The Great Barrier Reef is seeing unprecedented coral bleaching

The southern-hemisphere summer that ended in February is likely to have been the deadliest in the reef’s history. What made it so was the unprecedented thermal stress—a measure of both high temperatures and their duration—it endured. In the reef’s southern third, where the stress was worst, sea-surface temperatures peaked at 2.5°C above average. It will be months before the consequences are clear, but some corals are already succumbing. Up to half may die, estimates Neal Cantin of the Australian Institute of Marine Science (AIMS).