The decline of coral reefs from ocean temperature extremes: Are we close to peak bleaching?

Simon D. Donner, Harmony Martell, Cheryl Logan

Research output: Contribution to conferencePresentationpeer-review

Abstract

Ocean temperature extremes have been linked to mass mortality and phase shifts in coastal ecosystems, including warmwater coral reefs, kelp forests, and temperate reefs. Warm-season marine heat waves (MHWs), attributed to climate change, have led to widespread episodes of warmwater coral bleaching and associated mortality including the global-scale events observed from 2014 through 2017. For twenty years, modeling studies have consistently concluded that even moderate warming scenarios (e.g., RCP4.5) will make the prolonged sea surface temperature extremes that can cause mass bleaching an annual or bi-annual occurrence on reefs worldwide before the 2050s. These projections of increasingly frequent and severe bleaching conditions are often misinterpreted as indicating that bleaching itself will become increasingly frequent and severe as the decades pass. However, projected bleaching "conditions" only translate into actual mass bleaching and associated mortality if susceptible taxa are present in abundance. As the climate continues to warm, and corals continue to die from heat stress, the world could reach a point in time where the extent of bleaching and associated mortality peaks, and begins to decline.

In this presentation, we use the results of a new global ecological-evolutionary projection model to demonstrate that the world is likely to reach "peak bleaching", defined as the year that the global area of bleaching-related coral mortality reaches a maximum, in the next two decades. The model results suggest the annual mortality from bleaching worldwide will peak and then decline due to previous loss of coral cover and the survival of resistant taxa and genotypes, but that the timing of the peak depends on assumptions about the baseline to which corals are acclimated. These findings raise questions about the `shifting baselines' for coral reefs and other coastal systems threatened by marine heat waves, both in terms of the baseline used for computed future warm season MHWs and our own baseline expectations of ecosystem status.
Original languageAmerican English
StatePublished - Dec 2020

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