Do coral reef forecasts agree on how much coral will remain in 2050?

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Models of the future of coral reefs include threshold models (TM) and multivariate environmental variability models (VM), which differ in how they account for environmental variation across space and time. Researchers used a VM based on General Additive Model (GAM) methods to examine empirical relationships between coral cover and 15 potentially influential variables, using data from 905 sites pooled into 318 reef cells in the Western and Central Indian Ocean Provinces.

Six environmental variables and one fisheries management variable were selected as significant, including sea surface temperature (SST) distribution shapes, dissolved oxygen, calcite and fisheries management. Common predictive variables—including cumulative degree-heating weeks (DHW), pH, maximum light, SST bimodality and rate of rise, and two multivariate metrics—were weak or not significant predictors of coral cover.

Using the seven top VM variables, the researchers established a spatially resolved 2020 baseline for future coral cover predictions across 11,678 reef cells of approximately 6.25 km² each, spanning 13 ecoregions and four fisheries management categories. They compared the models’ 2050 predictions under high and low Relative Concentration Pathway scenarios (CMIP5; RCP8.5 and RCP2.6), using the four available future-cast SST variables.

The TM’s relationship between excess heat (DHW) and coral mortality predicted considerably lower coral cover in 2050 than the VM. For reef cells with more than 25% coral cover in 2020, and assuming proposed optimal fisheries management was achieved, the TM predicted declines of 81% under RCP8.5 and 58% under RCP2.6, compared with 29% and 20%, respectively, for the VM. Despite the differences, the models’ coral cover predictions overlapped in two environmental regions in the southern equatorial current region of the Indian Ocean. The researchers expect historical and future patterns of acute and chronic stresses to be more influential than cumulative heat stress in predicting coral cover, and say the VM accounts for these patterns better than the TM.