Climate change is increasingly threatening coral reefs through recurring mass bleaching events. Research indicates that thermal tolerance—the ability of corals to withstand heat stress—can increase over time, potentially through shifts in coral community composition, genetic adaptation, or acclimatization. However, the rate at which thermal tolerance can increase remains unclear, and many future projections for coral reefs often don’t include the adaptive capacities of corals.
Since 1985, Palau has experienced Degree Heating Week (DHW) conditions associated with mass bleaching events, with significant bleaching occurring in 1998 and 2010. In 2017, Palau’s reefs did not bleach, despite high DHW levels and similar light intensity as previous bleaching events. In this study, researchers used Palau as a model to simulate 13 rates of thermal tolerance increase (ranging from 0 to 0.3°C per decade) to assess how rising heat tolerance might affect bleaching over time. By comparing their model predictions with historical bleaching data, they identified a 0.1°C per decade increase in thermal tolerance as the most likely scenario.
Using various emission scenarios, the researchers modeled bleaching trajectories for Palau’s reefs. Without an increase in thermal tolerance, these reefs are projected to experience high-frequency bleaching by 2040-2050. High-frequency bleaching is defined as mass bleaching events (DHW > 8°C-weeks) occurring two or more times per decade, which does not provide sufficient recovery time for coral ecosystems.
The authors determined that a 0.1°C per decade increase in coral thermal tolerance could mitigate high-frequency bleaching in low-emissions scenarios. In high-emissions scenarios, the increase in thermal tolerance could delay high-frequency bleaching by 10 to 20 years. However, by the end of the century, most reefs would still be at risk of frequent bleaching.
Implications for managers
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While increasing coral thermal tolerance shows some ability of coral reefs to adapt to climate change, it is still essential to reduce carbon emissions to protect coral reefs.
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Remote protected reefs with few local stressors are still vulnerable to mass bleaching and managers should include climate management strategies.
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Strategic local management actions, such as improving water quality, reducing ecological threats, and assisted evolution, could leverage the natural capacity for ecosystem adaptation to support further increases in thermal tolerance (e.g., maintaining rates of 0.1°C or increasing beyond this rate) and help improve reef futures.
Author: Lachs, L, S.D. Donner, P.J. Mumby, J.C. Bythell, A. Humanes, H.K. East and J.R. Guest
Year: 2023
Nature Communications 14: 4939 doi: 10.1038/s41467-023-40601-6
