslashandburnagricul-tureinthetropicalrainforests),andisaddedtothe244GtCemittedbytheburningoffossilfuels.TheterrestrialsinkforanthropogenicCO2of101GtCisnotdirectlyverifiable,butisderivedfromthedifferencebetweencumulativeemissions(244+140=384GtC)andthecombinationofatmosphericincrease(165GtC)andoceanicsinks(100+18=118GtC).
Rivers0.8Weathering0.2Weathering0.2Surfaceocean900+18Intermediateanddeepocean37100+100Surfacesediment1500.2507070.60.4391122.2206.4Marinebiota3Fossilfuels3700-244Atmosphere597+165ReservoirsizesinGtCFluxesandratesinGtCperyear90.21.6101Landsink2.6Landusechange1.6Respiration119.6GPP120
Vegetation,soilanddetritus2300+101-140
>Chapter0230IronisacrucialnutrientforplantsandthesecondmostabundantchemicalelementonEarth,althoughthegreatestportionbyfarislockedintheEarth"score.Manyregionshavesufficientironforplants.Inlargeregionsoftheocean,however,ironissoscarcethatthegrowthofsingle-celledalgaeislimitedbyitsabsence.Iron-limitationregionsincludethetropicaleasternPacificandpartsoftheNorthPacific,aswellastheentireSouthernOcean.Theseoceanregionsarerichintheprimarynutrients(macronutrients)nitrateandphosphate.Theiron,however,whichplantsrequireonlyinverysmallamounts(micronutrients),ismissing.ScientistsrefertothesemarineregionsasHNLCregions(highnutrient,lowchlorophyll)becausealgalgrowthhereisrestrictedandtheamountoftheplantpigmentchlorophyllisreducedaccordingly.Researchusingfertiliza-tionexperimentshasshownthatplantgrowthinalloftheseregionscanbestimulatedbyfertilizingthewaterwithiron.Becauseplantsassimilatecarbon,carbondioxidefromtheatmosphereisthuscon-vertedtobiomass,atleastfortheshortterm.Ironfertilizationisacompletelynaturalphenomenon.Forexam-ple,iron-richdustfromdesertsisblowntotheseabythewind.Ironalsoenterstheoceanswiththemeltwateroficebergsorbycontactofthewaterwithiron-richsedimentsontheseafloor.ItispresumedthatdifferentwindpatternsandadryeratmosphereduringthelasticeageledtoasignificantlyhigherinputofironintotheSouthernOcean.Thiscould,atleastinpart,explaintheconsiderablyloweratmosphericCO2levelsduringthelasticeage.Accordingly,modernmodellingsimulationsindicatethatlarge-scaleironfertilizationoftheoceanscoulddecreasethepresentatmosphericCO2levelsbyaround30ppm(partspermillion).Bycomparison,humanactivitieshaveincreasedtheatmosphericCO2levelsfromaround280ppmtoapresent-dayvalueof390ppm.Marinealgaeassimilatebetweenathousandandamilliontimeslessironthancarbon.Thusevenverylowquantitiesofironaresufficienttostimulatetheuptakeoflargeamountsofcarbondioxideinplants.UnderfavourableconditionslargeamountsofCO2canbeconvertedwithrelativelylittleiron.ThisraisestheobviousideaoffertilizingtheoceansonalargescaleandreducingtheCO2concen-trationsintheatmospherebystorageinmarineorganisms(seques-tration).Whenthealgaedie,however,andsinktothebottomandaredigestedbyanimalsorbrokendownbymicroorganisms,thecar-bondioxideisreleasedagain.InordertoevaluatewhetherthefixedFertilizingtheoceanwithiron2.2>Ironisacrucialnutrientforalgae,anditisscarceinmanyoceanregions,whichinhibitsalgalgrowth.Ifthewaterisfertilizedwithironthereisarapidincreaseinalgae.Microscopicinvestigationsofwatersamplestakenbytheresearchvessel"Polarstern"clearlyshowthatalgaeinthisiron-poorregionproliferatequicklyafterironfertilization.Aroundthreeweeksafterfertilizationthemarinealgalcommunitywasdominatedbyelongate,hard-shelleddiatoms.
31Howclimatechangealtersoceanchemistry >Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.
