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Sensitivities of Marine Carbon Fluxes to Ocean Change

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Sensitivities of Marine Carbon Fluxes to Ocean Change Sensitivities of marine carbon fluxes to ocean change Ulf Riebesell1, Arne Ko¨ rtzinger, and Andreas Oschlies Marine Biogeochemistry, Leibniz Institute of Marine Sciences, IFM-GEOMAR, Du¨sternbrooker Weg 20, 24105 Kiel, Germany Edited by Hans Joachim Schellnhuber, Potsdam Institute for Climate Impact Research, Potsdam, Germany, and approved August 31, 2009 (received for review December 29, 2008) Throughout Earth’s history, the oceans have played a dominant role in the climate system through the storage and transport of heat and the exchange of water and climate-relevant gases with the atmosphere. The ocean’s heat capacity is Ϸ1,000 times larger than that of the atmosphere, its content of reactive carbon more than 60 times larger. Through a variety of physical, chemical, and biological processes, the ocean acts as a driver of climate variability on time scales ranging from seasonal to interannual to decadal to glacial–interglacial. The same processes will also be involved in future responses of the ocean to global change. Here we assess the responses of the seawater carbon- ate system and of the ocean’s physical and biological carbon pumps to (i) ocean warming and the associated changes in vertical mixing and overturning circulation, and (ii) ocean acidification and carbonation. Our analysis underscores that many of these responses have the potential for significant feedback to the climate system. Because several of the underlying processes are interlinked and nonlinear, the sign and magnitude of the ocean’s carbon cycle feedback to climate change is yet unknown. Understanding these processes and their sen- sitivities to global change will be crucial to our ability to project future climate change. climate change ͉ marine carbon cycle ͉ ocean acidification ͉ ocean warming he ocean is presently undergoing mospheric carbon dioxide (8) and has a sidered separately. For conceptual simpli- major changes. Over the past 50 mean residence of a few hundred years. fication, however, we will here focus on years, the ocean has stored 20 Hence, small changes in this pool, caused, physical impacts on the solubility pump times more heat than the atmo- for example, by biological responses to only before discussing impacts on the bio- Tsphere (1). The Arctic Ocean surface lay- ocean change, would have a strong effect logical pump in a later section. ers have recently experienced a pro- on atmospheric CO2. Counteracting the Rising atmospheric CO2 leads to higher nounced freshening (2), and although organic carbon pump in terms of its effect sea-surface temperature (SST) and a con- there is still considerable uncertainty in on air–sea CO2 exchange is a process current reduction in CO2 solubility. It is current observational estimates (3), cli- termed the carbonate counter pump (9), estimated that this positive SST feedback mate models run under increasing atmo- also known as the alkalinity pump. The will reduce the oceanic uptake of anthro- spheric CO2 levels essentially all predict a formation of CaCO3 shell material by cal- pogenic carbon by 9–15% [Ϸ45–70 giga- slowdown of the Atlantic meridional over- cifying plankton and its sinking to depth tons carbon (Gt C)] by the end of the 21st turning circulation (MOC), which is part lowers the DIC and alkalinity in the sur- century (10–13). Global warming will also of the global thermohaline circulation (4). face ocean, causing an increase in CO2 intensify the hydrological cycle. The direct Since preindustrial times, the ocean has partial pressure. It is worth noting that the dilution effect on CO2 solubility is small also taken up Ϸ50% of fossil fuel CO2, organic and inorganic carbon pumps rein- compared with the temperature effect and which has already led to substantial force each other in terms of maintaining a is expected to largely cancel out at the changes in its chemical properties (5). vertical DIC gradient, whereas they are global scale as enhanced precipitation in Carbon fluxes from the sea surface into counteractive with respect to their impact one area tends to be balanced by en- the ocean interior are often described in on air–sea CO2 exchange. hanced evaporation in another area. Over- terms of a solubility pump and a biologi- Although the range of potential all, the expected effect of direct dilution cal pump (6). The abiotic solubility pump changes in the solubility pump and chemi- on anthropogenic CO2 uptake is of uncer- is caused by the solubility of CO2 increas- cal responses of the marine CO2 system is tain sign but estimated to be much smaller ing with decreasing temperature. In known reasonably well, our understanding than 1% (13). A potentially more signifi- present climate conditions, deep