Calcium carbonate production response to future ocean warming and acidification

[摘要] Anthropogenic carbon dioxide (CO₂) emissions are acidifying the ocean,affecting calcification rates in pelagic organisms, and thereby modifyingthe oceanic carbon and alkalinity cycles. However, the responses of pelagiccalcifying organisms to acidification vary widely between species,contributing uncertainty to predictions of atmospheric CO₂ and theresulting climate change. At the same time, ocean warming caused by risingCO₂ is expected to drive increased growth rates of all pelagicorganisms, including calcifiers. It thus remains unclear whetheranthropogenic CO₂ emissions will ultimately increase or decreasepelagic calcification rates. Here, we assess the importance of thisuncertainty by introducing a dependence of calcium carbonate (CaCO₃)production on calcite saturation state (Ω_CaCO3) in anintermediate complexity coupled carbon-climate model. In a series of modelsimulations, we examine the impact of several variants of this dependence onglobal ocean carbon cycling between 1800 and 3500 under two differentCO₂ emissions scenarios. Introducing a calcification-saturation statedependence has a significant effect on the vertical and surface horizontalalkalinity gradients, as well as on the removal of alkalinity from the oceanthrough CaCO₃ burial. These changes result in an additional oceanicuptake of carbon when calcification depends on Ω_CaCO3 (of up to270 Pg C), compared to the case where calcification does not depend onacidification. In turn, this response causes a reduction of global surfaceair temperature of up to 0.4 °C in year 3500. Different versions of themodel produced varying results, and narrowing this range of uncertainty willrequire better understanding of both temperature and acidification effectson pelagic calcifiers. Nevertheless, our results suggest that alkalinityobservations can be used to constrain model results, and may not beconsistent with the model versions that simulated stronger responses ofCaCO₃ production to changing saturation state.

[学科分类] 地球化学与岩石