33Howclimatechangealtersoceanchemistry
WarmwaterIceColdwater
1.7>Thewatermoleculeisasymmetricalandisthereforeoppositelychargedatitstwoends(left).Thisiscalledadipole.Itthusbehavesdifferentlyfromothersubstancesinmanyways.Iceislessdense(top)andfloatsonthesurface.Freshwaterhasitsgreatestdensityatfourdegrees(bottom),andsinkstothebottom.Thisisthenoverlainbywarmwater(middle).Saltywaterhasdifferentcharacteristics.-++
>Chapter0118mostimportantfactorsintheanswertothisquestion.The freezingof waterin thepolar convectionregions alsoplaysacentralrole.Becauseiceonlycontainsaboutfivetenthsofapercentsalt,itleavesbehindaconsiderableamountofsaltinthewaterwhenitfreezes,whichincreases thesalinity ofthe surroundingocean waterandthusincreasesitsdensity.Thewatermassproducedbyconvection inthe Arcticis calledthe NorthAtlantic DeepWater (NADW).TheglobalconveyorbeltConvectionalsooccursintheAntarcticregions.Becauseoftheirevenhighersalinity,thewatermassesproducedheresinkallthewaytotheseafloor.ThisiscalledtheAntarcticBottomWater(AABW),anditflowsacrosstheoceanfloorhalfwayaroundtheglobeintotheNorthAtlantic. TheAABW isalso thedeep waterlayer thatthethick intermediateNADW overlieswhen itsinks bycon-vection.TheNADWformsintheGreenlandandLabra-dor Seas.Figure 1.8schematically illustratesits flowpathandthereturnflowofwarmwaterinthenear-surfacelayers, inthe globalconveyor beltof thermohalinecircu-lation.TheNADW,andespeciallytheAABW,remaininthedeepoceanforanamazinglylongtime.Radioactivecarbon-isotopedatingofthedeepwatersindicatesthatfromthetimeofsinkingintothedeepuntilitsreturntothesurface,aperiodofseveralhundredorevenupto1000 yearswill pass.Formostofthistimethewaterremainsinthecolderdeepregionsofthethermohalineconveyorbeltbecausetheretheflowrateisslow,ataroundonetothreekilo-metresperday,duetoitshighdensity.Theamountofwater involvedin thiscycle istruly immense.Its volumeis around400,000 cubickilometres, whichis equivalenttoaboutonethirdofthetotalwaterintheocean.Thisisenoughwatertofillaswimmingpool400kilometreslong,100kilometreswide,andtenkilometresdeep.Theoceanicconveyorbelttransportsabout20millioncubicmetresofwaterpersecond,whichisalmost5000timestheamountthatflowsoverNiagaraFallsinNorthAmerica.ConcernsaboutthebreakdownoftheGulfStreamTherehasbeenagreatdealofdiscussionabouttheextenttowhichclimatechangecouldinfluencethermohalinecirculationanditsturnoverprocessesintheAtlantic.After all,convection athigh latitudescould beweakenedbyanthropogenic(causedbyhumans)warmingoftheatmosphereandtheaccompanyingdecreaseinsurface-Thereisnootherareaintheoceanwherethesurfacewaterfindsitswaysoquicklyintothedeepasintheconvectionareas,andatnootherplacedochangesattheseasurfaceorintheatmospherebecomesorapidlyapparentintheocean"sinterior,forexample,intheincreasedcarbondioxidelevelsinthewaterasaresultofhighercarbondioxideconcentrationsintheatmos-phere.Convectionconnectstwodistinctlydifferentcomponentsoftheocean:thenear-surfacelayersthatareincontactwiththevariableatmos-phericfieldsofwind,radiationandprecipitation,andthedeepregionsoftheocean.Atthesurface,currents,temperatureandsalinityfluctuateonascaleofweekstomonths.Butatgreaterdepthstheenvironmentalcondi-tionschangeattimescalesofdecadesorcenturies.Intheconsistentlywarmoceanicregionsofthetropics(thewarmregionsoftheEarthbetween23.5degreesnorthand23.5degreessouthlatitude)andthesubtropics(theregionsbetween23.5and40degreesinthenorth-ernandsouthernhemispheres),thereisnoexchangebetweenthesurfaceanddeepwatersthatiscomparabletopolarconvection.Thisisbecause,averagedovertheyear,thereisanetradiationexcessofthesurface-layerwaters.Thewarmwater,withaminimumtemperatureoftendegreesCel-sius,hasarelativelylowdensityandfloatsasawarmlayerontopofthedeeper,colderwatermasses.Thetwolayersaredistinctlyseparatedwithnogradualtransitionbetweenthem.Attheboundarywheretheymeetthereisasharptemperaturejump,andthereforealsoanabruptdensitydifferencethatinhibitspenetrationoftheheattogreaterdepths.Thewarmsurfacelayerhasanaveragethicknessofseveralhundredmetres,whichisrela-tivelythincomparedtothetotaldepthoftheoceans.InverywarmoceanregionssuchasthewesternequatorialPacific,thereishardlyanyverticalmixingatall.Nearertothepoles,however,thereismoreverticalmixingoftheoceansandlayeringislesswell-defined.Becausethereisnoabrupttem-peratureanddensitychangethere,changesintheseasurfacecanbetrans-mittedtotheinteriordepthsoftheocean.Buttheconvectionareasarestilltheexpresselevatortothedeep.Thepathofwaterintothedeepocean
19Theworldoceans,globalclimatedriversTheworldwideoceancurrentsofthethermohalinecircu-lationsystemareextremelycomplex.Theflowofcold,salinesurfacewater(blue)downwardandtowardtheequatorcanonlybeclearlyrecognizedintheAtlantic.Warmsurfacewater(red)flowsintheoppositedirection,towardthepole.Inotherareasthecurrentrelationshipsarenotasclear-cutastheyareintheGulfStreamsystem(betweenNorthAmericaandEurope).TheCircumpolarCurrentflowsaroundAntarctica,anddoessothroughoutthetotaldepthofthewatercolumn.Thesmallyellowcirclesinthepolarregionsindicateconvectionareas.Thedarkareasarecharacterizedbyhighsalinityandthewhiteareasbylowsalinity.Saltyareasaremostlylocatedinthewarmsubtropicsbecauseofthehighevaporationrateshere.
