2200

pH value 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1

Unfavorable prognosis: According to simulations of researchers at the Max Planck Institute in Hamburg, the pH value of the oceanic surface layer will be considerably lower in 2200 than in 1950, visible in the color shift within the area shown in red (left). The oceans are becoming more acidic. FOCUS_Geosciences

When the Air Turns the Oceans Sour

Human society has begun an ominous large-scale experiment, the full consequences of which will not be foreseeable for some time yet. Massive emissions of man-made carbon dioxide are heating up the Earth. But that’s not all: the increased concentration of this greenhouse gas in the atmosphere is also acidifying the oceans. Tatiana Ilyina and her staff at the Max Planck Institute for Meteorology in Hamburg are researching the consequences this could have.

TEXT TIM SCHRÖDER

hey’re called “sea butterflies” are warming the Earth like a green- because they float in the house. Less well known is the fact that ocean like a small winged the rising concentration of carbon diox- creature. However, pteropods ide in the atmosphere also leads to the belong to the gastropod class oceans slowly becoming more acidic. T of mollusks. They paddle through the This is because the oceans absorb a large water with shells as small as a baby’s portion of the carbon dioxide from the fingernail, and strangely transparent atmosphere. Put simply, the gas forms skin. Though tiny, they are tremen- carbonic acid in the water. The more dously important because there are bil- carbon dioxide humans expel into the

lions of them. Fish – whales, too – air, the more CO2 is dissolved in water, swallow them down in vast quantities. and the more carbonic acid forms. The Pteropods are nautical nibbles. pH level, which indicates how alkaline American zoologists simply call or acidic the water is, falls. That be- them the “potato chips of comes a problem for the pteropods and the sea.” But it seems that other living creatures because, at lower the supply of pteropods pH values, they can hardly generate is endangered. Climate any calcium carbonate for constructing change, or more pre- their shells. What consequences this cisely, ocean acidifica- will have for life and the food chain in tion, may pose serious the sea is still largely unknown. problems for them in Today, the pH value of seawater is the future. 8.1, making it, strictly speaking, a weak It is common base rather than an acid. The term acid- 1950 knowledge today that ification applies nevertheless, because

Photo: Deutches Klimarechenzentrum (2) carbon dioxide emissions the pH value has fallen toward acidic

3 | 13 MaxPlanckResearch 19 FOCUS_Geosciences

» By absorbing carbon dioxide from the atmosphere, the sea decreases the greenhouse effect.

by a bit more than 0.1 since the start of gions of the ocean acidify? What effect cause this strongly influences climate the industrial revolution. Although 0.1 will high levels of carbon dioxide have change: by absorbing carbon dioxide doesn’t sound like much, the pH value on the survival of phytoplankton, the from the atmosphere, the ocean reduc- scales logarithmically, so it actually rep- microscopic algae that are the basis for es the greenhouse effect. resents an increase in the acidity of wa- all life in the oceans? Will the uptake Ilyina’s specialty is simulating the ter by about 30 percent already. of carbon dioxide slow down in the fu- biogeochemistry of the oceans. This ture? All of these questions still need includes the chemical processes that SEARCHING FOR ANSWERS to be answered. occur in seawater, the seafloor and be- WITH THE COMPUTER Tatiana Ilyina is attempting to find tween substances that marine organ- the answers with the aid of the com- isms absorb and release. Carbon is an When water reacts with carbon diox- puter. The oceanographer works at the important biogeochemical compo- ide, it can be expressed in a simple Max Planck Institute for Meteorology nent, and carbon dioxide therefore has chemical equation. However, the oce- in Hamburg, where simulations of the a major impact on the biogeochemis- anic pathway of carbon dioxide and Earth’s climate and calculations of cli- try. Ilyina has spent a lot of time in the the carbon it contains is so complex mate change have been carried out for past few years translating biogeochem- that researchers haven’t yet been able many years. Ilyina’s work focuses pri- ical processes into mathematical rules to fully estimate how the oceans will marily on the carbon pathway from and incorporating them into the cli- change. How quickly will various re- the atmosphere into the oceans, be- mate models of her colleagues, espe-

Sea butterflies (Limacina helicina) are predatory marine snails and represent a keystone species of plankton. Ocean acidification etches their shells made of calcium carbonate. Photo: SPL-Agentur Focus

