Lengthening of warm periods increased the intensity of warm-season marine heatwaves over the past 4 decades

Marine heatwaves (MHWs), periods of anomalously warm sea surface temperature (SST) which can have significant impacts on marine ecosystems, have increased in frequency and severity over recent decades. Many coastal systems (e.g., coral reefs) are particularly vulnerable to warm-season heat stress when temperature can exceed organisms' thermal thresholds and lead to mass mortality. While many studies have examined the change of the warm-season heat stress occurrence over time, e.g., for coral reefs, there has been less analysis of the thermal properties of heat stress events. Here we examine the trend in the characteristics of warm-season heat stress (referred to as warm-season MHWs) at the global-scale from 1985 to 2019, using multiple metrics for each of duration, peak intensity, accumulated heat stress and heating rates. The results show that warm-season MHWs have become more frequent, longer-lasting, featured higher peak intensity and accumulated heat stress across most of the ocean over the past 4 decades. Furthermore, decomposition of the trends in warm-season MHW properties shows that the increased accumulated heat stress was predominantly driven by the increased duration rather than the increased intensity. The results contribute to improving the understanding of warm-season MHWs, which may help inform the prediction of their impacts on marine ecosystems as well as marine conservation and management under climate change.

Automated summary

Marine heatwaves (MHWs) have increased in frequency and severity globally over recent decades. Warm-season MHWs have become more frequent, longer-lasting, featured higher peak intensity and accumulated heat stress. The increased accumulated heat stress is predominantly driven by the increased duration.