When coral gets too hot, it expels its colorful algae and turns white. We could be seeing this occur on an unprecedented scale. Ethan Daniels/Shutterstock

Worldwide record-breaking temperatures are having an impact on coral reefs. Reports are now coming in of widespread bleaching on the northernmost section of the Great Barrier Reef (GBR), amongst the most pristine coral regions on the planet. A large survey has been launched to determine the scale of the problem, although researchers say we may not know the severity for some time.

When corals become stressed, they expel the photosynthetic algae that gives them both their color and the nutrients they need to live. This turns the corals white, referred to as bleaching. Bleaching can be triggered by local factors, such as pollution, but the most common cause is overheating. “The evidence is that most corals are living close to the edge of their thermal tolerance,” Dr. Tyrone Ridgway of the University of Queensland told IFLScience. “A protracted bout of warm temperatures can push them over.”

A protracted bout of warm temperatures is what the world has had, with the current El Nino combining with human-induced global warming. Last year, the National Oceanic and Atmospheric Administration (NOAA) declared the third ever global coral bleaching event in response to damage in the Caribbean and Hawaii. Since then, bleaching has reached Fiji and New Caledonia, Ridgeway told IFLScience, before arriving at the world's largest reef system.

The bleaching of corals is bad news for this guy. BMCL/Shutterstock

During the first global coral bleaching event, which coincided with the 1997-98 El Nino, bleaching affected half of the GBR. “It's important to remember that bleaching doesn't mean a coral dies,” Ridgway told IFLScience. “It's only if the heat stress persists that we see mortality.” Previous outbreaks have been associated with the death of about 5 percent of coral.

Although this year's temperatures have been far higher worldwide than during previous outbreaks, Ridgway said it was too early to tell if the bleaching would be in proportion. “At the moment, the majority of what has been reported has been north of Cooktown,” he said. “But that doesn't mean we won't see it elsewhere.” The northern reaches of the GBR are the least polluted, making them some of the healthiest reefs in the world. If they succumb, nowhere may be safe.

How the Great Barrier Reef should look. Brian Kinney/Shutterstock

The Australian government has announced funding for emergency surveys to measure the damage. Ridgway will be part of this, noting that the Global Change Institute (GCI), where he is based, has collected data on more than 200 kilometers (124 miles) of reef that will “provide a very good baseline” of coral cover, against which observations can be compared.

The GCI has been part of similar surveys of Hawaiian reefs, conducted before and during the current bleaching event. “We'll be going back to do a post-bleaching survey,” Ridgway said, adding, “We've already seen significant mortality there.”

While the intensity may not yet be known, Ridgway said the way damagingly hot water has spread around the globe was very similar to the pattern in the first global bleaching event. Based on this, “the Indian Ocean is likely to be next. The Maldives reefs are already experiencing an exceptionally warm period.”