Calcium carbonate production response to future ocean warming and acidification

Abstract

Abstract. Anthropogenic carbon dioxide (CO 2 ) 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 CO 2 and the resulting climate change. At the same time, ocean warming caused by rising CO 2 is expected to drive increased growth rates of all pelagic organisms, including calcifiers. It thus remains unclear whether anthropogenic CO 2 emissions will ultimately increase or decrease pelagic calcification rates. Here, we assess the importance of this uncertainty by introducing a dependence of calcium carbonate (CaCO 3 ) production on calcite saturation state ( CaCO 3 ) in an intermediate complexity coupled carbonclimate 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 CO 2 emissions scenarios. Introducing a calcificationsaturation 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 CaCO 3 burial. These changes result in an additional oceanic uptake of carbon when calcification depends on CaCO 3 (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 CaCO 3 production to changing saturation state.

Current CaCO3dissolution at the seafloor caused by anthropogenic CO2

“…We note that calcifying organisms exhibit various responses to elevated pCO 2 conditions, due to the influence of other climate change-related effects, such as the warming of waters that counteracts acidification (21)(22)(23)(24). In the absence of unequivocal evidence of an immediate increase or decrease in the total calcification rate in the pelagic oceans, we assume that the flux of calcitic particles reaching the CCD (F) has remained invariant since the preindustrial era.…”

Quantifying the impact of ocean acidification on our future climate

“…These idealised modifications to the marine BGC cycle are summarised in Table 2 and described in more detail below. The modifications are designed to provide a ) was kept constant, which implies the simulation includes a temperature effect on PIC production because the PIC production would increases as a warming ocean increases POC production (Pinsonneault et al, 2012;Schmittner et al, 2008).…”

Simulated effect of calcification feedback on atmospheric CO2 and ocean acidification

“…The first type of parameterization follows the Michaelis-Menten function based on Pinsonneault et al 37 :…”

“…Where denotes the specified maximum value of (the CaCO 3 : POC production ratio), and is a half-saturation constant 37 38 . Different model versions based on this parameterization are denoted as “series S” ( Table 1 ).…”

“…Different model versions based on this parameterization are denoted as “series S” ( Table 1 ). In series S, the values of are selected to be 0.07, 1.5 and 20, which covers the range of values used by Pinsonneault et al 37 . For each , we calculate the corresponding value of by using the modeled preindustrial global mean sea surface calcite saturation state of 5.2 to ensure that the preindustrial surface mean CaCO 3 : POC production ratio in each model version has the same value of 0.018, i.e., the fixed constant used in the original model.…”