In the hottest days of Australia's summer, just after a full moon, the Great Barrier Reef experiences an underwater snowstorm.

Along the massive stretch of reefs corals release millions of sticky eggs and sperm that float up to the water's surface and combine. They will eventually form larvae. Some will float down and settle on the nearby ocean floor, while others might be swept along by a current.

New research published Wednesday in Nature finds that warmer waters are making it harder for corals to reproduce en masse. In fact, following a major coral bleaching event in 2017 the amount of reproductive material collected in the water after a mass spawning event was down by 89 percent in the Great Barrier Reef in 2018. It will take five to 10 years for coral to fully recover, the researchers estimate.

After mass bleaching events, coral reefs struggle to recover, the research shows. That decline in new young is also hitting some corals harder than others, meaning the composition of reefs could be drastically altered by climate change.

Gathering data

Corals that reproduce by releasing a mass of reproductive material into the water are called spawners. Most species of coral in the Great Barrier Reef reproduce in this way, but some, a group labeled brooders, reproduce by releasing larvae that settle nearby.

To assess which species of adult coral were the most impacted, researchers conducted underwater surveys by draping tape over reef beds and measuring the topography. To sample larval recruitment, they set out panels across the Great Barrier Reef in the days following a mass spawning event.

“We put out a thousand settlement panels,” says Terry Hughes, a coral reef scientist from James Cook University and the lead researcher. “Between the north and southern tip, that’s a distance of 1,800 miles.”

In previous surveys, they gathered panels that each contained 50 to 100 gametes. This year, he says, “the most common numbers were between zero and one.”

Less time to recover

Warm temperatures and pollution trigger coral to expel the algae that lives in their tissues. This algae provides food to each coral polyp. When water doesn't cool or pollution doesn't dissipate, the algae doesn't return to the coral, leaving them to starve to death. Weather patterns like El Niño can make already warm waters unbearable for coral.

Coral bleaching was first recorded in the early 1980s, but the Great Barrier Reef has experienced four mass bleaching events that have devastated huge swaths of the reef. The first was in 1998 and the second in 2002. From 2002 to 2016, corals were able to recover what had been lost to bleaching.

“We were lucky to have a 14-year gap between the second and third,” says Hughes. “And unlucky to have no gap after 2016.”

Half of the Great Barrier Reef Is Dead 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 *MOST SEVERE SCORE (SOME REEFS SURVEYED IN MORE THAN ONE BLEACHING). LAUREN E. JAMES AND CLARE TRAINOR, NGM STAFF ART: MATTHEW TWOMBLY. SOURCES: ARC CENTRE OF EXCELLENCE FOR CORAL REEF STUDIES; NOAA CORAL REEF WATCH; ROBIN BEAMAN, JAMES COOK UNIVERSI- TY; AUSTRALIAN HYDROGRAPHIC OFFICE; GEOSCIENCE AUSTRALIA; AUSTRALIAN INSTITUTE OF MARINE SCIENCE; RAY BERKELMANS AND OTHERS, CORAL REEFS 23, 2004; © OPENSTREETMAP CONTRIBUTORS, AVAILABLE UNDER OPEN DATABASE LICENSE: OPENSTREETMAP.ORG/COPYRIGHT Half of the Great Barrier Reef Is Dead 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 Direction of view HEAT STRESS 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. Great Barrier Reef Far Northern Management Area Degree heating week (DHW) combines intensity and duration of heat stress into a single number. 16 12 8 4 Great Barrier Reef Marine Park Heat stress in 2016 killed 80% of coral in this section of the reef. Australia Brisbane Sydney 2014 2015 2016 2017 Canberra Mackay Reef bleaching severity Proportion of individual reef in 1998, 2002, or 2016 event* Extreme (more than 60%) Moderate (30–60%) Low (10–29%) Townsville New Guinea Cairns Cooktown Australia Gulf of Papua Far Northern Management Area Worse than expected Bleaching in 2016 occurred so rapidly that scientists had to retool their predictions for how much heat the reef could endure. Lockhart River Cape York Peninsula N Saibai Island New Guinea Stressed Healthy Bleached Enlarged at right 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. Zooxanthellae algae Decay A healthy partnership Algae feed the coral; coral provides shelter and nutrients to the algae. Relationship breakdown Coral stressed from overly hot water expels the algae, causing the coral to starve. The skeleton of dying coral decays once exposed. SCALE VARIES IN THIS PERSPECTIVE. DISTANCE FROM SAIBAI ISLAND TO CAPE YORK IS 90 MILES. *MOST SEVERE SCORE (SOME REEFS SURVEYED IN MORE THAN ONE BLEACHING). LAUREN E. JAMES AND CLARE TRAINOR, NGM STAFF. TERRAIN RENDERING: CHARLES PREPPERNAU. ART: MATTHEW TWOMBLY. SOURCES: ARC CENTRE OF EXCELLENCE FOR CORAL REEF STUDIES; NOAA CORAL REEF WATCH; ROBIN BEAMAN, JAMES COOK UNIVERSITY; AUSTRALIAN HYDROGRAPHIC OFFICE; GEOSCIENCE AUSTRALIA; AUSTRALIAN INSTITUTE OF MARINE SCIENCE; RAY BERKELMANS AND OTHERS, CORAL REEFS 23, 2004; © OPENSTREETMAP CONTRIBUTORS, AVAILABLE UNDER OPEN DATABASE LICENSE: OPENSTREETMAP.ORG/COPYRIGHT

Back-to-back bleaching events in 2016 and 2017 devastated the Great Barrier Reef, and research published by Hughes last year in the journal Science suggests the gap between extreme warming is getting shorter.

“It's like getting hit by a serious disease every couple of years, or at such short intervals that you don't have time to recover in between,” study author and marine biologist Julia Baum told National Geographic at the time.

A rocky future

The estimated 10 years corals need to recover only holds true if no other mass bleaching events occur in that time. Hughes says it's unlikely for that to occur in a warming world.

“I’m reasonably positive that we’ll still have reefs [in the future],” says Hughes. “But already we’re seeing the shifts that are occurring with one degree of warming. If we go to two or more, then the reefs will become more and more degraded and more unrecognizable.”

Certain species of spawning corals, like Acropora corals, were more impacted. Recruitment for that species declined by 93 percent. Acroporas are shaped like tables; they're responsible for much of the reef's three dimensionality, and researchers say they support thousands of other species.