The letter is behind the journal’s subscription wall, so I’m providing excerpts here:
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 (–27°C), 6 January], at Central Park in New York (4°F (–15.6°C), 7 January], and at many other stations. 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.
There is a section on recent research pointing to a connection between expanded open water on the Arctic Ocean in summers and extreme winter weather in the temperate zone of the Northern Hemisphere. The authors write:
Sea-ice losses during late summer may indeed lead to regional changes in Arctic climate. 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. 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 [paper] 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. 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. 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…