Climate change, coral bleaching and the future of the world's coral reefs

TLDR

Sea temperatures in tropical regions have risen by nearly 1 °C over the past century, accelerating at 1–2 °C per century, and this warming has triggered mass coral bleaching that exceeds corals’ thermal tolerance, leading to widespread loss of zooxanthellae and significant reductions in live coral cover worldwide. The study examines coral bleaching from biochemical, physiological, and ecological perspectives. The authors use outputs from four runs of three global climate models to project future sea‑temperature changes and predict how bleaching frequency and intensity will evolve over the next century. Projected warming will exceed reef‑building corals’ thermal tolerances annually within the next few decades, making 1998‑level bleaching events common within 20 years, while corals’ acclimation capacity is already surpassed and adaptation too slow, implying that unchecked warming will lead to widespread loss and degradation of coral reefs worldwide.

Abstract

Sea temperatures in many tropical regions have increased by almost 1°C over the past 100 years, and are currently increasing at ~1–2°C per century. Coral bleaching occurs when the thermal tolerance of corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Mass coral bleaching has occurred in association with episodes of elevated sea temperatures over the past 20 years and involves the loss of the zooxanthellae following chronic photoinhibition. Mass bleaching has resulted in significant losses of live coral in many parts of the world. This paper considers the biochemical, physiological and ecological perspectives of coral bleaching. It also uses the outputs of four runs from three models of global climate change which simulate changes in sea temperature and hence how the frequency and intensity of bleaching events will change over the next 100 years. The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years. Most information suggests that the capacity for acclimation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world’s reefs. The rapidity of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.