Global coral reef thermal stress: patterns, trends, and emerging regimes

Abstract We present a global assessment of thermal stress indicators at the scale of individual coral reefs (∼ 25 km) spanning the time interval 1982–2024. We use 8 different metrics based on 4 different indicators of thermal stress: frequency, duration, degree heating days (DHD), and heating rate (HR) of marine heat waves (MHWs). While all indicators showed increasing trends at the global scale, we unveil clearly distinct regional patterns. A cluster analysis suggests a categorization of the reefs into four distinct groups: frequent mild-stress, episodic mild-stress, persistent high-stress, and rapidly intensifying stress. Each group is characterized by a different combination of thermal stressors and has a distinctive geographical coherence. We also quantified the duration, DHD, and HR of the longest MHW that occurred at each reef location, unveiling that on more than 90% of the sites, the longest MHW occurred after 2015.