A new study warns it has become a “highly altered, degraded system.”
Once upon a time, there was a city so dazzling and kaleidoscopic, so
braided and water-rimmed, that it was often compared to a single living
body. It clustered around a glimmering emerald spine, which astronauts
could glimpse from orbit. It hid warm nooks and crannies, each a nursery for
new life. It opened into radiant, iris-colored avenues, which tourists crossed
oceans to see. The city was, the experts declared, the planet’s largest living
structure.
Then, all at once, a kind of invisible wildfire overran the city. It consumed its
avenues and neighborhoods, swallowed its canyons and branches. It
expelled an uncountable number of dwellers from their homes. It was
merciless: Even those who escaped the initial ravishment perished in the
famine that followed.
Many people had loved the city, but none of them could protect it. No
firefighters, no chemicals, no intervention of any kind could stop the
destruction. As the heat plundered the city of its wealth, the experts could
only respond with careful, mournful observation.
All of this recently happened, more or less, off the east coast of Australia.
The Great Barrier Reef—which, at 1,400 miles long, is the longest and
largest coral reef in the world—was blanketed by dangerously hot water in
the summer of 2016. This heat strangled and starved the corals, causing
what has been called “an unprecedented bleaching event.”
Though that bleaching event was reported at the time, scientists are just
starting to understand how catastrophically transformative it was. A new
paper, published Wednesday in the journal Nature, serves as a kind of
autopsy report for the debacle.
After inspecting every one of its reefs, and surveying them on an almost
species-by-species basis, the paper reports that vast swaths of the Great
Barrier Reef were permanently transformed in the summer of 2016. The
reef’s northern third, previously its most pristine section, lost more than half
of its corals. Two of its most recognizable creatures—the amber-colored
staghorn corals, and the flat, fanlike tabular corals—suffered the worst
casualties.
But damage was widespread out across the entire ecosystem.
“On average, across the Great Barrier Reef, one in three corals died in nine
months,” said Terry Hughes, an author of the paper and the director of the
ARC Center of Excellence for Coral Reef Studies, the Australian
government’s federal research program devoted to corals.
“You could say [the ecosystem] has collapsed. You could say it has
degraded. I wouldn’t say that’s wrong,” Hughes said. “A more neutral way of
putting it is that it has transformed into a completely new system that looks
differently, and behaves differently, and functions differently, than how it
was three years ago.”
“It’s a confirmation of our worst fears,” said John Bruno, a marine biologist
at the University of North Carolina who was not involved in the study.
Yet it was not the end of troubles for the Great Barrier Reef. In the summer
months of 2017, warm waters again struck the reef and triggered another
bleaching event. This time, the heat hit the reef’s middle third. Hughes and
his team have not published a peer-reviewed paper on that event, but he
shared early survey results with me.
Combined, he said, the back-to-back bleaching events killed one in every
two corals in the Great Barrier Reef. It is a fact almost beyond
comprehension: In the summer of 2015, more than 2 billion corals lived in
the Great Barrier Reef. Half of them are now dead.
What caused the devastation? Hughes was clear: human-caused global
warming. The accumulation of heat-trapping pollution in the atmosphere
has raised the world’s average temperature, making the oceans hotter and
less hospitable to fragile tropical corals.
A researcher assesses one area of the Great Barrier Reef that escaped with
only minor damage after the 2016 bleaching event. (Gergely Torda / ARC
Center of Excellence for Coral Reef Studies)
“People often ask me, ‘Will we have a Great Barrier Reef in 50 years, or 100
years?’ And my answer is, yes, I certainly hope so—but it’s completely
contingent on the near-future trajectory of greenhouse-gas emissions,”
Hughes said.
The Paris Agreement on climate change aims to prevent the world’s average
temperature from rising by 2 degrees Celsius (3.6 degrees Fahrenheit).
“One degree of that warming has already occurred since industrialization,”
Hughes said. “That 1 degree has obviously made things uncomfortable for
reefs—most reefs have bleached three or four times since 1998.”
“But the global-average target includes both land and sea, the poles and
the tropics,” he continued. “The sea is warming at a slower rate than the
land, and the tropics are warming more slowly than higher latitudes near the
poles. So far, we’ve seen less than 1 degree of warming in the ocean—about
0.7 degrees Celsius. If we go to 2 degrees, we’ll see another 0.55 degrees
on average in the tropics. I think that’s possibly doable in terms of still
having functional coral reefs, but as we’ve already seen, the mix of species
will be very different than it was just two years ago.”
It is about as hopeful a note as you can get out of him. And there’s one
glaring problem: The world is currently not on track to meet the Paris
Agreement goals. And the United States announced it would leave the treaty
last year.
The new paper also advances its own idea of what the future of the Great
Barrier Reef will look like. In short, it is highly unlikely that the Great Barrier
Reef—the northern reef, especially—will resemble its old self in the lifetime
of any living person.
“It’s like clear-cutting a redwood forest,” said Bruno, the marine biologist. “In
10 years, you’re going to have a lot of stuff on the ground, but you’re not
going to have the old-growth forest back. Some of these corals were 10, 30
years old, but a lot of them were centuries old. In 100 years—if there is no
more warming—they could return. But it’s a long time.”
“The most likely scenario,” write the authors of the paper, “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.”
“Novel, heat-tolerant reef assemblages”: That doesn’t exactly evoke the
dazzling rainbow vistas familiar to viewers of Blue Planet or even Finding
Nemo.
