|As nicely illustrated the recent IPCC Fifth Assessment (Rhein et al. 2013; right), anthropogenic CO2 emissions are increasing the concentration of carbon dioxide (CO2) in Earth’s atmosphere, with about 30% (Mikaloff-Fletcher, 2006) of those emissions being taken up by the ocean. The surface ocean is slightly basic with an average pH (inverse log of hydrogen ion concentration; -log([H+])) of about 8, where a pH of
7 is neutral – lower than 7 being acidic, and higher being basic. Much of the ocean’s ability to soak up added CO2, or acid buffering capacity, comes from the weathering of alkaline rocks into dissolved salts, with most of the preindustrial ocean alkalinity in the form of bicarbonate ions (HCO3–, about 80%) and carbonate ions (CO32-; about 20%). When CO2 dissolves in seawater, it also reacts as an acid with the water molecule (CO2 + H2O -> H2CO3 -> H+ + HCO3– -> 2H+ + CO32-). Acidification decreases CO32- while HCO3– and H+ increase, making the ocean less efficient at taking up additional CO2. Ocean CO2 absorption rates and ultimate saturation limits depend on the regionally variable interplay of atmospheric exchange, ocean chemistry, and ocean circulation moving carbon into the interior. Both biological and climate change feedbacks also play important roles (Ciais et al., 2013).
There are many causes for concern regarding ocean acidification with respect to NOAA’s stewardship of living marine resources. Natural ocean acidification regionally impacts shellfish, coral reefs and other resources through reduction in the propensity for calcite and aragonite minerals to avoid dissolution. Human induced acidification is projected to exacerbate these impacts and may reduce fish respiration efficiency, change biodiversity through CO2 fertilization of photosynthesis in selectively benefiting slow-growing forms, and change availability of pH-sensitive nutrients and pollutants. There are many avenues of ongoing research to assess potential potential impacts. Further resources on ocean acidification and areas of research can be found at the IPCC Fifth Assessment Working Group I Report on the Scientific Basis and Working Group II report on Ocean Impacts, NOAA’s Ocean Acidification Program, the US Ocean Carbon and Biogeochemistry Program, and European Project on Ocean Acidification sponsored Fact Sheet on Ocean Acidification.
IPCC AR5 WG1 FAQ 3.2, Figure 1: A smoothed time series of atmospheric CO2 mole fraction (in ppm) at the atmospheric Mauna Loa Observatory (top red line), surface ocean partial pressure of CO2 (pCO2; middle blue line), and surface ocean pH (bottom green line) at Station ALOHA in the subtropical North Pacific north of Hawaii for the period from1990?2011 (after Doney et al., 2009; data from Dore et al., 2009). The results indicate that the surface ocean pCO2 trend is generally consistent with the atmospheric increase but is more variable due to large-scale interannual variability of oceanic processes.
GFDL Acidification Research
Ocean acidification research is a critical aspect of GFDL’s work in Future Carbon Uptake and Biogeochemical Change. GFDL has a long history of studying the ocean’s biological, solubility and carbonate pumps that control of air-sea partitioning of CO2 (e.g. Sarmiento and Toggweiler, 1984). In coordination with NOAA’s Ocean Acidification Program, GFDL scientists have recently built Earth System Models of the coupled carbon-climate system that model historical and projected ocean acidification under various emission scenarios out to the year 2100. Further, we have partnered with international teams of researchers to study the major stressors on open ocean ecosystems under climate change including warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton. These changes will be amplified. In a recent paper (Bopp et al. 2013), ten Earth System Models contributed to the Coupled Model Inter-comparison Project 5 (CMIP5) were used to assess how these stressors may evolve over the course of the 21st century. Two GFDL models were included in this study, GFDL-ESM2G and GFDL-ESM2M (Dunne et al. 2012; 2013). Specific to ocean acidification, this study found strong agreement in the surface acidification of approximately 0.35 pH units over this century under the highest emissions scenario (RCP8.5; Meinshausen et al. 2011). Additionally, projected acidification was found to have its maximum not at the surface where CO2 is being taken up from the atmosphere, but rather a few hundred meters below. In a second study (Resplandy et al., 2013), utilized the superior representation of thermocline biogeochemistry in GFDL’s ESM2M in this class of models to demonstrate how this strongest expression of ocean acidification manifests in the upper tropical thermocline. In this region, remineralization of organic material in the biological pump release CO2 and reduces buffer capacity to acidification. Enhanced stratification under climate warming also shoals the thermocline and enhances the subsurface remineralization signal. This mechanism of subsurface amplification of ocean acidification was also described in a regional study of the California Current using an early prototype of GFDL’s ESM (Rykaczewski and Dunne, 2010).
GFDL’s research on ocean acidification is ongoing both in representing the global and regional historical and projected future expression of acidification in more advanced and higher resolution models and in incorporating emerging scientific understanding of the mechanisms and impacts and feedbacks of acidification in ocean biogeochemistry and ecosystems.
Figure 7 and 8 from Bopp et al. (2013) illustrating projected changes in CO32- under various emission scenarios (Left) and the Depth and Magnitude of the Maximum pH change under the highest emission scenario considered (RCP8.5; Right):
- Bopp, L, and John P Dunne, et al., October 2013: Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models. Biogeosciences, 10(10), DOI:10.5194/bg-10-6225-2013.
- Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013: Carbon and Other Biogeochemical Cycles. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Dunne, John P., Jasmin John, Elena Shevliakova, Ronald J Stouffer, John P Krasting, Sergey Malyshev, P C D Milly, Lori T Sentman, Alistair Adcroft, William F Cooke, Krista A Dunne, Stephen M Griffies, Robert W Hallberg, Matthew J Harrison, Hiram Levy II, Andrew T Wittenberg, Peter Phillipps, and Niki Zadeh, April 2013: GFDL’s ESM2 global coupled climate-carbon Earth System Models Part II: Carbon system formulation and baseline simulation characteristics.Journal of Climate, 26(7), DOI:10.1175/JCLI-D-12-00150.1.
- Resplandy, L, L Bopp, James C Orr, and John P Dunne, June 2013: Role of mode and intermediate waters in future ocean acidification: analysis of CMIP5 models. Geophysical Research Letters, 40(12), DOI:10.1002/grl.50414.