30% of coral died in 'catastrophic' 2016 heatwave

30% of coral died in 'catastrophic' 2016 heatwave

"This is different from coral bleaching in the Great Barrier Reef or Okinawa, which is caused by unusual warming of water temperatures", Takahashi said.

"The longer-term transition that we have been predicting now for many years as a effect of global warming is well under way", added Hughes, the director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, Australia.

"We've been saying for a long time that these bleaching events are going to get more frequent and the effect of that will be that the mix of species will change", Professor Hughes said. "It also shows climate change threatens the diversity that is the hallmark of coral reefs".

Global heat and coral bleaching began to increase in 2014 and continued through 2017; this event meant that marine heat waves causing bleaching struck three-quarters of the world's coral reefs and that the heat waves that cause corals to die struck nearly a third, the researchers said.

Such events, though, tend to carve a swathe through the reef, perhaps 50 to 100 kilometres wide, with patchier damage than mass bleaching.

Upcoming papers will examine the impacts of the latter event - which mainly hammered the middle section of the Reef - and the scope for recovery.

In 2016 and 2017, the Great Barrier Reef was pummelled by two successive summers in which ocean temperatures far exceeded normal for key portions of the reef - and stayed that way for a considerable time. Over the entire Great Barrier Reef, coral cover declined by 30 percent during that time.

As a result of the damage, the world's largest coral reef is unlikely to recover in the distant future. "One of the big things we saw likely to happen elsewhere is this homogenization of coral ecosystems". "If their diet disappears, so do they", Hughes said.

A full 30 percent of the coral died in the first year of the heat wave, 2016, and the rest succumbed the following year in another marine heat wave, Earther reported. They then used satellite data to calculate what are called "degree heating weeks" - a measurement of the intensity and duration of corals' exposure to heat stress that was developed by Coral Reef Watch.

Although reef scientists have been predicting the increased frequency and severity of bleaching events for two decades, this paper has some surprising and alarming results. Generally, the higher the DHW, the higher the expected coral death.

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But in March 2016, as ocean temperatures in the northern section of the reef hit record highs, half of the corals killed during the bleaching event died nearly immediately.

"When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die".

Furthermore, the deaths did not always occur in the way that was expected - in which zooxanthellae abandon the corals and don't return fast enough, leading to starvation.

The Symbiodinium algae there have adapted to tolerate the warmest ocean waters in the world, and it appears this may also make them more resilient to the stresses of changing temperatures.

The Great Barrier Reef experienced an extended marine heat wave in 2016 that caused massive coral bleaching and die-off.

"The heat exposure was so extreme they actually were cooked and died very quickly", said Hughes.

Prof Hughes warned work needs to start urgently to protect the corals which are left.

"If we continue on the trajectory we're going, [the reef] will not be the same ecosystem it has been".

The thresholds "are lower than we thought they would be", Hughes said. "Coral reefs provide tens of billions of dollars to economies and protect shorelines and infrastructures around the world".

"I think if we can achieve that or close to it, we'll still have a reef but it won't look the same as the reef looked two years ago", he said.

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