>Chapter0120atmosphericlow-pressuresystems,whichcanoftenbeseveral hundredkilometres wide.These mesoscaleeddiesformwhenwaterflowsbetweenregionswithlargedensityortemperaturedifferences.Theycanbeclearlyrecognizedonsatellitephotographs.Investigationshaveshownthattheynotonlyoccurattheoceansurfaceas,forexample,intheNorthAtlanticarea,butcanalsobelocated atgreat depthsof thousandsof metres,e.g. offthecoastofBrazil.Becauseoftheirstronginfluenceonthelarge-scale heattransport, thesedeep-sea eddiesalso playan importantrole inlong-term climateprocesses.Variableanddynamic-theinfluenceofwindAlongwithconvection,windsalsoprovideanimportantcontributionindrivingtheoceancurrents.Incombina-tionwiththedivertingforcecausedbytheEarth"srota-tion(Coriolisforce)andtheshapeoftheoceanbasins,winds determinethe characteristicpatterns ofthe world-widesystemofsurfacecurrents.Especiallystrikingarethe largegyres thatextend acrossentire oceanbasins, forexamplebetweenAmericaandEurope.ThesesurfacecurrentsincludetheGulfStreamintheAtlanticOcean,whichisdrivenbothbywindandthethermohalineforce, aswell asthe Kuroshioin thePacific Ocean,whoseintensity justdecreases withdepth.TheGulfStreamisarelativelyfastcurrent.AlongthecoastofNorthAmericaitreachesaspeedofaround3.6kilometresperhourattheseasurface,whichisacasualwalkingspeed.Itextendsdowntoadepthofaround 2000metres, wherethe speedis aroundten timesslowerbecausetheinfluenceofthewindislessandthedensityofthewaterisgreater.Nevertheless,thewindcaninfacthaveadirectinfluencedowntogreatdepths.Typicalwindconditionscanchangeforextendedtimeperiods.Forexample,thenormallysteadytradewindscanblowfromadifferentdirectionformonthsatatime,causingchangesintheupwellingofwatermasses,andcreatingwavesandcurrentsintheocean"sinteriorthatresonateatdepthfordecades.Thesewavescanalsochangetheoceantemperatureandthusalsotheregionalclimate.Fromsatellitesthesewavesareperceivedasslowly movingups anddowns ofthe oceansurface.Furthermore,incertainregionstheprevailingwindscausepersistentupwellinganddownwellingmotion.Insomeareasthewindsdrivesurfacewatersawayfromthe landmasses, allowingcold waterfrom greaterdepthstoriseinitsplace.Thesurface-watertemperaturesintheseareasarethereforeespeciallylow.Importantup-wellingregionsareoftenfoundonthewesternmarginsof continentswhere thewinds blowparallel tothe coast(Chile, California,Namibia). Inthe southernhemisphere,forexample,becauseoftheCoriolisforce,thewaterispushed tothe leftaway fromthe coastwhen thewind isblowingfromthesouth.Thisproducesarollingmotioninthewater,wherebythewateronthesurfacemovesawayfromthecoastandwaterrisestoreplaceitfrombelow.Thisupwellingwaterisusuallyrichinnutrients,which iswhy manyupwelling regionsare alsoabundantwith fish.Theocean-aglobalstorehouseforheatIn additionto hugemasses ofwater, largeocean currentsalsotransportenormousamountsofheataroundtheglobe. Similarto theway thewater tankin aheating sys-1.9>SatellitephotographoftheGulfStreamanditseddies.Warmareasarered,coldareasareblue.TheCoriolisforceTheEarth"srotationcausesallfreelinearmotionontheEarth,suchasairorwatercurrents,tobedivertedtooneside.ThedivertingforceiscalledtheCoriolisforceorCoriolisaccel-eration.Itworksinoppositedirectionsinthenorthernandsouthernhemispheres.TheCoriolisforceisnamedaftertheFrenchnaturalscientistGaspareGustavedeCoriolis(1792to1843),whoderiveditmathe-matically.
21Theworldoceans,globalclimatedriversTheworld"slargeoceancurrentsarealsoinfluencedbytheprevailingwinds.Warmoceancurrentsarered,andcoldcurrentsareshowninblue.
