LETTERS
edited by Jennifer Sills
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COMMENTARY
LETTERS I BOOKS I POLICYFORUM I EDUCATION FORUM I PERSPECTIVES
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Simple rules,
complex structures
Somites, size, and place
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Global Warming and Winter Weather
IN MID-JANUARY, A LOBE OF THE POLAR VORTEX SAGGED SOUTHWARD OVER THE CENTRAL
and eastern United States. All-time low temperature records for the calendar date were set
at O’Hare Airport in Chicago [–16°F (–8°C), 6 January], at Central Park in New York [4°F
(–15.6°C), 7 January], and at many other stations (1). Since that event, several substantial snow
storms have blanketed the East Coast. Some have been touting such stretches of extreme cold
as evidence that global warming is a hoax, while others have been citing them as evidence that
global warming is causing a “global weirding” of the weather. In our view, it is neither.
As climate scientists, we share the prevailing view in our community that human-induced
global warming is happening and that, without mitigating measures, the Earth will continue to
warm over the next century with serious consequences. But we consider it unlikely that those
consequences will include more frigid winters.
Distinguishing between different kinds of extreme weather events is important because
the risks of different kinds of events are affected by climate change in different ways. For
example, a rise in global mean temperature will almost certainly lead to an increase in the
incidence of record high temperatures. Global warming also leads to increases in atmospheric water vapor, which increases the likelihood of heavier rainfall events that may cause
flooding. Rising temperatures over land lead to increased evaporation, which renders crops
more susceptible to drought. As the atmosphere and oceans warm, sea water expands and
glaciers and ice sheets melt. In response, global sea-level rises, increasing the threat of
coastal inundation during storms.
In contrast to the above examples, the notion that the demise of Arctic sea ice during
summer should lead to colder winter weather over the United States seems counterintuitive.
But that is exactly what an influential study has suggested (2). The authors hypothesize that
global warming could perturb the polar vortex in a manner that renders the flow around it
more wavy, leading to an increased incidence of both extreme warmth and extreme cold in
temperate latitudes. It’s an interesting idea,
but alternative observational analyses and
simulations with climate models have not
confirmed the hypothesis, and we do not
view the theoretical arguments underlying it
as compelling [see (3–6)].
Other studies have suggested that the loss
of Arctic sea ice may influence the atmospheric circulation in mid-latitudes during summer [e.g., (7)]. Sea-ice losses during late summer may indeed lead to regional
changes in Arctic climate [e.g., (5, 8)]. But
tremendous natural variability occurs in the
large-scale atmospheric circulation during
all seasons, and even in summer, the links
between Arctic warming and mid-latitude
weather are not supported by other observational studies (6). The lag between decreases
in sea-ice extent during late summer, and
changes in the mid-latitude atmospheric
circulation during other seasons (when the
recent loss of sea ice is much smaller) needs
to be reconciled with theory.
Summertime sea-ice extent in the Arctic
has been remarkably low since 2007, and the
ensuing years have been marked by some
notable cold air outbreaks. It was this coincidence that prompted Francis and Vavrus (2)
to link the cold air outbreaks to global warming. But coincidence does not in itself constitute a strong case for causality. Cold air outbreaks even more severe than occurred this
winter affected the United States in the early
1960s, the late 1970s (most notably 1977),
and in 1983, back when the Arctic sea ice was
thicker and more extensive than it is today
[e.g., (9)]. Over the longer time span of 50
to 100 years, it is well established that there
has been a decrease in the rate at which low
temperature records are being set relative to
all-time high temperature records at stations
across the United States (10). For the present
at least, we believe that statistics based on the
longer record are more indicative of what the
future is likely to bring.
The research linking summertime Arctic
sea ice with wintertime climate over temperate latitudes deserves a fair hearing. But to
make it the centerpiece of the public discourse
Icy blast. Arctic winds flowed down to North America in January, causing record-breaking cold temperatures. Image
shows streamlines of wind at the 500 mbar level at 1:00 a.m. Eastern Standard Time on 7 January 2014. Red indicates faster speeds.
