Declining coral calcification to enhance twenty-first-century ocean carbon uptake by gigatonnes

Abstract

The sensitivity of coral reefs to climate change is well established. As the oceans warm and acidify, the calcification of coral reefs declines with net calcium carbonate dissolution projected under even moderate emissions trajectories. The impact of this on the global carbon cycle is however yet to be accounted for. Here, we use a synthesis of the sensitivity of coral reef calcification to climate change, alongside reef distribution products to estimate alkalinity and dissolved inorganic carbon fluxes resulting from reductions in reef calcification. Using a global ocean biogeochemical model, we simulate the impact on ocean carbon uptake under different emissions scenarios, accounting for uncertainty in present-day calcification rates. Reductions in net coral reef carbonate production can enhance the ocean carbon sink by up to 1.25 GtCO₂ y^-1 by midcentury (0.48 GtCO₂ y^-1 median estimate) with cumulative ocean carbon uptake up to 13% greater by 2300 (7% median estimate). Our findings indicate that accounting for the coral reef feedback in projections will increase estimates of the remaining carbon budget associated with global warming thresholds, as well as the likelihood that net zero emissions can be achieved without negative emissions.

Keywords: calcification; carbon; climate; coral reef; feedback.

Fig. 1.

Global declines in coral reef carbonate production and the associated fluxes of alkalinity and DIC. The (A), estimated global coral reef net ecosystem calcification (TgC y⁻¹ and PgCaCO₃ y⁻¹) and the (B), simulated global flux of alkalinity (Pmol y⁻¹) and DIC associated with carbonate production decline in each RCP. Black lines represent the historical unperturbed rate of global carbonate production (30, 150, and 300 TgC y⁻¹), with the same line type used for corresponding RCP simulations and the assumed carbonate historical production rate denoted in subscript.

Fig. 2.

Enhancement of the global ocean anthropogenic carbon sink due to reductions in coral reef carbonate production. The simulated (A), annual ocean anthropogenic carbon uptake (PgC y⁻¹), (B), enhancement due to coral reef degradation (PgC y⁻¹), (C), cumulative anthropogenic ocean carbon uptake (PgC) and (D), enhancement due to coral reef degradation (PgC). Anomalies in ocean carbon uptake due to coral reef carbonate production declines are shown relative to their respective RCP. No decline in coral reef carbonate production is simulated prior to 2005. Data-based estimates of the ocean anthropogenic carbon sink in the 2000s are shown in magenta in panel A with error bars representing 1 SD (2). The gray bar in panel A corresponds to the period shown in Fig. 3.

Fig. 3.

Carbon uptake enhancement extends beyond the regions of coral reefs. The (A), mean ocean anthropogenic carbon uptake in 2081 to 2099 of RCP4.5 and the (B), coral reef-driven anomaly in ocean anthropogenic carbon uptake in 2081 to 2099 of RCP4.5₁₅₀. Stippling indicates the distribution of coral reefs.