20 MaxPlanckResearch 3 | 13 cially into the standard ocean chemis- The findings from Ilyina’s group are try model known as HAMOCC, which also important for the upcoming report has been well established for many of the Intergovernmental Panel on Cli- years now. The computer simulations mate Change (IPCC). The Max Planck have since produced several interest- Institute for Meteorology in Hamburg ing insights. is one of about twenty research institu- Researchers like Tatiana Ilyina al- tions around the world whose climate ways try to simplify reality in their sim- simulations form the scientific basis of ulations, yet still include the key pro- this report. The different simulations cesses in their models. In the case of are so important because each of them biogeochemistry, this is very challeng- yields slightly different results. ing because carbon has many path- The experts of the Intergovernmen- ways. Even the transport of the carbon tal Panel on Climate Change (IPCC) are dioxide into the sea is complex, be- responsible for distilling something cause how much gas dissolves in seawa- like a scientific consensus from these ter, among other factors, depends, for models. This international climate instance, on the water temperature. model comparison project is called Cold water absorbs more gas than warm Coupled Model Intercomparison Proj- water. The Arctic Ocean, as the simula- ect Phase 5 (CMIP5). For a long time, tions show, is thus likely to acidify con- simulations with climate models fo- siderably faster over the coming de- cused only on currents and tempera- cades than, for example, the subtropical tures. After the influence of ocean bio- and tropical regions of the oceans. geochemistry was recognized, biogeo- chemical computations were included CARBON DIOXIDE WITH THE in CMIP5 for the first time. “Although WEIGHT OF 500,000 CARS the models differ slightly from one an- other, they all agree that, from a glob- 20 μm

When carbon dioxide (CO2) from the al perspective, the oceans are acidify- air passes into the sea, it reacts with wa- ing,” says Ilyina. Ocean acidification threatens corals (Millepora spec., above), important for reef formation, and ter (H2O) to form carbonic acid. This, Deviations arise primarily when the in turn, reacts with the carbonate ions various models treat individual regions the pelagic microscopic diatom algae (Arachnoi- discus spec., below). 2- (CO3 ) naturally contained in water to – that is, when regional details become - form bicarbonate (HCO3 ). Since CO2 is important. Simulations show that there quickly converted into ions and thus will be winners and losers among the water that floats on top of the deeper, becomes unavailable for the exchange phytoplankton. Several calculations cold water. The mixing between these with the air above, the ocean can con- even predict that phytoplankton in bodies of water will then be slower, as

tinually absorb large quantities of CO2. some ocean regions, such as the Arctic, the boundary between warm and cold

The oceans swallow a mass of CO2 will flourish more than previously. The acts like a separation layer. The result equivalent to the weight of 500,000 au- reason: the receding ice allows more is that the transport of deep, nutrient- tomobiles every day. The various ions sunlight to penetrate the water, which rich water to the surface is interrupt- are in equilibrium and can be trans- is exactly what plankton algae need, ed. However, these nutrients are es- formed into one another when envi- just like plants on land. sential for the growth of the phyto- ronmental conditions change. The pH However, most of the simulations plankton. The data from Ilyina’s re- value also influences this equilibrium. predict the opposite: that ocean acidi- search group also indicate a decline in Ilyina’s simulations showed that fication will, on the whole, bring disad- phytoplankton. acidification shifts the equilibrium to- vantages to these sea creatures. The The decline of the phytoplankton ward bicarbonate. For many sea crea- mechanisms behind this are still not population affects not only the food tures, the accompanying decline in car- completely understood, but some ex- chain in the oceans; it also directly af- bonate ions may be fatal, because car- periments show that the growth of al- fects the carbon cycle. The phytoplank- bonate is an important component of gae may be impaired. Accordingly, the ton absorb large amounts of carbon di- mollusk and snail shells. If more and quantity of plankton is expected to de- oxide and transform it into biomass.

more CO2 enters the oceans, the quan- cline in the coming decades. When the algae die, they sink into the

tity of carbonate ions could decline so In addition, a physical effect could deep. In this way, large amounts of CO2 far that calcium carbonate production also contribute to the algae starving. are ultimately withdrawn from the up- is impaired. If the amount of carbonate The surface layer of the oceans will per layers of the ocean and removed

falls further still, shells made of calci- warm up further due to global warm- from the CO2 exchange with the atmo-

