Effects Of Carbon Dioxide & Climate Change On Ocean Acidification & Carbonate Saturation - AGU

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Effects of carbon dioxide and climate change on ocean acidification and carbonate mineral saturation

Long Cao
Department of Atmospheric Sciences, University of Illinois,
Urbana, Illinois, USA

Ken Caldeira
Department of Global Ecology, Carnegie Institution,
Stanford, California, USA

Atul K. Jain
Department of Atmospheric Sciences, University of Illinois,
Urbana, Illinois, USA

<1> We use an earth system model of intermediate complexity to show how consideration of climate change affects predicted changes in ocean pH and calcium carbonate saturation state. Our results indicate that consideration of climate change produces second-order modifications to ocean chemistry predictions made with constant climate; these modifications occur primarily as a result of changes in sea surface temperature, and climate-induced changes in dissolved inorganic carbon concentrations. Under a CO2 emission scenario derived from the WRE1000 CO2 stabilization concentration pathway and a constant climate, we predict a 0.47 unit reduction in surface ocean pH relative to a pre-industrial value of 8.17, and a reduction in the degree of saturation with respect to aragonite from a pre-industrial value of 3.34 to 1.39 by year 2500. With the same CO2 emissions but the consideration of climate change under a climate sensitivity of 2.5°C the reduction in projected global mean surface pH is about 0.48 and the saturation state of aragonite decreases to 1.50. With a climate sensitivity of 4.5°C, these values are 0.51 and 1.62, respectively. Our study therefore suggests that future changes in ocean acidification caused by emissions of CO2 to the atmosphere are largely independent of the amounts of climate change.

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<12> In this study we predict future changes in ocean chemistry as a result of increased CO2 concentrations and accompanying climate change. With prescribed CO2 emissions calculated from the WRE1000 CO2 concentration pathway and a constant climate, we predict a 0.47 unit reduction in surface ocean pH relative to a pre-industrial value of 8.17, and a reduction in the degree of saturation with respect to aragonite from a pre-industrial value of 3.34 to 1.39 by year 2500. Ocean acidification (lowering of pH and carbonate saturation state) will almost certainly adversely impact marine biota through a variety of mechanisms . In particular, it will likely pose a great threat to the survival of calcifying organisms such as corals and foraminifera . The consideration of climate change produces a modest modification to the predicted ocean chemistry mainly through changes in sea surface temperature and climate-induced DIC concentrations. With a climate sensitivity of 2.5°C and 4.5°C our simulations show that relative to constant climate simulations, the inclusion of climate change further reduces projected global mean surface pH by 0.01 and 0.04, and increases projected saturation state of aragonite by 0.11 and 0.23 respectively by year 2500.

<13> McNeil and Matear <2006> reports that the effects of climate change on surface ocean pH are negligible from a coupled climate-carbon cycle simulation driven by the IS92a atmospheric CO2 concentration pathway. With prescribed CO2 concentrations we project negligible climatic effects on surface pH (Table 1), consistent with their study. In this case, the indirect DIC effect almost cancels the direct temperature effect (not shown), leading to a negligible net climatic effect on pH. However, with prescribed CO2 emissions, we find that consideration of climate change has a pronounced effect on surface pH (namely, to cause a greater decrease in pH, as seen from Figure 2c); the direct temperature effect dominates the indirect DIC effect (as explained above) (Figure 3a).

<14> We also show that climate change results in less reduction in ocean pH and aragonite saturation state in the deep ocean due to reduced North Atlantic overturning circulation and increased ocean stratification. The reduced acidification in the deep ocean is of interest since the deep ocean biota may be more sensitive to pH changes than surface biota . Nevertheless, we conclude, based on our simulation results, that climate change exerts a second order control on ocean chemistry. The changes in ocean acidification and saturation state of calcium carbonate minerals caused by CO2 emissions, and the resulting increases in atmospheric CO2 concentration, are insensitive to the amounts of climate change.

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