How often does heat stress return on coral reefs? Return intervals of bleaching-level events and the narrowing recovery window in 214 reef regions, 1986–2025
Mercan resiflerinde ısı stresi ne sıklıkla geri dönüyor? 214 resif bölgesinde ağartma düzeyindeki olayların dönüş aralıkları ve daralan toparlanma penceresi, 1986–2025
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Abstract
Whether coral reefs can recover after mass bleaching events depends on the time between two events. This study treats the recurrence of heat stress as an event-history problem. Using the daily degree heating week (DHW) series of the 214 regional virtual stations of the US National Oceanic and Atmospheric Administration's Coral Reef Watch programme, 8,560 station-years between 1986 and 2025 are examined. The annual prevalence of bleaching-level (DHW ≥ 4 °C-weeks) and severe (DHW ≥ 8 °C-weeks) events, the time between events and the stress-free recovery windows are estimated with the Kaplan–Meier method; the trend in event probability is estimated with logistic models. The annual share of stations experiencing bleaching-level heat stress rose from 15% in 1986–1995 to 60% in 2016–2025; the share with severe stress rose from 3.6% to 37%. The median time from a severe event to the next fell from 7 years for intervals starting in 1986–1999 to 2 years for those starting in 2010–2019. In 2016–2024 the probability that a station with severe stress experienced severe stress again in the following year was 49%. Between 2023 and 2025, 91% of stations experienced severe stress at least once and 66% reached their record value in these three years. A sensitivity analysis with DHW reconstructed from daily hotspot series shows that thermal tolerance would have to rise by 0.16–0.20 °C per decade to keep event frequency at its 1986–1995 level; this is about twice the tolerance gain reported in field studies. The findings indicate that, in terms of the frequency of heat stress, the recovery window is closing in most reef regions.
Keywords
Coral bleaching; Degree heating week; Return interval; Survival analysis; Recovery window; Thermal tolerance; Marine heatwaves
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