Potential precursor to another die-off

The Great Barrier Reef in Australia has long been one of the world’s most magnificent natural wonders, so enormous it can be seen from space, so beautiful it can move visitors to tears. But the reef, and the profusion of sea creatures living near it, are in profound trouble. Huge sections of the Great Barrier Reef, stretching across hundreds of miles of its most pristine northern sector, were recently found to be dead, killed last year by overheated seawater. More southerly sections around the middle of the reef that barely escaped then are bleaching now, a potential precursor to another die-off that could rob some of the reef’s most visited areas of colour and life.

Telling sign of sea’s health
“We didn’t expect to see this level of destruction to the Great Barrier Reef for another 30 years,” said Terry P Hughes, director of a government-funded Center for Coral Reef Studies at James Cook University in Australia and the lead author of a paper on the reef published in the journal Nature. “In the north, I saw hundreds of reefs — literally two-thirds of the reefs were dying and are now dead.”

The damage to the Great Barrier Reef, one of the world’s largest living organisms, is part of a global calamity that has been unfolding intermittently for nearly two decades and seems to be intensifying. In the paper, dozens of scientists described the recent disaster as the third worldwide mass bleaching of coral reefs since 1998, but by far the most widespread and damaging.

The state of coral reefs is a telling sign of the health of the seas. Their distress and death are yet another marker of the ravages of global climate change. If most of the world’s coral reefs die, as scientists fear is increasingly likely, some of the richest and most colourful life in the ocean could be lost, along with huge sums from reef tourism. In poorer countries, lives are at stake: Hundreds of millions of people get their protein primarily from reef fish, and the loss of that food supply could become a humanitarian crisis.

With this latest global bleaching in its third year, reef scientists say they have no doubt about the responsible party. They warned decades ago that the coral reefs would be at risk if human society kept burning fossil fuels at a runaway pace, releasing greenhouse gases that warm the ocean. Emissions continued to rise, and now the background ocean temperature is high enough that any temporary spike poses a critical risk to reefs. “Climate change is not a future threat,” Terry said. “On the Great Barrier Reef, it’s been happening for 18 years.”

Corals require warm water to thrive, but they are exquisitely sensitive to extra heat. Just two or three degrees Fahrenheit of excess warming can sometimes kill the tiny creatures. Globally, the ocean has warmed by about 1.5 degrees Fahrenheit since the late 19th century, by a conservative calculation, and a bit more in the tropics, home to many reefs. An additional kick was supplied by an El Nino weather pattern that peaked in 2016 and temporarily warmed much of the surface of the planet, causing the hottest year in a historical record dating to 1880. It was obvious last year that the corals on many reefs were likely to die, but now formal scientific assessments are coming in.

Combating local threats
The paper in Nature documents vast coral bleaching in 2016 along a 500-mile section of the reef north of Cairns, a city on Australia’s eastern coast. Bleaching indicates that corals are under heat stress, but they do not always die and cooler water can help them recover. Subsequent surveys of the Great Barrier Reef, conducted late last year after the deadline for inclusion in the Nature paper, documented that extensive patches of reef had in fact died, and would not be likely to recover soon, if at all. Terry led those surveys. He said that he and his students cried when he showed them maps of the damage, which he had calculated in part by flying low in small planes and helicopters.

Terry said he hoped the die-off this time would not be as serious as last year’s, but “back-to-back bleaching is unheard-of in Australia.” The central and southern part of the reef had already been badly damaged by human activities like dredging and pollution. The Australian government has tried to combat these local threats with its Reef 2050 plan, restricting port development, dredging and agricultural runoff, among other risks.

But Terry’s research found that, given the high temperatures, these national efforts to improve water quality were not enough. “The reefs in muddy water were just as fried as those in pristine water,” Terry said. “That’s not good news in terms of what you can do locally to prevent bleaching — the answer to that is not very much at all. You have to address climate change directly.”

Australia’s conservative government also continues to support fossil fuel development, including what many scientists and conservationists see as the reef’s most immediate threat — a proposed coal mine, expected to be among the world’s largest, to be built inland from the reef by the Adani Group, a conglomerate based in India. “The fact is, Australia is the largest coal exporter in the world, and the last thing we should be doing to our greatest national asset is making the situation worse,” said Imogen Zethoven, campaign director for the Australian Marine Conservation Society.

Australia relies on the Great Barrier Reef for about 70,000 jobs and billions of dollars annually in tourism revenue, and it is not yet clear how that economy will be affected by the reef’s deterioration. Even in hard-hit areas, large patches of the Great Barrier Reef survived, and guides will most likely take tourists there, avoiding the dead zones.

Not necessarily extinction
The global reef crisis does not necessarily mean extinction for coral species. The corals may save themselves, as many other creatures are attempting to do, by moving toward the poles as the Earth warms, establishing new reefs in cooler water. But the changes humans are causing are so rapid, by geological standards, that it is not entirely clear if coral species will be able to keep up. And even if the corals do survive, that does not mean individual reefs will continue to thrive where they do now.

Within a decade, certain kinds of branching and plate coral could be extinct, reef scientists say, along with a variety of small fish that rely on them for protection from predators. “I don’t think the Great Barrier Reef will ever again be as great as it used to be — at least not in our lifetimes,” said C Mark Eakin, a reef expert with the National Oceanic and Atmospheric Administration, in Silver Spring, Maryland.