Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host. Here, using newly developed geochemical proxies (δ11B and B/Ca), we show that Porites corals from natural reef environments exhibit a close (r2 ∼0.9) antithetic relationship between dissolved inorganic carbon (DIC) and pH of the corals’ calcifying fluid (cf). The highest DICcf (∼ × 3.2 seawater) is found during summer, consistent with thermal/light enhancement of metabolically (zooxanthellae) derived carbon, while the highest pHcf (∼8.5) occurs in winter during periods of low DICcf (∼ × 2 seawater). These opposing changes in DICcf and pHcf are shown to maintain oversaturated but stable levels of carbonate saturation (Ωcf ∼ × 5 seawater), the key parameter controlling coral calcification. These findings are in marked contrast to artificial experiments and show that pHcf upregulation occurs largely independent of changes in seawater carbonate chemistry, and hence ocean acidification, but is highly vulnerable to thermally induced stress from global warming.

Ocean warming and acidification threaten the future growth of coral reefs. This is because the calcifying coral reef taxa that construct the calcium carbonate frameworks and cement the reef together are highly sensitive to ocean warming and acidification. However, the global-scale effects of ocean warming and acidification on rates of coral reef net carbonate production remain poorly constrained despite a wealth of studies assessing their effects on the calcification of individual organisms. Here, we present global estimates of projected future changes in coral reef net carbonate production under ocean warming and acidification. We apply a meta-analysis of responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change to estimate the net carbonate production rates of 183 reefs worldwide by 2050 and 2100. We forecast mean global reef net carbonate production under representative concentration pathways (RCP) 2.6, 4.5, and 8.5 will decline by 76, 149, and 156%, respectively, by 2100. While 63% of reefs are projected to continue to accrete by 2100 under RCP2.6, 94% will be eroding by 2050 under RCP8.5, and no reefs will continue to accrete at rates matching projected sea level rise under RCP4.5 or 8.5 by 2100. Projected reduced coral cover due to bleaching events predominately drives these declines rather than the direct physiological impacts of ocean warming and acidification on calcification or bioerosion. Presently degraded reefs were also more sensitive in our analysis. These findings highlight the low likelihood that the world’s coral reefs will maintain their functional roles without near-term stabilization of atmospheric CO2 emissions.

Severe, global-scale thermal stress events like those of 1998 and 2016, are becoming more frequent and intense, potentially compromising the future of coral reefs. Here we report the effects of the 1998 bleaching event on coral calcification as well as the composition of the calcifying fluid (cf) from which corals precipitate their calcium carbonate skeletons. This was investigated by using the Sr/Ca, Li/Mg (temperature), and boron isotopes (δ11B) and B/Ca (carbonate chemistry) proxies in a Porites sp. coral. Following the summer of 1998 the coral exhibited a prolonged period (~18 months) of reduced calcification (~60%) and a breakdown in the seasonality of the geochemical proxies. However, the maintenance of elevated dissolved inorganic carbon (DICcf; >×2 seawater) and pHcf (>8.3 compared to seawater ~8.0) even during severe stress of 1998 indicate that a minimum threshold of high aragonite saturation state (Ωcf) of ~14 (~×4 seawater), is an essential pre-requisite for coral calcification. However, despite maintaining elevated levels of Ωcf even under severe stress, coral growth is still impaired. We attribute this to reductions in either the effective active volume of calcification and/or DICcf as bleaching compromises the photosynthetically fixed carbon pool available to the coral.