>Chapter0122The Atlanticand PacificOceans eachcarry aroundonepetawattofheatnorthwardfromthetropicsandsub-tropics.Bycomparison,thesharemovedbytheIndianOcean isnegligible.InthissystemtheAtlantichasauniquefunctionamong theworld"s oceans.It isthe onlyocean basinthattransports heatnorthward throughoutits length,even inthe southernhemisphere. Europeansall benefitfrom thenorthwardtrend,thankstotheGulfStreamandtheNorthAtlanticCurrent.TheclimateintheregionoftheNorth Atlanticis comparativelymild, especiallyin north-westEurope,includingGermany.Thewintersinotherregionsatthesamelatitudearenotablycolder.InCana-da,forexample,thewintertemperaturesarearoundtendegreesCelsiuslowerthaninWesternEurope.Butitisnottheoceancirculationalonethatcausesthemildclimate.Aircurrentsalsocontributesignificantlytothisphenomenon.Thedistributionofmountainranges,par-ticularlytheRockyMountains,whichrunfromnorthtosouthalongthewestcoastofNorthAmerica,togetherwith theinfluence ofthe Coriolisforce, causesthe forma-tion ofvery stable,large-scale vorticesin theatmospherecalled standingplanetary waves.Such avortex liesabovethe USAbecause theRocky Mountainsact asan obstacletodivertlargeairmasses.AsaconsequencethewindsarepredominantlywesterlyovertheAtlanticcarryingrelativelymildairtonorthwestEurope,andfendoffthecold fromthe east.TheuncertainfutureofseaiceSeaiceintheArcticregionshasasignificantimpactonheatexchangebetweentheatmosphereandocean,because itacts asan insulatinglayer toprevent heatfrom0-40-804080120Wattspersquaremetre-1201.11>Heatexchangebetweentheatmosphereandtheseasur-face(inwattspersquaremetre)isveryvariabledependingontheoceanregion.Positivevaluesindicateabsorptionofheatbytheocean,whichischaracteristicofthetropics,andnegativevaluesindicateaheatloss,whichistypicalforthenorthernlatitudes.Inthehigharcticregions,however,heatlossisrela-tivelylowbecausetheseaiceactsasaninsulatinglayerandpreventsheatescapingfromthewater.
23Theworldoceans,globalclimatedriversOceanscontributetotheglobaltransportofheatwithdifferentintensities.Inthesouthernhemisphere,onlytheAt-lantictransportsheattothenorth(positivevalues).Theequa-torliesatzerodegrees.TheAtlanticandPacificeachcarryaroundonepetawattofheatasfaras20degreesnorthlatitude.Furthertothenorth,theAtlanticcarriesmorethanthePacific.TheIndianOcean,ontheotherhand,makesanegligiblecon-tributiontonorthwardheattransport.
>Chapter0124incomingsunlight.Thiseffectisenhancedwhentheiceiscoveredwithsnow.TheseaicethereforeinfluencestheradiationbalanceoftheEarthandthusplaysanimportant rolein theclimate system.Theimpactofseaiceonclimateisfurtheramplifiedbyitsinsulatingeffectbetweentheatmosphereandocean.Itinhibitstheexchangeofheatandwindenergybetweentheatmosphereandoceanconsiderably.Theatmosphereisthereforemuchcolderabovethesea-icesurface thanabove theopen ocean.This hasthe effectofincreasingtheair-temperaturedifferencebetweenthetropics,subtropics,andthepolarregions.Inwarmerregions theair hasa greatertendency torise, whichlow-ers theair pressuresignificantly. Bycontrast, invery coldregionstheairisheavier,andhighpressurezonesarecreated. Accordingly,the compensatingair flowbetweenhighandlowpressureareasisstrongand,inconcertwiththeCoriolisforce,createsstrongerwestlywindsinthemiddlelatitudes.Ofcourse,seaicealsoinfluencesconvectionprocessesintheocean,andthustheforma-tionofdeepandbottomwater.Seaicethereforeplaysanimportantroleinthelarge-scaleoceancirculation,especially withregard tothermohaline circulation.Itisnotyetknownhowglobalwarmingaffectstheformationofseaiceandtherelatedprocesses.Icemeltswhenitbecomeswarmer.Butitisdifficulttopredictwhateffectthishasonthecurrents.Inanycase,allclimatemodelspredictanaccelerationofwarmingintheArcticwithacontinuingriseintrace-gasconcentra-tions.Inaddition,observationsindicateacleardecreaseinArcticsea-icecoverinrecentdecades.Thisispartlyrelatedtoapositivefeedbackmechanismcalledtheice-albedofeedback.Lightsurfaceshaveaveryhighalbedo.Whentheseaiceretreatsasaresultofglobalwarming,albedodecreasesandmoresolarenergyisavailable,whichleadstoadditionalwarming,andmeltsmoreice.Thisprocessprimarilyoccursatthemarginsofthesea1.13>Asarule,icebergsconsistoffreshwaterorcontainonlysmallamountsofsalt.Becauseoftheirslightlylowerdensitycomparedtoseawater,asmallfractionextendsabovethewater.Thelargestpartisbelowthesurface.
25Theworldoceans,globalclimatedrivers >Chapter0226 Howclimatechangealtersoceanchemistry2
27Howclimatechangealtersoceanchemistry<
>Massiveemissionsofcarbondioxideintotheatmospherehaveanimpactonthechemicalandbiologicalprocessesintheocean.Thewarmingofoceanwatercouldleadtoadestabilizationofsolidmethanedepositsontheseafloor.BecauseoftheexcessCO2,theoceansarebecomingmoreacidic.ScientistsaremakingextensivemeasurementstodeterminehowmuchofthehumanmadeCO2isbeingabsorbedbytheoceans.Importantcluesareprovidedbylookingatoxygen.
