Recent Ocean Heat Waves Have "Forever" Altered Great Barrier Reef

By Chelsea Harvey, E&E News

21 April 18

 

At least 30 percent of corals died off in 2016, and more extreme heat is likely in store

record-breaking marine heat wave, which scorched the waters off the Australian coast in 2016, has changed the Great Barrier Reef “forever,” scientists say. Not only has the reef suffered extensive die-offs, but the types of corals that remain are different and less diverse than they were before.

The findings published yesterday in Nature by more than a dozen researchers are the product of months of aerial and underwater surveys, starting in the spring of 2016. They suggest that about 30 percent of coral cover across the entire Great Barrier Reef was lost between March and November 2016. And in the northern sector of the reef, where heat stress was highest, more than half the coral cover died within eight months.

It’s an “unprecedented” event in the reef’s recorded history, the scientists note—and it’s possible some parts of the reef may never be the same again.

“The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs,” co-author Andrew Baird of Australia’s James Cook University said in a statement. As a result, only a few “tough” species are remaining in some of the most heavily affected parts of the reef.

The changes are the result of a massive coral bleaching event, brought on by higher temperatures. Healthy corals have a symbiotic relationship with tiny algae, which live inside them and give them their bright colors. But when corals experience heat stress, they expel their algae, turning a bleached white color in the process.

Bleaching isn’t necessarily a death sentence for corals. Given enough time to recover, they can regrow their algae. But prolonged warm periods may kill corals before they have time to return to normal.

Back-to-back bleaching events can also cause additional damage to already-weakened corals that haven’t had sufficient time to recover. In the Great Barrier Reef’s case, a second marine heat wave in 2017 is believed to have caused additional damage on top of the mortalities reported in 2016.

The new research reaffirms a growing conclusion among coral experts that some coral species are naturally better at weathering heat stress than others. On the Great Barrier Reef, the surveys suggest that many areas have seen a decline in fast-growing species, such as staghorn and tabular corals, with slower-growing, simpler species left behind.

Those findings present both good and bad news for the reef. On the one hand, the survival of slower-growing, hardier coral species suggests that there’s still a future for the reef, although it may look different from before.

On the other hand, many of the surviving corals are much simpler in terms of their physical structure than their faster-growing counterparts, which were notable for their complex, branching 3-D designs. The simpler corals may provide less variety in habitat for fish and other organisms that populate the reef.

Prospects for a full recovery—back to the way the reef used to be, that is—are slim, the researchers say. Even if some of the faster-growing species started to make a comeback, it would still take at least a decade for them to return to their previous levels. It’s likely more heat waves will occur in the meantime, causing more damage before the reef has fully rebuilt.

In fact, a study published last week in Nature Communications finds that marine heat waves have increased worldwide by 54 percent since the 1920s (Climatewire, April 11). And much of that increasing trend has occurred in the last few decades.

And other recent research, published by some of the same authors who conducted the Great Barrier Reef study, suggests that coral bleaching events are also happening more frequently. Around the world, severe bleaching events have increased nearly fivefold since the 1980s (Climatewire, Jan. 5). These events now occur about once every six years on average, scientists suggest—which is worrying, considering that most reefs require a decade or more to fully recover from a severe bleaching episode.

Both the increase in heat waves and bleaching events are strongly linked to climate change, the studies suggest. And the Great Barrier Reef is hardly the only reef around the world that’s suffering the consequences. The 2016 heat wave around Australia was just one part of a massive global bleaching event, which affected about 70 percent of the world’s reefs between 2014 and 2017.

But the iconic Great Barrier Reef may present one of the biggest warnings about the future of corals worldwide. Continued efforts to meet international climate targets and keep global warming below a 2-degree temperature threshold are more critical than ever, the researchers note.

In the meantime, the survey results underscore the importance of determining which coral species fare best under stress, and why. They may be the future of coral reefs in a warming world.

On the Great Barrier Reef, the corals that have survived so far “are tougher than the ones that died,” lead study author Terry Hughes of James Cook University said in a statement. “We need to focus urgently on protecting the glass that’s still half full, by helping these survivors to recover.”