The intense El Niño event that started in 2015 drove global air temperatures to new records, helped by the long trend of human-driven warming. But the air wasn't the only thing affected. El Niño is fundamentally about Pacific Ocean temperatures, and those were exceptionally hot as well. One of the unfortunate results of this was a massive bleaching of the corals of the Great Barrier Reef.

While the damage to corals looked dramatic at the time, appearances aren't the same as data, and they don't give a comprehensive view of the damage, much less the corals' ability to recover from the bleaching. Now, a large Australian-US team of researchers has provided a comprehensive overview of the damage to and recovery of the Great Barrier Reef. The results are grim, showing that mass coral die offs started at lower temperatures than we had expected. The overview also shows that the entire composition of sections of the Great Barrier Reef have changed and are unlikely to recover any time soon.

Bleached to death

The corals that build reefs are actually a collaboration between animals (the coral proper) and single-celled algae that form a symbiotic relationship with corals, providing them with nourishment. At high temperatures, this relationship breaks down and causes the corals to lose their photosynthetic guest. The reefs turn white, giving bleaching its name. And, if recovery doesn't happen quickly enough, the corals will starve, causing a mass die off. Complicating matters, different species of coral will bleach at different temperatures and recover at different rates.

As a result, there's no single outcome of a bleaching event. It depends on what was growing on the reef to start with, as well as the dynamics of the recovery.

But by all appearances, the 2016 bleaching event on the Great Barrier Reef was a severe one. Temperatures were extraordinarily high, especially on the northern portions of the reef, and the resulting bleaching was extensive.

The team behind the new work did a thorough job of investigating the event and its aftermath. Data on ocean temperatures at a 5km resolution were obtained from NOAA, and the full extent of the bleaching was tracked via aerial surveys. Researchers also visited 63 individual reefs to identify the coral species present and then returned to the site eight months later to determine the degree to which these species had survived and were recovering from the bleaching event.

There's a lot of bad news here, and it's difficult to know where to start. We'll begin with how much of the mortality from bleaching has been viewed as arising from the longer-term process of starvation. This gives the corals at least the prospect of recovery. But here, corals were already dying during the initial survey.

Corals also died at lower temperatures than expected. Their risk is tracked through a value called "degree heating weeks," which registers the highest water temperature they were exposed to for at least a week (registered as a number of degrees above the average temperature of the area). It had been thought that fatality could be avoided until 6°C degree heating weeks. Instead, lethal effects appeared in a degree heating week of as little as 3°C.

On top of that, the corals continued to be lost over the ensuing eight months. "Many of the surviving coral colonies continue to die slowly even after recovery of their algal symbionts," the researchers write, "because they have lost extensive patches of tissue, are injured and fragmented, and because corals weakened by bleaching are susceptible to subsequent outbreaks of disease." Overall, reefs in the northern section of the Great Barrier Reef lost half their corals by eight months. Across the entire Great Barrier Reef, nearly a third of the corals died from this bleaching event.

Species specificities

Overall, individual reefs within the Great Barrier Reef experienced a huge range of temperatures, ranging from no significant change up to 10°C degree heating weeks. And the authors conclude that the effects were non-linear. At lower temperatures (degree heating weeks of less than 4°C), even though bleaching could affect up to a quarter of the corals, and some died, there was little to no loss of coral cover at eight months.

But things changed rapidly beyond that. At a 4°C degree heating week, there was a 40 percent decline. And, by the time the warmth of a degree heating week went above 8°C, more than 80 percent of the coral was dead at eight months.

At the intermediate temperatures, the loss of corals wasn't evenly distributed. The ones that suffered the most were fast-growing species, like staghorns and tabular corals. At 6°C degree heating weeks, these ended up mostly dead, shifting the entire ecology of the reef to one focused on slower-growing species. These tend to be physically simpler and so don't provide as much cover or complexity to the other species in the environment. Strikingly, these temperatures affected nearly 30 percent of the individual reefs that make up the Great Barrier Reef, indicating that the 2016 bleaching was a transformative event.

In addition to physically changing the reefs, this transformation makes them less resilient. Slower-growing corals will, not surprisingly, repopulate a reef more slowly. As temperatures continue to rise, the chances increase that recovery will still be in progress when another bleaching event occurs.

Unfortunately, this has already happened. "The Great Barrier Reef experienced severe bleaching again in early 2017, causing additional extensive damage" the researchers note. "The most likely scenario, therefore, is that coral reefs throughout the tropics will continue to degrade over the current century until climate change stabilizes, allowing remnant populations to reorganize into novel, heat-tolerant reef assemblages." Of course, at the moment, there's no indication of when our current climate change might stabilize, so this reorganization could be more than a century away.

Nature, 2018. DOI: 10.1038/s41586-018-0041-2 (About DOIs).