Oh, Global Warming, Where Art Thou? How the recent cold outbreaks in the US are consistent with, not in contradiction of, the expected impacts of climate change.
During the months of January and February, the eastern North America experienced numerous extreme winter weather events. In early January, many locations in the Midwestern United States and Canada were colder than both the North and South Poles; the extreme cold is evident in the plot of January through February temperature anomalies (deviations from average) in the plot below (source: NOAA/NCEP) . In late January and mid-February, winterstorms severely impacted areas more accustomed to heat waves, hurricanes and tornadoes; the cities of Atlanta, Raleigh-Durham and Birmingham, along with the Outer Banks of North Carolina, ground to a halt.
The recent spate of extreme cold has led many to ask, "But what about global warming? Aren't our winters supposed to be getting warmer?" Our climate, and thus weather, is primarily driven by the difference in temperature between the equator, where radiation from the sun is near constant, and the poles, where solar radiation varies strongly over the course of the year. This difference in temperature results in high altitude winds which blow from west to east; these winds, the jet stream, separate the cold polar air from the more temperate tropical and mid-latitude air. In general, the greater the temperature contrast between the equator and pole, the stronger and more zonal (west to east) the jet stream. When the difference in temperature between the equator and the pole decreases, the jet stream slackens and takes on more of a wavy pattern, with larger north to south meanders. These north to south meanders in the jet stream allow for cold Arctic air to penetrate further south, down into the United States.
One of the primary consequences of climate change is that the Arctic is warming much quicker than the equator; this is well established by the International Panel on Climate Change (IPCC). This Arctic amplification is caused by melting ice, which reveals ocean. This now exposed ocean absorbs solar radiation, instead of reflecting it as ice would, leading to additional warming and continuing ice melt. A significantly warmer Arctic, coupled with little to no appreciable temperature changes at the equator, would lead to a decrease in the temperature difference between the equator and the pole, which would in turn, lead to a weaker jet steam with more north to south meanders. Therefore, under climate change, one could expect an increase in the frequency of Arctic air outbreaks during the winter in the continental United States.
Category: Climate/natural disasters: Climate change, Floods/storms
Location: North America