water of biological responses to ocean change is cant impact of changes in the hydrological forms at high latitudes. As a result, still in its infancy. Such responses relate cycle on the oceanic CO2 uptake can arise volume-averaged ocean temperatures are both to possible direct effects of rising at high latitudes in the North Atlantic: lower than average sea-surface tempera- atmospheric CO2 through ocean acidifica- Here, reduced surface salinities, together tures. The solubility pump then ensures tion (decreasing seawater pH) and ocean with higher SSTs, would lower the density that, associated with the mean vertical carbonation (increasing CO2 concentra- of surface waters and thereby may inhibit temperature gradient, there is a vertical tion), and indirect effects through ocean the formation of deep waters. This lower- gradient of dissolved inorganic carbon warming and changes in circulation and ing in turn would reduce meridional pres- (DIC). This solubility-driven gradient ex- mixing regimes. These changes are ex- sure gradients and tend to slow down the plains Ϸ30–40% of today’s ocean surface- pected to impact marine ecosystem struc- thermohaline-driven part of the meridi- to-depth DIC gradient (7). ture and functioning and have the poten- onal, overturning circulation. Climate A key process responsible for the re- tial to alter the cycling of carbon and model simulations indeed predict a gen- maining two thirds of the surface-to-depth nutrients in the surface ocean with likely DIC gradient is the biological carbon feedbacks on the climate system. pump. It transports photosynthetically Author contributions: U.R., A.K., and A.O. wrote the paper. fixed organic carbon from the sunlit sur- Changes in the Solubility Pump. Alterations The authors declare no conflict of interest. face layer to the deep ocean. Integrated in the physical state of the ocean will af- This article is a PNAS Direct Submission. over the global ocean, the biotically medi- fect both the solubility pump and the bio- 1To whom correspondence should be addressed. E-mail: ated oceanic surface-to-depth DIC gradi- logical pump. For finite-amplitude pertur- [email protected]. ent corresponds to a carbon pool 3.5 bations, changes in both pumps interact This article contains supporting information online at www. times larger than the total amount of at- nonlinearly and therefore cannot be con- pnas.org/cgi/content/full/0813291106/DCSupplemental. 20602–20609 ͉ PNAS ͉ December 8, 2009 ͉ vol. 106 ͉ no. 49 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0813291106 Downloaded by guest on September 25, 2021 SPECIAL FEATURE: PERSPECTIVE eral weakening of the North Atlantic MOC for the 21st century when forced with increasing greenhouse gas concentra- tions (4, 14). Complementary to the simu- lated weakening of the Atlantic MOC, climate models also predict an intensifica- tion of the ocean circulation in the South- ern Ocean, resulting from a larger warm- ing in mid and low latitudes compared with the waters around Antarctica. In consequence, the meridional pressure gra- dients are predicted to increase both across the Antarctic Circumpolar Current (15) and in the atmosphere above the Southern Ocean (16). Estimates of the total impact of the circulation changes on the solubility pump differ considerably among different models, predicting a re- duction in global oceanic carbon uptake Fig. 1. Trends and projections of changes in the properties of the marine CO2 system in cold (blue) and warm (red) surface waters between 1750 and 2100 under the prescribed atmospheric CO2 increase. (A) Equilibrium ء by some 3–20% (17). A major part of the CO2 partial pressure (pCO2) and pH. (B) Equilibrium DIC and TA. (C) Concentrations of the three species CO2, ء Ϫ 2Ϫ large uncertainty can be attributed to our HCO3 , and CO3 (note that the concentration of CO2 is multiplied by a factor of 10. (D)CO2-uptake ratio and incomplete understanding of the Southern saturation states (⍀) of calcite and aragonite. (E) Temperature sensitivities of pCO2 and pH. (F) Chemical (i.e., Ocean’s role as a carbon sink. CO2) sensitivities of pCO2 and pH. See SI Text for details on the calculation and assumptions made therein. Chemical Response of the Marine CO2 Sys- tem. A significant portion of the carbon To illustrate changes in the properties units (Fig. 1A). This increase in the hy- ϩ released by humankind in the Anthropo- of the marine CO2 system during the drogen ion (H ) concentration of up to cene era has been—and will continue to course of the 21st century, we need to 250%, also referred to as ‘‘ocean acidifica- be—redistributed to the world ocean. The prescribe the development of atmospheric tion,’’ is currently of growing concern ocean owes its huge CO2-uptake capacity CO2 concentrations. With vastly differing among marine biologists. Also note that to the presence of the
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