>Chapter0228ThemutabilityofcarbonCarbonistheelementoflife.Thehumanbodystructureisbasedonit,andotheranimalandplantbiomasssuchasleavesandwoodconsistpredominantlyofcarbon(C).Plantsonlandandalgaeintheoceanassimilateitintheformofcarbondioxide(CO2)fromtheatmosphereorwater,andtransformitthroughphotosynthesisintoenergy-richmoleculessuchassugarsandstarches.Car-bonconstantlychangesitsstatethroughthemetabolismoforganismsandbynaturalchemicalprocesses.CarboncanbestoredinandexchangesbetweenparticulateanddissolvedinorganicandorganicformsandexchangedwiththetheatmosphereasCO2.Theoceansstoremuchmorecarbonthantheatmosphereandtheterrestrialbiosphere(plantsandanimals).Evenmorecarbon,how-ever,isstoredinthelithosphere,i.e.therocksontheplanet, includinglimestones (calciumcarbonate, CaCO3).Thethreemostimportantrepositorieswithinthecontextofanthropogenicclimatechange-atmosphere,terrestrial biosphereand ocean- areconstantly exchang-ingcarbon.Thisprocesscanoccurovertimespansofupto centuries,which atfirst glanceappears quiteslow. Butconsidering thatcarbon remainsbound upin therocks ofthe Earth"scrust formillions ofyears, thenthe exchangebetween theatmosphere, terrestrialbiosphere andoceancarbonreservoirscouldactuallybedescribedasrelativelyrapid. Todayscientists canestimate fairlyaccurately howmuchcarbonisstoredintheindividualreservoirs.Theocean,witharound38,000gigatons(Gt)ofcarbon(1gigaton=1billiontons),contains16timesasmuchcarbonastheterrestrialbiosphere,thatisallplantandtheunderlyingsoilsonourplanet,andaround60timesasmuchasthepre-industrialatmosphere,i.e.,atatimebeforepeoplebegantodrasticallyaltertheatmosphericCO2contentby theincreased burningof coal,oil andgas.Atthattimethecarboncontentoftheatmospherewasonlyaround600gigatonsofcarbon.Theoceanisthere-fore thegreatest ofthe carbonreservoirs, andessentiallydeterminestheatmosphericCO2content.Thecarbon,however,requirescenturiestopenetrateintothedeepocean,becausethemixingoftheoceansisaratherslow(Chapter1).Consequently,changesinatmosphericcar-boncontentthatareinducedbytheoceansalsooccuroveratimeframeofcenturies.Ingeologicaltimethatisquite fast,but froma humanperspective itis tooslow toextensively bufferclimate change.Withrespecttoclimatechange,thegreenhousegasCO2isofprimaryinterestintheglobalcarboncycle.Today,weknowthattheCO2concentrationintheatmos-pherechangedonlyslightlyduringthe12,000yearsbe-tweenthelasticeageandtheonsetoftheindustrialrevolutionatthebeginningofthe19thcentury.Thisrela-tivelystableCO2concentrationsuggeststhatthepre-industrialcarboncyclewaslargelyinequilibriumwiththeatmosphere.Itisassumedthat,inthispre-industrialequilibrium state,the oceanreleased around0.6 gigatonsofcarbonperyeartotheatmosphere.Thisisaresultoftheinputofcarbonfromlandplantscarriedbyriverstotheoceanand,afterdecompositionbybacteria,releasedintotheatmosphereasCO2,aswellasfrominorganiccarbonfromtheweatheringofcontinentalrockssuchaslimestones.Thistransportpresumablystilloccurstodayatratesessentiallyunchanged.SincethebeginningofTheoceans-thelargestCO2-reservoir>Theoceansabsorbsubstantialamountsofcarbondioxide,andtherebyconsumealargeportionofthisgreenhousegas,whichisreleasedbyhumanactivity.Thisdoesnotmean,however,thattheproblemcanbeignored,becausethisprocesstakescenturiesandcannotpreventtheconsequencesofclimatechange.Furthermore,itcannotbepredictedhowthemarinebiospherewillreacttotheuptakeofadditionalCO2.
29HowclimatechangealtersoceanchemistryThecarboncycleinthe1990swiththesizesofthevariousreservoirs(ingigatonsofcarbon,GtC),aswellastheannualfluxesbetweenthese.Pre-industrialnaturalfluxesareshowninblack,anthropogenicchangesinred.Thelossof140GtCintheterrestrialbiospherereflectsthecumulativeCO2emis-sionsfromland-usechange(primarilyslashandburnagricul-tureinthetropicalrainforests),andisaddedtothe244GtCemittedbytheburningoffossilfuels.TheterrestrialsinkforanthropogenicCO2of101GtCisnotdirectlyverifiable,butisderivedfromthedifferencebetweencumulativeemissions(244+140=384GtC)andthecombinationofatmosphericincrease(165GtC)andoceanicsinks(100+18=118GtC).