Photos: 123RF (above), AWI-Michels (below) um carbonate may even dissolve. ing. This would form a warm body of sphere for long periods. Climatologists

3 | 13 MaxPlanckResearch 21 FOCUS cline inthecomingyears,”saysIlyina. 22 wil areas become much these more widespread. 2200, Through Ocean. Pacific the of depths inthe only located were dissolved calcite inwhich In19 regions blue carbonate. the calcium of form –amineral calcite to respect with supersaturated are areas red the while undersaturation, of acondi display regions blue (right). The 2200 and (left) 1950 for ocean inthe Ωvalue) the by (indicated saturation Carbonate strong istheeffectofphysicaltrans- transport ofCO the phytoplanktonandbiological in this.”Howgreatistheinfluenceof what factorsplaythemostcriticalrole “Unfortunately, wedon’t yetknow capacity toabsorbCO “Our simulationsshowthattheoceans’ A RAINMAKER SULFUR COMPOUNDIS ably inthefuture. could weakenthistransportconsider- quantities ofCO mixing takingplace,transportinglarge today, thereisstrongcirculation and well mixedwateroftheNorthAtlantic the deepocean.Within therelatively crease ofCO centration declines. it willshrinkifthephytoplanktoncon- refer totheoceansasacarbonsink.But Enhanced thermaloceanstratification MaxPlanckResearch MaxPlanckResearch Physics, too,playsaroleinthede- _Geosciences » 2 beingsequesteredinto will indeeddeclineinthecomingyears. Our simulationsshowthattheoceans’capacitytoabsorbCO 2 2 intothedeep?How intothedeepocean. 3 | 13 2 willindeedde- arina Six:simulatingthefuturerelease collaboration withhercolleagueKath- experiment thatIlyinaconductedin iscanbeseenfromafurther chemistry simulations. intoalgorithmsforclimate chemistry processes involvedinoceanbiogeo- right toolsfortransformingcomplex admits. Theclimatologistalsohasthe simply enjoyconstructingmodels,”she experience inconstructingmodels.“I pher, butshealsohasmanyyearsof for this.Notonlyissheanoceanogra- na istherightpersoninplace delivers plausibleresults.”Tatiana Ilyi- ulation stillcorrespondstorealityand ters, wehavetomakesurethatthesim- this: “Ifweswitchoffcertainparame- oceanic carboncycle.Theproblemis lations todeterminetheirroleinthe cus onindividualaspectsofhersimu- ter play? of theionsandpHvaluewa- port? Whatrole Just howimportantoceanbiogeo- Ilyina nowwantstoattemptfo- 902200 1950 does theequilibrium Anyone whohastrudgedthroughsea- and transportofdimethylsulfidegas. from theoceanislargestsource of there. Emissionofdimethylsulfide tributes totheformationofclouds ly, viaseveralintermediatesteps,con- sphere abovetheoceansandultimate- rainmakers. Itriseshighintotheatmo- pose. Dimethylsulfideisoneofthe generated whenalgaedieanddecom- the mustysmellofthissubstance.Itis weed onthebeachesofBalticknows one considersthatthecarboncyclein- ships becomeevenmoreintricatewhen ification astheeviltwins.Therelation- fer toglobalwarmingandoceanacid- fully understood.Researchers nowre- concentrations intheoceanarenotyet the Earthbecomeswarmer. flect theSun’s energy. Consequently, This resultsinfewerclouds,whichre- decreases duetooceanacidification. of dimethylsulfideintotheatmosphere sulfur intheatmosphere. The consequencesofincreasedCO The simulationshowsthattheflux 2 tion 50, l Ω value 0 1 2 3 4 5 6 7 saturation saturation Under- Super- 2

