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NY Times, Jan. 8, 2018
Why So Cold? Climate Change May Be Part of the Answer
By HENRY FOUNTAIN
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As bitter cold continues to grip much of North America and helps spawn
the fierce storm along the East Coast, the question arises: What’s the
influence of climate change?
Some scientists studying the connection between climate change and cold
spells, which occur when cold Arctic air dips south, say that they may
be related. But the importance of the relationship is not fully clear yet.
The Arctic is not as cold as it used to be — the region is warming
faster than any other — and studies suggest that this warming is
weakening the jet stream, which ordinarily acts like a giant lasso,
corralling cold air around the pole.
“There’s a lot of agreement that the Arctic plays a role, it’s just not
known exactly how much,” said Marlene Kretschmer, a researcher at the
Potsdam Institute for Climate Impact Research in Germany. “It’s a very
complex system.”
The reason a direct connection between cold weather and global warming
is still up for debate, scientists say, is that there are many other
factors involved. Ocean temperatures in the tropics, soil moisture, snow
cover, even the long-term natural variability of large ocean systems all
can influence the jet stream.
“I think everyone would agree that potentially the warming Arctic could
have impacts on the lower latitudes,” said Rick Thoman, climate services
manager with the National Weather Service in Fairbanks, Alaska. “But the
exact connection on the climate scale is an area of active research.”
Much of the Northern Hemisphere is cold this time of year (it’s winter,
after all). Cold snaps have occurred throughout history — certainly long
before industrialization resulted in large emissions of greenhouse
gases. And as with any single weather event, it’s difficult to directly
attribute the influence of climate change to a particular cold spell.
But scientists have been puzzled by data that at first seems
counterintuitive: Despite an undeniable overall year-round warming
trend, winters in North America and Europe have trended cooler over the
past quarter-century.
“We’re trying to understand these dynamic processes that lead to cold
winters,” Ms. Kretschmer said.
She is the lead author of a study published last fall that looked at
four decades of climate data and concluded that the jet stream — usually
referred to as the polar vortex this time of year — is weakening more
frequently and staying weaker for longer periods of time. That allows
cold air to escape the Arctic and move to lower latitudes. But the study
focused on Europe and Russia.
“The changes in very persistent weak states actually contributed to cold
outbreaks in Eurasia,” Ms. Kretschmer said. “The bigger question is how
this is related to climate change.”
Timo Vihma, head of the polar meteorology and climatology group at the
Finnish Meteorological Institute, explained that warmer air in the
Arctic reduces the temperature difference between it and lower latitudes
and weakens the polar vortex.
“When we have a weak temperature gradient between the Arctic and
mid-latitudes, the result is weaker winds,” he said.
Ordinarily the jet stream is straight, blowing from west to east. When
it becomes weaker, Dr. Vihma said, it can become wavy, “more like a big
snake around the Northern Hemisphere.”
The weaker winds are more susceptible to disturbances, such as a zone of
high pressure that can force colder air southward. These “blocking”
high-pressure zones are often what creates a severe cold spell that
lingers for several days or longer.
The current cold snap has been in place for more than a week, and the
cold air on Wednesday was moving east and colliding with a mass of
warmer air from the Atlantic Ocean. That created a storm known as a
“bomb cyclone.”
In a bomb cyclone, the temperature difference between the two air masses
leads to a steep and rapid — meteorologists often use the term
“explosive” — drop in atmospheric pressure. The air starts to move and,
aided by the earth’s rotation, begins to rotate. The swirling air can
bring high winds and a lot of precipitation, often in the form of snow.
That could happen this time — depending on the track of the storm, parts
of the Northeast were expecting heavy snow. But one impact of the storm
is even more clear: After it eventually moves off to the north and west,
it should draw even more cold polar air into the eastern half of the
United States, continuing the big chill.
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