Rivers0.8Weathering0.2Weathering0.2Surfaceocean900+18Intermediateanddeepocean37100+100Surfacesediment1500.2507070.60.4391122.2206.4Marinebiota3Fossilfuels3700-244Atmosphere597+165ReservoirsizesinGtCFluxesandratesinGtCperyear90.21.6101Landsink2.6Landusechange1.6Respiration119.6GPP120
Vegetation,soilanddetritus2300+101-140
>Chapter0230IronisacrucialnutrientforplantsandthesecondmostabundantchemicalelementonEarth,althoughthegreatestportionbyfarislockedintheEarth"score.Manyregionshavesufficientironforplants.Inlargeregionsoftheocean,however,ironissoscarcethatthegrowthofsingle-celledalgaeislimitedbyitsabsence.Iron-limitationregionsincludethetropicaleasternPacificandpartsoftheNorthPacific,aswellastheentireSouthernOcean.Theseoceanregionsarerichintheprimarynutrients(macronutrients)nitrateandphosphate.Theiron,however,whichplantsrequireonlyinverysmallamounts(micronutrients),ismissing.ScientistsrefertothesemarineregionsasHNLCregions(highnutrient,lowchlorophyll)becausealgalgrowthhereisrestrictedandtheamountoftheplantpigmentchlorophyllisreducedaccordingly.Researchusingfertiliza-tionexperimentshasshownthatplantgrowthinalloftheseregionscanbestimulatedbyfertilizingthewaterwithiron.Becauseplantsassimilatecarbon,carbondioxidefromtheatmosphereisthuscon-vertedtobiomass,atleastfortheshortterm.Ironfertilizationisacompletelynaturalphenomenon.Forexam-ple,iron-richdustfromdesertsisblowntotheseabythewind.Ironalsoenterstheoceanswiththemeltwateroficebergsorbycontactofthewaterwithiron-richsedimentsontheseafloor.ItispresumedthatdifferentwindpatternsandadryeratmosphereduringthelasticeageledtoasignificantlyhigherinputofironintotheSouthernOcean.Thiscould,atleastinpart,explaintheconsiderablyloweratmosphericCO2levelsduringthelasticeage.Accordingly,modernmodellingsimulationsindicatethatlarge-scaleironfertilizationoftheoceanscoulddecreasethepresentatmosphericCO2levelsbyaround30ppm(partspermillion).Bycomparison,humanactivitieshaveincreasedtheatmosphericCO2levelsfromaround280ppmtoapresent-dayvalueof390ppm.Marinealgaeassimilatebetweenathousandandamilliontimeslessironthancarbon.Thusevenverylowquantitiesofironaresufficienttostimulatetheuptakeoflargeamountsofcarbondioxideinplants.UnderfavourableconditionslargeamountsofCO2canbeconvertedwithrelativelylittleiron.ThisraisestheobviousideaoffertilizingtheoceansonalargescaleandreducingtheCO2concen-trationsintheatmospherebystorageinmarineorganisms(seques-tration).Whenthealgaedie,however,andsinktothebottomandaredigestedbyanimalsorbrokendownbymicroorganisms,thecar-bondioxideisreleasedagain.InordertoevaluatewhetherthefixedFertilizingtheoceanwithiron2.2>Ironisacrucialnutrientforalgae,anditisscarceinmanyoceanregions,whichinhibitsalgalgrowth.Ifthewaterisfertilizedwithironthereisarapidincreaseinalgae.Microscopicinvestigationsofwatersamplestakenbytheresearchvessel"Polarstern"clearlyshowthatalgaeinthisiron-poorregionproliferatequicklyafterironfertilization.Aroundthreeweeksafterfertilizationthemarinealgalcommunitywasdominatedbyelongate,hard-shelleddiatoms.
31Howclimatechangealtersoceanchemistry >Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.
33Howclimatechangealtersoceanchemistry
Howclimatechangealtersoceanchemistry2
27Howclimatechangealtersoceanchemistry<
>Massiveemissionsofcarbondioxideintotheatmospherehaveanimpactonthechemicalandbiologicalprocessesintheocean.Thewarmingofoceanwatercouldleadtoadestabilizationofsolidmethanedepositsontheseafloor.BecauseoftheexcessCO2,theoceansarebecomingmoreacidic.ScientistsaremakingextensivemeasurementstodeterminehowmuchofthehumanmadeCO2isbeingabsorbedbytheoceans.Importantcluesareprovidedbylookingatoxygen.
