Scientists have discovered a number of factors that led to the mass coral bleaching last year — an event that left Australia’s Great Barrier Reef devastated.

Even during summers that face excessive heat with an El Nino event, such as in 1998, there was no impact from coral bleaching in the Torres Strait and only slight bleaching was discovered in the northern Great Barrier Reef, according to Eric Wolanski, a professor at James Cook University in Australia.

“So, the extensive coral bleaching in these areas during the summer of 2016 was an unwelcome surprise,” Wolanski said.

Back in early April, scientists recorded severe coral bleaching surveys for the second time in 12 months.

“This is the fourth time the Great Barrier Reef has bleached severely – in 1998, 2002, 2016, and now in 2017,” James Kerry from James Cook University in Australia said. “It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016.”

An aerial survey of the northern Great Barrier Reef in 2016 revealed 90 percent of the reefs in some areas were harshly bleached. Wolanski claimed the satellite data demonstrated the heating of the 2016 El Nino took initially took place in the Gulf of Carpentaria. Certain areas in the water reached an exceptionally high 34 degrees Celsius, or 93 degrees Fahrenheit.

The water traveled east in the direction of the Torres Strait reefs and south to the Great Barrier Reef. The residence time of the heated water in these areas was exceedingly long, increasing the thermal stress on the coral. All of these factored led the solar heating to continue.

“Examining surface currents suggests that the North Queensland Coastal Current in the Coral Sea, which would normally flush and cool the Northern Great Barrier Reef, actually did the opposite,” Wolanski said. “It reversed course and brought very warm water to the Northern Great Barrier Reef.”

Altogether, these processes create the perfect thermal storm, he said.