The map above, based on data from NOAA’s Coral Reef Watch, shows areas with varying levels of heat stress. The amount of heat stress is an indicator of bleaching and its potential intensity. Photo: NASA

The bleaching of the Great Barrier Reef reached “unprecedented” levels last year. Such events have happened in the past, and large swaths of the coral in the unique ecosystem have managed to survive stressful events such as in 1998 and 2002.

However, this year has brought yet more bleaching for the second year in a row – and the Australian researchers who are currently trying to gauge the extent of the damage say climate change is the culprit.

The latest survey, compiled from aerial and underwater scans, is published by the National Coral Bleaching Taskforce in the journal Nature today.

“The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year,” they write. “Immediate global action to curb future warming is essential to secure a future for coral reefs.”

The coral researchers are from James Cook University, the Australian Institute of Marine Science, CSIRO, the Great Barrier Reef Marine Park Authority, the University of Queensland, the University of Sydney, the University of Western Australia, and the ARC Centre of Excellence for Coral Reef Studies.

But even as the latest look at coral bleaching is published, the scientists are grappling with a sequel: the damage done over the last few scorching months.

Coming off the hottest months Down Under (which are December through February in the Southern Hemisphere), the scientists are hoping to find that the bleaching is not as severe as last year.

“We’re hoping that the next two to three weeks will cool off quickly, and this year’s bleaching won’t be anything like last year,” said Terry Hughes, of the ARC Centre, the lead author, and head of the taskforce. “The severity of the 2016 bleaching was off the chart.”

“With rising temperatures due to global warming, it’s only a matter of time before we see more of these events. A fourth event after only one year is a major blow to the Reef.”

The Reef, off the northeast coast of Australia, is protected from local water pollution and overfishing. But ocean temperatures cannot be modulated at a local, or even national, level.

Coral naturally has bright colors from microscopic algae that live inside the coral. But when the water temperature gets too hot, the algae is driven out and the coral turns white. At that point, it becomes vulnerable to disease, and mortally susceptible to further stress.

The National Oceanic and Atmospheric Administration was also involved in the work. The latest data from the agency’s Coral Reef Watch shows a week of stressors on the Reef and other parts of the South Pacific during the last week in February. The resulting map indicates that mortality is likely up and down the Great Barrier Reef this year.

“Unlike past global bleaching events that lasted less than 12 months, this even it in its 33rd month and shows no signs of stopping,” said Mark Eakin, the Coral Reef Watch coordinator. “It has been the longest, most widespread, and most damaging coral bleaching event ever recorded.”