“I regret that I agree with the disturbing prognosis,” said Rebecca Vega
Thurber, a coral scientist at Oregon State University who was not connected
to the paper, in an email. “Unless we see evidence that these mass thermal
anomalies are becoming less frequent or severe (which is counter to the
evidence at hand), it is likely that the Great Barrier Reef (and reefs
worldwide) will continue to degrade.”
How did this catastrophe happen so quickly? In part, this is just what heat
can do. In late 2015, the Pacific Ocean lurched into one of the largest El Niño
events ever recorded. El Niño is the shared name for a set of global weather
symptoms that, in general, raise global temperatures and switch
precipitation patterns.
El Niño–like conditions had struck the Great Barrier Reef thousands of times
before, but the 2016 event had an accomplice: the slow, inexorable rise in
global temperatures caused by climate change. Combined, global warming
and El Niño sent ocean temperatures around the world soaring. Coral reefs
in every ocean and hemisphere suffered in the blast.
It can be easy to forget, when gazing at their hard and gnarled exterior, that
corals are animals. They do not make food on their own; like chimpanzees,
golden retrievers, and sea cucumbers, corals have to find something to eat.
But they approach this problem in a unique way. Tiny, photosynthetic algae
live in coral tissue. The algae turn sunlight into food for corals, and corals
give them a place to live in return. This symbiotic relationship between
animal and plant undergirds every interaction in the tropical coral reef.
Warm water severs this symbiosis. When corals are exposed to heat, they
expel the colorful algae from their tissue. To human eyes, this causes them
to lose their color—they “bleach”—but it also robs them of their food
source. If temperatures do not soon return to safe levels, the corals starve
and die.
At least, that is the conventional thinking. About 50 percent of all the coral
that perished in the 2016 bleaching event died in the autumn and winter,
long after temperatures had returned to normal. Those corals never
regained their algae after evicting them, and they slowly starved to death.
“But about half of the corals that died did so in March, at the peak of
summer temperatures,” Hughes told me. “We were surprised that about half
of that mortality occurred very quickly.”
In other words, some corals did not even survive long enough to starve.
“They died instantly, of heat stress,” Hughes said. “They cooked.”
Based off their work observing all of the Great Barrier Reef’s 3,863
constituent reefs by satellite, and then surveying many of them in person,
Hughes’s team also advances new guides for predicting when the reef as a
whole will bleach. They use a unit called degree-heating weeks, which
multiplies the length of a heating episode (measured in weeks) by the size
of its temperature anomaly. So if the temperature of a reef rises 2 degrees
Celsius above average—and that warm spell lasts for two weeks—then the
spell was four degree-heating weeks. If the temperature of a reef rises by 4
degrees Celsius, but only for one week, then the spell also was also four
degree-heating weeks.
Corals are shockingly fragile to even small amounts of warmth, they found.
At two degree-heating weeks, most corals in a local area will mildly bleach.
At four degree-heating weeks, corals in an area will start to die. And if a
local reef hits six degree-heating weeks, it will suffer “catastrophic
mortality” comparable to what happened in 2016, Hughes said.
These distinctions are more than academic. The International Union for
Conservation of Nature, the global organization that declares species
“endangered” or “threatened,” is currently creating a similar scale for
ecosystems. Its leaders hope that soon individual ecosystems might be
discussed as “endangered,” in the same way that giant pandas and
mountain gorillas are.
“I would classify the Great Barrier Reef, and all coral reefs around the world,
as being endangered due to climate change,” Hughes said.
This finding, while a central finding of coral-reef science, may take a long
time to sink in in Australia. “A lot of people believed that because [the Great
Barrier Reef] was super diverse and had no pollution and was remote, it was
resistant to bleaching,” said Bruno. But the scale of the 2016 event shows
that “bleaching has nothing to do with virtue or management or having a
commitment to resilience.”
“It’s just warming,” he said. “Once it warms, it’s toast.”
The Great Barrier Reef is almost late to this moment: Reefs around the world
have already gone through a similar transformation in the past several
decades.
“Coral cover in the Caribbean has been trashed for 20 years,” Bruno told
me. “But it wasn’t like that in the ’80s. In the mid-’80s, you could go
snorkeling in Florida and it was like flying over a Kansas wheat field—golden
coral for acres and acres and acres.”
“Now that’s totally gone,” he said. “Coral cover in the Florida Keys is like at 3
percent.”
“This single event represents the mass destructive force that global climate
change can have on coral reefs,” said Thurber. “Since the Great Barrier Reef
is, for many, our gold-standard reef, its precipitous decline is all the more
disturbing to those studying both pristine corals reefs and those that are on
the fringe.”
The study fits into a streak of dreary findings for coral reefs. If the world
were to warm by an average of 2 degrees Celsius, then ocean temperatures
would consistently exceed their 2016 levels, even during non–El Niño years,
a recent study in Nature Climate Change found. In another study, released in
January, scientists surveyed observations of 100 coral reefs around the
world going back 35 years. They found that mass bleaching events now
strike five times more often than they did in the early 1980s.
Hughes said that the news will continue to get worse until governments and
their citizens focus on the “root cause” of the problem. The Great Barrier
Reef will continue to collapse and die until humanity stabilizes the amount of
greenhouse-gas pollution in the air. But fixing that problem will require
remaking the energy system, moving away from oil and gas and to solar,
wind, and other renewable sources.
“But the federal government of Australia, like the federal government of the
United States, still very much favors the continued development of the
fossil-fuel industry,” Hughes sighed. “And that, to me, is a complete policy
failure for the Great Barrier Reef.”"
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