Past the Precipice? Projected Coral Habitability Under Global Heating

Abstract

Coral reefs are rapidly declining due to local environmental degradation and global climate change. In particular, corals are vulnerable to ocean heating. Anomalously hot sea surface temperatures (SSTs) create conditions for severe bleaching or direct thermal death. We use SST observations and CMIP6 model SST to project thermal conditions at reef locations at a resolution of 1 km, a 16-fold improvement over prior studies, under four climate emissions scenarios. We use a novel statistical downscaling method which is significantly more skillful than the standard method, especially at near-coastal pixels where many reefs are found. For each location we present projections of thermal departure (TD, the date after which a location with steadily increasing heat exceeds a given thermal metric) for severe bleaching recurs every 5 years (TD5Y) and every 10 years (TD10Y), accounting for a range of post-bleaching reef recovery/degradation. As of 2021, we find that over 91% and 79% of 1 km² reefs have exceeded TD10Y and TD5Y, respectively, suggesting that widespread long-term coral degradation is no longer avoidable. We project 99% of 1 km² reefs to exceed TD5Y by 2034, 2036, and 2040 under SSP5-8.5, SSP3-7.0, and SSP2-4.5 respectively. We project that 2%-5% of reef locations remain below TD5Y at 1.5°C of mean global heating, but 0% remain at 2.0°C. These results demonstrate the importance of further improving ecological projection capacity for climate-vulnerable marine and terrestrial species and ecosystems, including identifying refugia and guiding conservation efforts. Ultimately, saving coral reefs will require rapidly reducing and eliminating greenhouse gas emissions.

Keywords: CMIP6; climate change; coastal; coral reefs; downscaling; projection.

Figure 1

Comparison between standard downscaling and BGL downscaling mean squared error (MSE, in degrees Celsius squared) estimated from validation against withheld 2018–2020 Multiscale Ultrahigh Resolution data in a central region of the Great Barrier Reef. This comparison was performed using SSP126 time series. Coral reef locations are indicated by the brown translucent masking. Note. The MSE improvement provided by the BGL downscaling method that is especially evident in near‐coastal regions. Averaged over coral reef locations, the standard downscaling method had MSE of 0.252°C² and the BGL method had MSE of 0.173°C², a reduction of 31%.

Figure 2

(left) Global mean surface air temperature anomaly projections, relative to an 1880–1900 baseline, from the CMIP6 ensemble mean (right) Mean SST averaged only over coral reef locations included in the analysis, with observational Multiscale Ultrahigh Resolution data before 2020 shown within the shaded region and the downscaled CMIP6 model ensemble projections after 2020. Colors correspond to emissions scenarios as indicated in the legend.

Figure 3

Global maps of thermal departure (top) The highest thermal threshold we considered, with the latest departure years, and the most optimistic climate scenario: TD5Y, 8 DHW₂₀₀₈ threshold, and SSP126 (bottom) The lowest thermal threshold we considered, with the earliest departure years, and the most pessimistic climate scenario: TD10Y, 8 DHW₁₉₈₈ threshold, and SSP585. Maps of other scenarios are shown in the Supporting Information S1.

Figure 4

Cumulative histograms of thermal departure as a function of year, for SSP126 (black), SSP245 (blue), SSP370 (green), SSP585 (red), for a five year heat event return timescale (TD5Y, top row) and a 10 year heat event return timescale (TD10Y, bottom row). The 1988 (left column) and 2008 (right column) climatological baselines are shown. Cyan and magenta horizontal lines show the 10% and 1% fractional levels respectively; colored vertical ticks on the y‐axis indicate crossings of these levels.