Half of the Great Barrier Reef has been bleached to death since 2016. Mass coral bleaching, a global problem triggered by climate change, occurs when unnaturally hot ocean water destroys a reef’s colorful algae, leaving the coral to starve. The Great Barrier Reef illustrates how extensive the damage can be: Thirty percent of the coral perished in 2016, another 20 percent in 2017. The effect is akin to a forest after a devastating fire. Much of the marine ecosystem along the reef’s north coast has become barren and skeletal with little hope of recovery.

New Guinea

Great Barrier Reef Marine Park

Australia

Brisbane

Sydney

Canberra

Gulf of Papua

New Guinea

Reef bleaching severity Proportion of individual reef in 1998, 2002, or 2016 event*

Extreme (more than 60%)

Cooktown

Moderate (30–60%)

Low (10–29%)

Cairns

Townsville

Australia

Mackay

150 mi

150 km

Worse than expected Bleaching in 2016 occurred so rapidly that scientists had to retool their predictions for how much heat the reef could endure.

HOW HOT FOR HOW LONG As climate change warms Earth’s oceans, underwater heat waves last longer. Coral species can’t withstand extended hot periods. They start to die off, which diminishes reef diversity. After heat stress becomes severe, as it did along the northern Great Barrier Reef in 2016, few species remain, and final die-off is rapid.

HEAT STRESS

Great Barrier Reef Far Northern Management Area

16 12 8 4

Heat stress in 2016 killed 80% of coral in this section of the reef.

2014

2015

2016

2017

Degree heating week (DHW) combines intensity and duration of heat stress into a single number.

NO TIME FOR RECOVERY Severe regional bleaching used to hit a given reef about every 27 years. Since the 1980s, the pace has accelerated­ to every six. Even in the best conditions, badly damaged reefs take at least 10 years to rebound. The Great Barrier Reef, struck two years in a row, may never fully recover.

A healthy partnership Algae feed the coral; coral provides shelter and nutrients to the algae.

Enlarged below

Relationship breakdown Coral stressed from overly hot water expels the algae, causing the coral to starve. The skeleton of dying coral decays once exposed.

Stressed

Healthy

Bleached

Zooxanthellae algae

Decay