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| A wildfire in Azusa, Calif., in 2016. New research has analyzed 27 extreme weather events from that year for links to climate change. Credit Gene Blevins/Reuters |
Extreme weather left its mark across the planet in 2016, the hottest year in recorded history.
Record heat baked Asia and the Arctic. Droughts gripped Brazil and
southern Africa. The Great Barrier Reef suffered its worst bleaching
event in memory, killing large swaths of coral.
Now climate scientists are starting to tease out which of last year’s calamities can, and can’t, be linked to global warming.
In a new collection of papers
published Wednesday in the Bulletin of the American Meteorological
Society, researchers around the world analyzed 27 extreme weather events
from 2016 and found that human-caused climate change was a “significant
driver” for 21 of them. The effort is part of the growing field of climate change attribution, which explores connections between warming and weather events that have already happened.
To
judge whether global warming made a particular extreme weather event
more likely to occur, scientists typically compare data from the real
world, where rising greenhouse gases have heated the planet over the
past century, against a modeled counterfactual world without those
rising emissions. This technique has gained broader acceptance among climate scientists in the last decade.
Here are five extreme weather events from 2016 that scientists now think were made more likely by global warming:
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| A researcher near Utqiagvik, Alaska. Two studies have found that unusually high temperatures in the Arctic “would not have been possible without human-caused climate change.” Credit David L. Ryan/The Boston Globe, via Getty Images |
Two
separate studies also found that unusually high temperatures across
Asia and the Arctic in 2016 “would not have been possible without
human-caused climate change.” Such forceful assertions are rare:
Typically, scientists will only go so far as to say that global warming
made an extreme weather event more likely to occur. In these cases, they
went further, finding that such extreme warmth could not have happened
in a world without rising emissions.
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| Bleached coral on the Great Barrier Reef off Cairns, Australia, in March. Credit Biopixel Pty Ltd., via Agence France-Presse — Getty Images |
Over the past two years, unusually warm waters in the Pacific have caused bleaching on the Great Barrier Reef,
a phenomenon in which coral expel vital algae from their tissue and
take on a ghastly white appearance. If the warm water temperatures
persist, many corals can die off, with dire consequences for the marine
ecosystems that depend on them.
Here,
scientists were more measured in putting all the blame on global
warming, in part because the impact of El Niño was tough to disentangle:
A study led by Sophie C. Lewis of Australian National University
concluded that human greenhouse gas emissions “likely increased the risk
of the extreme Great Barrier Reef event” by increasing thermal stress
in the ocean. The study also warned that bleaching risks were likely to
increase in the future.
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In the first few months of 2016, severe droughts and heat waves spread across much of southern Africa, triggering local food and water shortages that affected millions.
While
such “flash droughts” are often associated with El Niño, scientists now
say climate change plays an important role, too. A study led by Xing
Yuan of the Chinese Academy of Sciences found that flash droughts had
tripled in the region over the past 60 years, with global warming
“mainly responsible” for the trend.
Climate
change can’t be blamed for all recent dry spells, however. In a
separate study, researchers looked at a five-year drought in Northeast
Brazil but “could not find sufficient evidence that human-caused climate
change increased drought risk.”
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In 2016, wildfires burned about 8.9 million acres of western Canada and the United States, including a particularly destructive fire in Alberta that forced mass evacuations and destroyed 2,400 homes. Here, climate change most likely played a supporting role.
Researchers
at the University of Edinburgh found that global warming had made
“extreme vapor pressure deficits” five times more likely across the
region during the summer months — a measure of changes in atmospheric
moisture that is associated with the drying of vegetation and wildfire
risk. But this finding came with a caveat: The increased fire risk
linked to climate change did not hold for the month of May, when the
Alberta fire broke out.
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Over the past few years, a large patch of unusually warm water has appeared off the coast of Alaska, popularly known as “the blob.” These warm waters have allowed toxic algae blooms to spread across the region, killing seabirds by the thousands and forcing local fisheries to close.
A
new study, led by John E. Walsh of the University of Alaska, called the
blob “unprecedented” and argued that it “cannot be explained without
anthropogenic climate warming,” although natural factors such as El Niño
and atmospheric variability also played an important role. The study
also concluded that more such blobs were likely to occur with further
warming, which “will result in a profound shift for people, systems, and
species.”
Climate attribution remains easier for some weather events than others. Temperature records are the simplest to link to climate change.
But droughts — which are influenced by a complex interplay of
temperature, precipitation and soil moisture — can be trickier to
connect to warming trends. And hurricanes are more difficult still,
because they occur so rarely.
Overall,
however, attribution science has improved significantly since the
Bulletin of the American Meteorological Society began publishing its
annual investigations into weather extremes six years ago, said Heidi
Cullen, chief scientist at Climate Central, a news organization that
focuses on climate science.
“In
2011, people were still of the mind-set that you couldn’t attribute any
individual event to climate change,” Dr. Cullen said. “But with each
subsequent issue, people are able to say that climate change really is
increasing the risk” of certain extremes occurring.
Crucially,
however, the journal does not explicitly set out to prove links between
specific weather extremes and global warming. Instead, the editors
accept proposals to investigate certain weather events before the
results are known, in order to minimize publication bias.
In
some cases, scientists either ruled out or could not find a significant
role for climate change, effectively arguing that a given weather
extreme could just as likely have occurred in a world without global
warming. That was true of Brazil’s brutal drought, which was largely
influenced by El Niño, as well as a major snowstorm in the Mid-Atlantic
United States.
“A
few events from this past year were judged to have been of such a
magnitude that they would not have been possible in the climate of a few
hundred years ago,” said Martin P. Hoerling, a meteorologist at NOAA
who edited the collection. But, he added, “not everything is being made
demonstrably more severe because of climate change.”
In
the future, scientists are hoping to refine and standardize their
attribution methods, so that a community hit by a storm, wildfire or
other extreme event can learn much more quickly how that event might
have been swayed by global warming — and take steps to adapt.
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