Abstract
Abstract. Anthropogenic carbon dioxide (CO2) emissions are acidifying the ocean, affecting calcification rates in pelagic organisms, and thereby modifying the oceanic carbon and alkalinity cycles. However, the responses of pelagic calcifying organisms to acidification vary widely between species, contributing uncertainty to predictions of atmospheric CO2 and the resulting climate change. At the same time, ocean warming caused by rising CO2 is expected to drive increased growth rates of all pelagic organisms, including calcifiers. It thus remains unclear whether anthropogenic CO2 emissions will ultimately increase or decrease pelagic calcification rates. Here, we assess the importance of this uncertainty by introducing a dependence of calcium carbonate (CaCO3) production on calcite saturation state (ΩCaCO3) in an intermediate complexity coupled carbon-climate model. In a series of model simulations, we examine the impact of several variants of this dependence on global ocean carbon cycling between 1800 and 3500 under two different CO2 emissions scenarios. Introducing a calcification-saturation state dependence has a significant effect on the vertical and surface horizontal alkalinity gradients, as well as on the removal of alkalinity from the ocean through CaCO3 burial. These changes result in an additional oceanic uptake of carbon when calcification depends on ΩCaCO3 (of up to 270 Pg C), compared to the case where calcification does not depend on acidification. In turn, this response causes a reduction of global surface air temperature of up to 0.4 °C in year 3500. Different versions of the model produced varying results, and narrowing this range of uncertainty will require better understanding of both temperature and acidification effects on pelagic calcifiers. Nevertheless, our results suggest that alkalinity observations can be used to constrain model results, and may not be consistent with the model versions that simulated stronger responses of CaCO3 production to changing saturation state.
Highlights
Ocean uptake of atmospheric carbon dioxide (CO2) is having a profound effect on biochemical cycles and ocean ecosystems
The standard version of the model simulated a CaCO3 production rate of 0.6 Pg C yr−1 in the year 2000. This is consistent with the estimate of 0.6–1.6 Pg C yr−1 based on satellite and sediment trap data and of 0.4–1.8 Pg C yr−1 based on model predictions (Doney et al, 2009)
This study has shown that the response of the marine CaCO3 cycle to anthropogenic CO2 may hinge on the degree by which increased temperatures accelerate plankton growth, compared to the degree by which acidification hinders calcification
Summary
Ocean uptake of atmospheric carbon dioxide (CO2) is having a profound effect on biochemical cycles and ocean ecosystems. This, in turn, leads to a decrease in carbonate ion concentration ([CO23−]), a shoaling of both the calcium carbonate (CaCO3) saturation horizon and lysocline, and an alteration of CaCO3 stored in deep-sea sediments. This process, known as ocean acidification, has the potential to severely impact the biological carbon pumps, which influence the vertical alkalinity (ALK) and dissolved inorganic carbon (DIC) gradients in the ocean. Ocean acidification could affect the strength of the ocean as a carbon sink and, the rate and magnitude of global climate change.
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