Fingerprints of climate change are increasingly appearing in extreme weather


— Extremely low sea ice in the Bering Sea. Heavy
rainfall in the mid-Atlantic United States. Wildfires in northeast Australia.

Examinations of these and 16 other extreme
weather events that occurred in 2018 found that all but one were made more
likely due to human-caused climate change, scientists reported December 9 at a
news conference at the American Geophysical Union’s annual meeting.
Insufficient observational data made it impossible to assess the influence of
climate change on the one event, heavy rains in Tasmania.

The new report marks the third year in a row
that scientists have identified specific weather events that they said would
not have happened without human activities that are altering Earth’s climate.

The findings are part of a climate-attribution
special issue, called “Explaining
extreme events in 2018 from a climate perspective
published online December 9 in the Bulletin of the American Meteorological
. Over the 11 years that BAMS has published the special issue,
it’s included 168 studies examining specific weather events. Of those, 122, or 73
percent, found that climate change likely played some role in the event, the special
issue’s editor Stephanie Herring said at the news conference. In some cases, that means the event was more likely to
occur due to human actions; in a few studies, researchers have concluded that
the event would not have occurred without climate change. 

Finding a role is becoming more common; the
special issues tied to events in 2017
and 2016
found that climate change was linked to 95 percent of the events studied (SN: 12/14/17; SN: 12/11/18).

Rather than being a cause for despair,
identifying humankind’s role in these events should be seen as a way to provide
new data in the fight to prepare for climate change, BAMS Editor in Chief Jeff
Rosenfeld said in the news conference. “Instead of stoking fear, I think
having real quantifiable information like this puts that human fingerprint in
terms that we can actually act [on] without getting overwhelmed.”

Below are a few of the highlights of the new report.

Historic flooding

Extreme rainfall from January through
September 2018 led to flooding across Maryland, New Jersey, Pennsylvania,
Delaware, West Virginia and Washington, D.C. An analysis of 40 climate
simulations of both historical and future climate conditions showed that the intense
rainfall was 1.1
to 2.3 times more likely
as a result of climate change, a team
led by climate scientist Jonathan Winter of Dartmouth College reported.

To determine how the rains might be linked to
changing climate, scientists first examined daily rainfall measured at 63
stations in the mid-Atlantic region from 1920 to 2018. Rainfall totals from
January through September 2018 were the highest ever measured at 33 percent of
the stations, the team found — and in the top three at 62 percent of the
stations. That made 2018 precipitation a 1-in-99-year event.

Fierce fires

About 130 wildfires
that raged across northeast Australia
in November 2018
also bore the stamp of climate change, researchers say — although
deconstructing the cause of the fires is complex. Multiple contributing factors
likely added to the potential fuel for the fires, reports the team, led by
climate scientist Sophie Lewis of the University of New South Wales Canberra in
Australia. Those factors include low rainfall during the preceding spring
months as well as an extreme heat wave, both linked to climate change.

Deepwater National Park fire
Firefighters battle a blaze at Deepwater National Park in Queensland, Australia, on November 29, 2018. Climate change–related conditions, including an extreme heat wave, low spring rainfall and altered atmospheric circulation patterns, made the fires more likely.ROB GRIFFITH/AFP via Getty Images

Altered atmospheric circulation patterns, such
as a sustained low-pressure system south of the region and an unusual westerly
winds, likely made conditions more favorable to wildfires, the team reported.

Vanishing ice

Floating sea ice in the Bering Sea over the
winter of 2017 to 2018 was lower than
at any time in the last five decades
of satellite
observations, said climate scientist Walt Meier of the National Snow & Ice
Data Center in Boulder, Colo., who coauthored a study on climate change’s
impacts on the region.

The impacts of that low Bering Sea ice extent
have been well
), including massive seabird
), changes in fish populations and extensive coastal flooding in the
region. But now, scientists are confirming that the lack of ice that year was the
result of climate change.

The researchers simulated how the sea ice
would wax and wane from January to April in a world with no human-caused
greenhouse gas emissions — and therefore with atmospheric carbon dioxide levels
similar to those of 1850. The team ran 1,800 simulations of this “planet B”
scenario, and found only two instances in which the minimum sea ice extent
dipped down as low as the observed 2018 levels. That suggests that the observed
extent would be “nearly impossible” without human-caused climate change, Meier

By 2050, the low sea ice extent of 2018 will become the new normal, he added. And by 2100, any years with sea ice extents matching that of 2018 will again be outliers — they’ll be unusually large.

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