Increasing thermal stress for tropical coral reefs: 1871-2017

Area of Science:

• Marine Biology
• Climate Science
• Ecology

Background:

• Tropical corals are sensitive to thermal stress, living near their upper temperature limits.
• Coral bleaching, a breakdown of symbiosis, is exacerbated by rising sea temperatures and global warming.
• Mass coral bleaching events are increasingly linked to El Niño events and anthropogenic climate change.

Purpose of the Study:

• To assess historical thermal stress levels at 100 coral reef locations.
• To compare recent thermal stress with historical El Niño events and pre-industrial levels.
• To evaluate the adequacy of the 1.5°C global warming target for preventing future coral bleaching.

Main Methods:

• Analysis of historical sea surface temperature data to calculate the degree heating month index (DHMI).
• Comparison of DHMI values during the 2015-2016 El Niño with historical data from 1871-2017.
• Examination of thermal stress levels relative to pre-industrial and recent El Niño events.

Main Results:

• The 2015-2016 El Niño caused unprecedented thermal stress, exceeding that of the 1997-1998 El Niño and the 'pre-industrial' 1877-1878 El Niño.
• The DHMI during the 2015-2016 event was five times higher than the pre-industrial El Niño.
• Current levels of thermal stress demonstrate coral reef vulnerability to even modest global warming (~0.92°C).

Conclusions:

• Coral reefs are already highly vulnerable to current global warming levels.
• The 1.5°C Paris Agreement target is insufficient to prevent frequent mass bleaching events.
• Future coral reefs will likely differ significantly from those of the past due to persistent thermal stress.