Emergent increase in coral thermal tolerance reduces mass bleaching under climate change

Coral reefs are severely threatened by climate change, experiencing unprecedented declines due to marine heatwaves causing mass coral bleaching and mortality. The survival of these ecosystems hinges on increased thermal tolerance in coral assemblages – the temperature threshold beyond which thermal stress occurs. While some reefs have shown changes in bleaching susceptibility, the rate of natural thermal tolerance increase remains unknown, hindering accurate predictions of future bleaching trajectories. This study investigates the rate of this increase and its impact on future bleaching projections, focusing on a remote and well-studied coral reef system in Palau.

Existing literature demonstrates that coral bleaching resistance can increase after recurrent marine heatwaves, potentially through community composition turnover (shift toward stress-tolerant species), genetic adaptation (increased frequency of heat-resistant genes), and acclimatisation (increased heat resistance within an organism's lifetime). These mechanisms can operate at various scales (individual, species, assemblage, ecosystem) and often interact, making causal assignment challenging in field observations. Current bleaching projections often neglect or coarsely represent these adaptive mechanisms, typically assuming fixed increases in thermal tolerance. This simplification may be biologically unrealistic, as these adaptive processes are likely non-linear.

This study used a simulation modelling approach to quantify the natural increase in thermal tolerance of coral communities in Palau. Daily 5 km satellite sea surface temperature (SST) data were used, along with a 36-year dataset (1985-2020) from the NOAA Coral Reef Watch. Thirteen simulations were created, each with a different linear rate of thermal tolerance increase (0.0–0.3 °C/decade). Degree Heating Weeks (DHW), a metric representing accumulated heat stress, was calculated for each simulation. The most likely historic rate was identified by comparing the parsimony of DHW-derived bleaching predictions against historical bleaching observations using a Bayesian approach which accounted for spatial autocorrelation in bleaching data. High solar insolation's impact was also assessed. Future projections of mass coral bleaching under four contrasting global emissions scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) were generated using statistically downscaled data from 17 global circulation models (GCMs). The influence of different thermal tolerance enhancement rates on future bleaching trajectories was then evaluated.

Palau experienced mass coral bleaching in 1998 and 2010, but not in 2017, despite comparable heat stress (DHW) and light intensity. Simulations showed that a 0.1 °C/decade increase in thermal tolerance best explained the observed bleaching patterns. This rate was associated with the highest model parsimony, prediction success rate (>65%), and lowest misclassification rate. More rapid thermal tolerance increases led to over- or under-prediction of bleaching events, especially underestimating DHW in later years due to unrealistically high thermal tolerance thresholds. Future projections under the Paris Agreement scenario (SSP1-2.6) showed a substantial reduction in high-frequency bleaching (defined as ≥2 bleaching events >8 °C-weeks per decade) if the 0.1 °C/decade increase continues. High-frequency bleaching is projected to occur by 2040 under all emissions scenarios without thermal tolerance enhancement. With the 0.1 °C/decade increase, high-frequency bleaching is delayed by 10-20 years under higher emission scenarios (SSP2, SSP3, and SSP5) but ultimately occurs by 2100 under those higher emissions scenarios. More rapid thermal tolerance increases (0.2 and 0.3 °C/decade) would mitigate high-frequency bleaching in the Paris Agreement and middle-of-the-road scenarios, while only the 0.3 °C/decade rate could prevent it entirely under the middle-high emissions scenario. Even the most rapid increase could only delay, not prevent, high-frequency bleaching under the worst-case scenario (SSP5).

The study's findings highlight an emergent increase in coral thermal tolerance in Palau, potentially driven by species composition turnover, genetic adaptation, acclimatisation, or symbiont community changes. The lack of profound shifts in coral community composition suggests mechanisms beyond species turnover may have played a more significant role. While the 0.1°C/decade increase offers hope, it is insufficient to prevent high-frequency bleaching under high emission scenarios, underscoring the necessity of ambitious emission reductions. The research advances our understanding of coral resilience, offering insights into potential management strategies like assisted evolution to enhance thermal tolerance.

This study demonstrates an emergent increase in coral thermal tolerance in Palau at a rate of 0.1 °C/decade, reducing the severity of mass bleaching events. While this natural resilience offers hope, it is not sufficient to mitigate high-frequency bleaching under high-emission scenarios. Ambitious emission reduction targets, alongside potential management actions like assisted evolution, are crucial for preserving coral reefs. Future research should focus on determining the specific biological mechanisms behind this thermal tolerance increase and investigate the potential for accelerating this rate through management interventions.

The simulation model cannot account for hard physiological limits to thermal tolerance, potential trade-offs with other traits, or changes in responses as corals approach their upper thermal limit. The model assumes a constant rate of thermal tolerance increase, neglecting potential non-linearities and the impact of punctuated stress events. Furthermore, the analysis focuses on assemblage-level thermal tolerance, not considering species-specific variations.