>Chapter0228ThemutabilityofcarbonCarbonistheelementoflife.Thehumanbodystructureisbasedonit,andotheranimalandplantbiomasssuchasleavesandwoodconsistpredominantlyofcarbon(C).Plantsonlandandalgaeintheoceanassimilateitintheformofcarbondioxide(CO2)fromtheatmosphereorwater,andtransformitthroughphotosynthesisintoenergy-richmoleculessuchassugarsandstarches.Car-bonconstantlychangesitsstatethroughthemetabolismoforganismsandbynaturalchemicalprocesses.CarboncanbestoredinandexchangesbetweenparticulateanddissolvedinorganicandorganicformsandexchangedwiththetheatmosphereasCO2.Theoceansstoremuchmorecarbonthantheatmosphereandtheterrestrialbiosphere(plantsandanimals).Evenmorecarbon,how-ever,isstoredinthelithosphere,i.e.therocksontheplanet, includinglimestones (calciumcarbonate, CaCO3).Thethreemostimportantrepositorieswithinthecontextofanthropogenicclimatechange-atmosphere,terrestrial biosphereand ocean- areconstantly exchang-ingcarbon.Thisprocesscanoccurovertimespansofupto centuries,which atfirst glanceappears quiteslow. Butconsidering thatcarbon remainsbound upin therocks ofthe Earth"scrust formillions ofyears, thenthe exchangebetween theatmosphere, terrestrialbiosphere andoceancarbonreservoirscouldactuallybedescribedasrelativelyrapid. Todayscientists canestimate fairlyaccurately howmuchcarbonisstoredintheindividualreservoirs.Theocean,witharound38,000gigatons(Gt)ofcarbon(1gigaton=1billiontons),contains16timesasmuchcarbonastheterrestrialbiosphere,thatisallplantandtheunderlyingsoilsonourplanet,andaround60timesasmuchasthepre-industrialatmosphere,i.e.,atatimebeforepeoplebegantodrasticallyaltertheatmosphericCO2contentby theincreased burningof coal,oil andgas.Atthattimethecarboncontentoftheatmospherewasonlyaround600gigatonsofcarbon.Theoceanisthere-fore thegreatest ofthe carbonreservoirs, andessentiallydeterminestheatmosphericCO2content.Thecarbon,however,requirescenturiestopenetrateintothedeepocean,becausethemixingoftheoceansisaratherslow(Chapter1).Consequently,changesinatmosphericcar-boncontentthatareinducedbytheoceansalsooccuroveratimeframeofcenturies.Ingeologicaltimethatisquite fast,but froma humanperspective itis tooslow toextensively bufferclimate change.Withrespecttoclimatechange,thegreenhousegasCO2isofprimaryinterestintheglobalcarboncycle.Today,weknowthattheCO2concentrationintheatmos-pherechangedonlyslightlyduringthe12,000yearsbe-tweenthelasticeageandtheonsetoftheindustrialrevolutionatthebeginningofthe19thcentury.Thisrela-tivelystableCO2concentrationsuggeststhatthepre-industrialcarboncyclewaslargelyinequilibriumwiththeatmosphere.Itisassumedthat,inthispre-industrialequilibrium state,the oceanreleased around0.6 gigatonsofcarbonperyeartotheatmosphere.Thisisaresultoftheinputofcarbonfromlandplantscarriedbyriverstotheoceanand,afterdecompositionbybacteria,releasedintotheatmosphereasCO2,aswellasfrominorganiccarbonfromtheweatheringofcontinentalrockssuchaslimestones.Thistransportpresumablystilloccurstodayatratesessentiallyunchanged.SincethebeginningofTheoceans-thelargestCO2-reservoir>Theoceansabsorbsubstantialamountsofcarbondioxide,andtherebyconsumealargeportionofthisgreenhousegas,whichisreleasedbyhumanactivity.Thisdoesnotmean,however,thattheproblemcanbeignored,becausethisprocesstakescenturiesandcannotpreventtheconsequencesofclimatechange.Furthermore,itcannotbepredictedhowthemarinebiospherewillreacttotheuptakeofadditionalCO2.
29HowclimatechangealtersoceanchemistryThecarboncycleinthe1990swiththesizesofthevariousreservoirs(ingigatonsofcarbon,GtC),aswellastheannualfluxesbetweenthese.Pre-industrialnaturalfluxesareshowninblack,anthropogenicchangesinred.Thelossof140GtCintheterrestrialbiospherereflectsthecumulativeCO2emis-sionsfromland-usechange(primarilyslashandburnagricul-tureinthetropicalrainforests),andisaddedtothe244GtCemittedbytheburningoffossilfuels.TheterrestrialsinkforanthropogenicCO2of101GtCisnotdirectlyverifiable,butisderivedfromthedifferencebetweencumulativeemissions(244+140=384GtC)andthecombinationofatmosphericincrease(165GtC)andoceanicsinks(100+18=118GtC).
