New York Times - Kendra Pierre-Louis | Brad Plumer
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| A researcher examined bleached coral at Zenith Reef, in the northern section of the Great Barrier Reef, in November 2016.
Credit
Andreas Dietzel/ARC Centre of Excellence for Coral Reef Studies |
Before
we call rockfish, shrimp and crab “dinner,” some of these species call
coral reefs “home.” But those reefs, home to a quarter of all marine
fish species, are now increasingly threatened as rising ocean
temperatures accelerate a phenomenon known as coral bleaching.
Large-scale
coral bleaching events, in which reefs become extremely fragile, were
virtually unheard-of before the 1980s. But in the years since, according
to
a study published Thursday in the journal Science,
the frequency of coral bleaching has increased to the point that reefs
no longer have sufficient recovery time between severe episodes.
Jelle
Atema, a professor of biology at the Boston University Marine Program
who was not involved in the study, said the effects of more frequent
bleaching events were very difficult to predict because of the complex
networks of dependencies within reefs. But he said they could be
devastating.
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An aerial view of bleaching coral off the Australian coast in March 2016.
Credit
Terry Hughes/ARC Centre of Excellence for Coral Reef Studies |
“When
coral dies, it affects the shelter and food that sustain fish,
lobsters, shellfish, worms, etc. The same happens in a rain forest. When
the trees die, the animals and plants that have developed over
millennia die with them,” he said, before adding an analogy. “When a
country is ravaged by war, people die and migrate.”
During
bleaching events,
overheated seawater causes corals to part ways with symbiotic plantlike
organisms called zooxanthella that live inside of them. In addition to
giving coral reefs their bright colors, zooxanthella also provide corals
with oxygen, waste filtration, and up to 90 percent of their energy.
Absent zooxanthella, corals not only take on a ghostly pallor, hence the
term bleaching, but they are also more susceptible to death.
In
theory, coral reefs can recover from even a severe bleaching event.
Some of the coral will die off from increased disease susceptibility,
but once ocean temperatures drop again, many of the corals will start
growing back.
But that’s only if they’re given enough time.
Typically,
it takes 10 to 15 years for the fastest-growing corals to recover after
a severe bleaching event. Larger corals that provide shelter for bigger
fish can take even longer to grow back.
As bleaching events become more frequent, reefs are unlikely to get that needed reprieve. Earth’s average temperature
has increased 1 degree Celsius,
or 1.8 degrees Fahrenheit, above preindustrial levels, and the median
time between severe bleaching events is now just six years, the Science
study found.
Case in point: The Scott Reef, 180 miles off the coast of Northwestern Australia, had over the past few years
finally begun recovering from a major bleaching event in 1998,
with the fastest-growing corals inhabiting much of their earlier
territory. But the area was hit by bleaching again in 2016, causing
widespread mortality.
Before
1982-3, mass bleaching events across wide areas were nonexistent. That
year, reefs across the Tropical Eastern Pacific exposed to warm El Niño
year waters bleached. Coral reefs in Costa Rica, Panama and Colombia
experienced 70 to 90 percent mortality. Most reefs in the Galápagos
Islands, the cradle of Darwin’s theory of evolution, experienced 95
percent mortality.
While
many mass bleachings were prompted by El Niño events, which tends to
warm Pacific Ocean temperatures, the bleaching event that hit the Great
Barrier Reef in 2017 — the reef’s first back-to-back bleaching —
occurred at the beginning of a La Niña event, when ocean waters should
have been cooler. It’s a sign that global warming is steadily pushing up
ocean temperatures even in cooler years.
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A blenny on bleached coral. Reefs are home to about a quarter of all marine species.
Credit
Greg Torda/ARC Centre of Excellence for Coral Reef Studies |
“La
Niña periods today are actually warmer than El Niño periods were 40
years ago,” said Terry Hughes, a senior researcher who specializes in
coral reefs at James Cook University in Australia and the lead author of
the Science study.
“Coral
bleaching is caused by global warming full stop,” Dr. Hughes said.
“It’s not due to El Niño. We’ve had thousands of El Niño prior to 1983,
none of them caused bleaching. Bleaching is caused by the rising
baseline temperatures due to anthropogenic global warming.”
Scientists
have long warned that the effects of climate change will not
necessarily progress in a linear way as the planet warms. As Earth
crosses certain key temperature thresholds, severe and far-reaching
changes can unfold relatively rapidly, such as the collapse of ice
sheets or the die-off of key ecosystems.
All
evidence suggests that bleaching will only get more and more frequent
as the Earth continues to warm. By midcentury, climate models suggest,
most reefs will experience the sort of heat associated with severe
bleaching every year.
If
corals can’t adapt quickly enough, “we could be looking at the
effective loss of most of the world’s coral reefs,” said Mark Eakin, an
oceanographer who is coordinator of the Coral Reef Watch project at the
United States National Oceanographic and Atmospheric Administration.
The
Great Barrier Reef had two back-to-back bleaching events that killed
just about half of the corals along the length of the barrier reef. This
means half are still alive. Those corals are the source of larvae that
spawn future generations, which means that the reef moving forward will
have a distinctly different character than it had two years before the
bleaching event.
“The
ecological effect of more and more bleachings is that it’s changing the
mix of species in favor of the tougher corals that can survive
bleaching events and in terms of the corals that bounce back the
quickest,” said Dr. Hughes. “It’s changing the whole ecology of the
reefs.”
There
are a few things that can help make reefs more resilient to bleaching.
Humans can limit fertilizer and sewage runoff that damage coral. They
can avoid overfishing key herbivores like the rabbitfish that nurture
the reefs by clearing away excessive algae.
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A sea turtle in the Indian Ocean amid coral that succumbed to a 2016 bleaching event.
Credit
Kristen Brown/ARC Centre of Excellence for Coral Reef Studies |
Some researchers are experimenting
with even more radical techniques,
such as trying to breed coral that can thrive in warmer temperatures,
or looking at ways to pump cooler water into reefs to protect the coral
from overheating, or
even placing giant “shade cloths” over reefs.
Some
of these ideas are admittedly wild, Dr. Eakin said, and none of them
can ever be a substitute for reducing greenhouse gas emissions. “We
can’t act as if we can keep emitting carbon dioxide into the atmosphere
and just by tinkering around with corals in a lab we’re going to solve
the problem,” he said.
But
given that frequent bleaching is already underway, and given that at
least half a degree of additional global warming appears inevitable,
coral researchers are desperate for new ideas.
“We’ve
got to start taking steps that we haven’t thought about before — even
if they sound absolutely crazy,” Dr. Eakin said. “Because the stuff we
thought made sense will no longer work.”
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