Global declines in coral reef calcium carbonate production under ocean acidification and warming

Abstract

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.

Keywords: calcification; carbonate production; climate change; corals.

Fig. 1.

Processes involved in net carbonate production and accretion on reefs as well as the associated methods typically employed to measure this. +ve = positive contribution to accretion with solid lines; −ve = negative contribution with dashed lines. Processes in gray are not included in most carbonate budgets or here. Here, we project the effects of ocean acidification and warming on CCA and coral calcification, chemical components of bioerosion, and sediment dissolution. Only chemical components of bioerosion are included in hydrochemical measurements, while direct sediment production by bioeroders is also included here.

Fig. 2.

(A) Net carbonate production rates (kg CaCO₃ m⁻² ⋅ y⁻¹) and (B) potential vertical accretion rates (mm ⋅ y⁻¹), presently and under the interactive effects of ocean acidification and ocean warming. These data account for reduced future coral cover due to mass bleaching events across three ocean basins for the mean of each of 183 reefs. Scenarios shown are three RCP scenarios (2.6, 4.5, and 8.5) by 2050 and 2100. Medians, 75% quartiles, 95% whiskers, and outliers are presented. See SI Appendix, Figs. S1 and S3 for the accretion and carbonate production rates projected under each stressor singularly. For accretion without sediment dissolution, see SI Appendix, Fig. S2.

Fig. 3.

Location of study regions and their net carbonate production (kg CaCO₃ m⁻² ⋅ y⁻¹) under the following scenarios: (A) present-day and projections of the interactive effects of ocean acidification, warming, and mass coral bleaching by 2050 at (B) RCP2.6, (C) RCP4.5, and (D) RCP8.5 and by 2100 under (E) RCP2.6, (F) RCP4.5, and (G) RCP8.5 occurring at each of 183 reefs. Present-day Palmyra reef is higher than 15 kg CaCO₃ m⁻² ⋅ y⁻¹.

Fig. 4.

Examples of how the effects of ocean acidification, ocean warming, and mass coral bleaching are projected to impact net carbonate production through changes in bioerosion and net calcification of corals and of coralline algae. Displayed here are regions in the Atlantic (A and B), Indian (C and D), and Pacific Oceans (E and F) with high (A, C, and E) and low (B, D, and F) present-day net carbonate production. Scenarios are the same as in Fig. 1: present-day and RCP2.6, 4.5, and 8.5 in 2050 and 2100. Photo credits: A, B, and C were taken by Chris Perry; D was taken by Nicholas Graham; E was taken by Gareth Williams; and F was taken by Christopher Cornwall.