Rivers0.8Weathering0.2Weathering0.2Surfaceocean900+18Intermediateanddeepocean37100+100Surfacesediment1500.2507070.60.4391122.2206.4Marinebiota3Fossilfuels3700-244Atmosphere597+165ReservoirsizesinGtCFluxesandratesinGtCperyear90.21.6101Landsink2.6Landusechange1.6Respiration119.6GPP120
Vegetation,soilanddetritus2300+101-140
>Chapter0230IronisacrucialnutrientforplantsandthesecondmostabundantchemicalelementonEarth,althoughthegreatestportionbyfarislockedintheEarth"score.Manyregionshavesufficientironforplants.Inlargeregionsoftheocean,however,ironissoscarcethatthegrowthofsingle-celledalgaeislimitedbyitsabsence.Iron-limitationregionsincludethetropicaleasternPacificandpartsoftheNorthPacific,aswellastheentireSouthernOcean.Theseoceanregionsarerichintheprimarynutrients(macronutrients)nitrateandphosphate.Theiron,however,whichplantsrequireonlyinverysmallamounts(micronutrients),ismissing.ScientistsrefertothesemarineregionsasHNLCregions(highnutrient,lowchlorophyll)becausealgalgrowthhereisrestrictedandtheamountoftheplantpigmentchlorophyllisreducedaccordingly.Researchusingfertiliza-tionexperimentshasshownthatplantgrowthinalloftheseregionscanbestimulatedbyfertilizingthewaterwithiron.Becauseplantsassimilatecarbon,carbondioxidefromtheatmosphereisthuscon-vertedtobiomass,atleastfortheshortterm.Ironfertilizationisacompletelynaturalphenomenon.Forexam-ple,iron-richdustfromdesertsisblowntotheseabythewind.Ironalsoenterstheoceanswiththemeltwateroficebergsorbycontactofthewaterwithiron-richsedimentsontheseafloor.ItispresumedthatdifferentwindpatternsandadryeratmosphereduringthelasticeageledtoasignificantlyhigherinputofironintotheSouthernOcean.Thiscould,atleastinpart,explaintheconsiderablyloweratmosphericCO2levelsduringthelasticeage.Accordingly,modernmodellingsimulationsindicatethatlarge-scaleironfertilizationoftheoceanscoulddecreasethepresentatmosphericCO2levelsbyaround30ppm(partspermillion).Bycomparison,humanactivitieshaveincreasedtheatmosphericCO2levelsfromaround280ppmtoapresent-dayvalueof390ppm.Marinealgaeassimilatebetweenathousandandamilliontimeslessironthancarbon.Thusevenverylowquantitiesofironaresufficienttostimulatetheuptakeoflargeamountsofcarbondioxideinplants.UnderfavourableconditionslargeamountsofCO2canbeconvertedwithrelativelylittleiron.ThisraisestheobviousideaoffertilizingtheoceansonalargescaleandreducingtheCO2concen-trationsintheatmospherebystorageinmarineorganisms(seques-tration).Whenthealgaedie,however,andsinktothebottomandaredigestedbyanimalsorbrokendownbymicroorganisms,thecar-bondioxideisreleasedagain.InordertoevaluatewhetherthefixedFertilizingtheoceanwithiron2.2>Ironisacrucialnutrientforalgae,anditisscarceinmanyoceanregions,whichinhibitsalgalgrowth.Ifthewaterisfertilizedwithironthereisarapidincreaseinalgae.Microscopicinvestigationsofwatersamplestakenbytheresearchvessel"Polarstern"clearlyshowthatalgaeinthisiron-poorregionproliferatequicklyafterironfertilization.Aroundthreeweeksafterfertilizationthemarinealgalcommunitywasdominatedbyelongate,hard-shelleddiatoms.
31Howclimatechangealtersoceanchemistry >Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.
33Howclimatechangealtersoceanchemistry
>Chapter0232human-madeatmosphericCO2,andthisspecialpropertyofseawaterisprimarilyattributabletocarbonation,which, at10 percent, representsa significantproportionofthedissolvedinorganiccarbonintheocean.Intheocean,thecarbondissolvedintheformofCO2,bicarbo-nateandcarbonateisreferredtoasinorganiccarbon.Whenanewcarbonequilibriumbetweentheatmos-phere andthe worldocean isre-established inthe future,thentheoceanicreservoirwillhaveassimilatedaround80percentoftheanthropogenicCO2fromtheatmos-phere,primarilyduetothereactionwithcarbonate.Thebufferingeffectofdeep-seacalciumcarbonatesedimentsisalsoimportant.TheseancientcarbonatesneutralizelargeamountsofCO2byreactingwithit,anddissolvingtosomeextent.Thankstotheseprocesses,theoceanscould ultimatelyabsorb around95 percent ofthe anthro-pogenicemissions.Becauseoftheslowmixingoftheocean,however,itwouldtakecenturiesbeforeequilib-rium isestablished. Thevery gradualbuffering ofCO2bythereactionwithcarbonatesedimentsmighteventakemillennia. Fortoday"s situationthis meansthat amarkedcarbondisequilibriumbetweentheoceanandatmos-phere willcontinue toexist forthe decadesand centuriesto come.The worldocean cannotabsorb thegreenhousegasasrapidlyasitisemittedintotheatmospherebyhumans.Theabsorptivecapacityoftheoceansthroughchemicalprocessesinthewaterisdirectlydependentontherateofmixingintheworldocean.ThecurrentoceanicuptakeofCO2thuslagssignificantlybehinditschemical capacityas thepresent-day CO2emissions occurmuch fasterthan theycan beprocessed bythe ocean.MeasuringexchangebetweentheatmosphereandoceanFor dependableclimate predictionsit isextremely impor-tanttodetermineexactlyhowmuchCO2isabsorbedbytheoceansink.Researchershavethereforedevelopeda2.3>CementplantslikethisoneinAmsterdamare,secondtotheburningoffossilfuels,amongthemostsignificantglobalsourcesofanthropogeniccarbondioxide.ThepotentialforreducingCO2outputisaccordinglylargeintheseindustrialareas.