Early last year, the Great Barrier Reef in Australia underwent a once-in-a-lifetime calamity. Ocean temperatures around the reef rose far above normal. The extra heat turned corals bone-white and caused them to expel the symbiotic algae which feed them from inside their branches. Overcooked, starving, and vulnerable to disease, vast swaths of the world’s largest reef died.
The summer of 2016 remains one of the most severe coral bleaching and die-off events ever observed—a level of devastation that scientists didn’t expect to see until the 2050s. A new study argues that it will not remain a rare event for long. Even in simulations of the most hopeful global-warming scenarios, modern climate models suggest that ocean temperatures around the Great Barrier Reef will regularly surpass the devastating warmth of 2016.
In other words: A once-in-a-lifetime event will soon become the new normal.
The study, published Monday in Nature Climate Change, isn’t focused only on the Great Barrier Reef. It takes a broader look at how Australia will fare in two different climate-changed worlds: one where global temperatures rise by 2 degrees Celsius (3.6 degrees Fahrenheit); and another where humanity manages to keep the rise below 1.5 degrees Celsius (2.7 degrees Fahrenheit).
Things don’t look good—in either scenario. A climate-changed Australia will be a more sweltering, less biodiverse place. Toasty waters off its shores will evict the continent’s marine ecosystems, while, on land, summers that would once have broken records will become routine.
The study is important to researchers even beyond the land down under. Since the adoption of the Paris Agreement last year, the world’s nations have aspired to keep global temperatures from rising by more than 1.5 degrees Celsius.
It’s a nice idea—and a really tough goal to meet. The actual stated commitments of the Paris Agreement put the world on track for 3.4 degrees Celsius of warming. To have a good shot at avoiding 1.5 degrees of warming, the world would have to halt all carbon emissions by 2021, according to the U.K.-based think tank Carbon Brief. (Without the halt, we won’t actually reach the 1.5 degree threshold for a couple more decades, but by 2021 there will be enough carbon in the atmosphere to lock us in for that warming.)
In Australia, both 1.5 degrees and 2 degrees of warming will cause more heat waves and coral calamities. Take the risk of another 2016-level event in the Coral Sea, for instance. The paper finds that—in a two-degree world—there’s an 87-percent chance that 2016-levels of warmth will strike the Great Barrier Reef in any individual year.
“At two degrees Celsius of warming, last year’s event would actually be a bit cooler than average,” said Andrew King, a climate scientist at the University of Melbourne and one of the authors of the paper, in an email. “This poses a major problem for the survival of most of the Great Barrier Reef.”
Its prognosis barely improves on a slightly-less-warm planet. In a 1.5-degree world, there’s still about a two-thirds chance that any summer would bring 2016-level heat to the Great Barrier Reef. And even in the coolest, kindest of the scenarios—the “gentlest” version of a 1.5-degree world—the odds of 2016-level heat are just over 50 percent. The reef would bleach every other year.
“By the time we’re seeing bleaching temperatures there every year, there probably will not be a reef anymore. There’s only five or six times bleaching can happen before a reef is essentially dead,” said Ruth Gates, a coral biologist at the University of Hawaii and the president of the International Society for Reef Studies. She was not connected to the paper.
She said that in the few back-to-back bleaching events that scientists have observed in the wild—like the one in Kāne’ohe Bay, next to her lab, in 2014 and 2015—they find that the second year is even “more profound” in its consequences than the first. “It will be a magnification each time a coral bleaches,” she said. “You will lose a portion of the reef each time, and there comes a point where it’s no longer functionally a reef. ”
The authors also found ominous results when they modeled land temperatures. In 2013, Australia experienced a broiling, weeks-long series of heat waves dubbed the “Angry Summer.” Power systems buckled, wildfires ignited across the country, and the Bureau of Meteorology had to put a new color on weather maps to describe temperatures above 125 degrees Fahrenheit (52 degrees Celsius).
The paper argues that angry summers, too, will become the new normal. Even in a 1.5-degree world, they’ll have one every two years. In a two degree world, they’ll happen every three out of four.
If there is some good news in the paper, it’s that the models suggest that droughts will not become more common. Even in the most severe scenarios, the risk of a very-low-rainfall summer—similar to what the continent saw in 2006—gets only a tiny bit likelier. But because summers will still get hotter, droughts may get more severe, King told me. Evaporation, super-powered by the heat, will whisk away water that, in today’s climate, stays in rivers and lakes.
“The benefits of limiting global warming to 1.5 degrees Celsius, instead of 2 degrees Celsius, can be clearly understood for temperature extremes but are not always clear for regional precipitation extremes,” said Ambarish Karmalkar, a climate scientist at the University of Massachusetts Amherst, who was not part of the study, in an email.
This wasn’t surprising, he added, as it shows up in many other studies too. Earlier this year, Karmalkar looked at how North America would fare in warmer world. After two degrees of warming, he found that the Northeast will have wetter winters, but the Great Plains will have drier summers.
The Nature Climate Change study draws on more than 30 different models from across the world. It looked at more than 6,700 years of climate simulations to arrive at its estimates. “I was surprised by just how much more frequent these extreme heat events that we have … on the Great Barrier Reef will become, even if we manage to meet the Paris targets,” said King.
Gates agreed. “It’s troubling and confirmatory—people have been talking about this, but this is solid modeling,” she told me. “These simulations are directed specifically at the Great Barrier Reef, but I’m pretty sure if the exercise were done for a majority of other places, it would be a pretty similar conclusion.”
“It’s telling us that if we do not become very aggressive with mitigation, the chances of allowing coral reefs to continue to exist is… low,” she said. “We have to do something, or the new reality will be these temperatures.”
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