Graphics: MPI for Microbiology – Michael Böttinger Photo: MPI for Meteorology compounds. Noonec bering. Onewouldhaveto pourinto The resultsofthesimulation wereso- whether thiswouldbefeasible atall. geoengineering. Ilyinahas checked artificial mitigationofclimate change cium hydroxide,orhydratedlime. tional alkalinecompounds,suchascal- could fertilizetheoceanswithaddi- been atopicofdiscussion–whetherwe acidification canbesloweddownhas now, thequestionofwhetherocean pening formillionsofyears.Foryears the effectofCO line ionsneutralizeacids,thusbuffering rive fromerodedrockandotheralka- ly theopposite.Carbonateionsthatde- by acidificationofseawater, butexact- stance, limestone,aren’t accompanied ocean fromtheweatheringof,forin- Carbonate ionsthataresuppliedinthe asamodelforthis: process thatserves ification artificially. Thereisanatural simulate theideaofhaltingoceanacid- Tatiana Ilyina’s groupisdevelopingto cesses andtransferthemtoasimulation. comprehend thebiogeochemicalpro- ample showshowdifficultitistofully in theoceans.Mainly, however, theex- anobacteria profitfromtheaddedCO in theocean.Thereisnodoubtthatcy- vantageous ordisadvantageousforlife tainty yetwhetherthatwillprovead- Thus, moreCO ENHANCEMENT UNFORESEEABLE CONSEQUENCES OFALKALINITY for nutrition. pounds, suchasthosethatplantsuse eous N life formsthatareabletoconvertgas- tivity. Cyanobacteriaareoneofthefew trations drivethemtohighlevelsofac- but alotofCO was presentintheEarth’s atmosphere, oped duringatimewhenlittleoxygen dreds ofmillionsyears.Theydevel- blue-green algae. teria –thephytoplanktonknownas en by, amongotherthings,cyanobac- as thenitrogencycle.Thiscycleisdriv- fluences othercyclesintheocean,such lutely loveCO Researchers callthesemethodsof This alsoappliestothecalculations Cyanobacteria haveexistedforhun- 2 intoothernitrogencom- 2 . IncreasedCO 2 2 . Thesemicrobesabso- meansmorenitrogen 2 . Thishasbeenhap- an saywithcer- 2 concen- 2

and a massive intervention intheeco- and amassiveintervention would betoomuchofagoodthing, until thecurrentsdistributedit.That alkalinity wasaddedtotheseawater, climb to8.7at stone. Thatisquiteexpensive. some 70to100billiontonsoflime- ural weathering.Thiswouldrequire ions thataredepositedgloballybynat- the oceans200timesquantityof

discuss their latest results. latest their discuss Six Katharina and (left) Tatiana Ilyina oceans: the of future the Simulating ● ● ● ● TO THEPOINT ● first linkinthefood chainoftheoceans. like plants,utilizetheenergy ofsunlight through photosynthesis.Phytoplankton isthe Phytoplankton: Intergovernmental Panel onClimateChange(IPCC). worldwide. to the The conclusionswillbedistilledintoascientificconsensus for areport results bymore ofclimatesimulationsthatwere thantwenty institutions undertaken CMIP GLOSSARY Initially, though,thepHvaluewould

the atmosphere.Atsametime,oceanswouldreleaseless dimethyl sulfide, may declineasaresultofclimatechange,andtheoceansabsorb lessCO Model simulationsshowthatprimaryproductionbyphytoplankton intheoceans Ocean acidificationcausesfeedbackthatcanamplifythegreenhouseeffect. these shells. pH values,makingitharderforallorganismswithcalcareousshellstoconstruct The concentrationofcarbonateionsisdecreasinginseawaterwith changing theclimate,itisalsoacidifyingoceans. The increasingconcentrationofcarbondioxideintheatmosphereisnotonly and theclimate. have unforeseeableconsequencesforbiogeochemistry,theecology oftheoceans, Slowing downoceanacidificationbygeoengineeringcouldbeexpensive and reach theEarth’ssurface. impairing cloudformationandallowingmorethermalradiationfrom theSunto 5 : In the Coupled ModelIntercomparisonProjectPhase the locations Microorganisms intheocean,suchasgreen, brown andbluealgae,that, where the creatures wouldbeunforeseeable.For system. Theconsequencesforliving as thecarboncycle. destabilize individualprocesses,such monitor andcontroltheeffectsifthey climate aretoocomplexforhumansto and ships betweenbiogeochemistry search makes itclearthattherelation- much sense.Especiallysinceherre- kalinity enhancementwouldn’t make this reason,Ilyinathinksthatglobalal- 5 , climatologistsare examiningthe 3 | 13 MaxPlanckResearch MaxPlanckResearch

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