We here in Peterborough were caught in a cold snap for much of January and February, the coldest stretch in decades. Everywhere I went people were complaining about the winter and asking what happened to Global Warming. The weather reports were all talking about the Polar Vortex. What the heck is a Polar Vortex anyway?
Can we reconcile the apparent discrepancy between the coldest January in decades and global warming? I think so, and there is even pretty good evidence that the cold snap was caused by global warming.
One of the predicted and observed effects of global warming is that the extreme northern latitudes (the Arctic) are heating at almost twice the rate of the rest of the northern hemisphere. One result of this heating is that the extent (area) of Arctic sea ice compared to 30 years ago is about 50% of what it was and that its volume is about 80% less. As a result the temperature gradient in the atmosphere from south to north is smaller. This causes the jet stream, a strong flow of air flowing from west to east around the North Pole, to flow more slowly than previously and at the same time to have bigger waves in it that now extend much farther south. You probably saw some pretty slick graphical representations of these on TV weather reports in January and February, and there are more of them with better explanations like this one on the web.
The deeper, slower moving troughs in the jet stream funnelling cold air south are what has showed up in the media as the Polar Vortex. In Eastern North America we were caught in a long, slow moving trough of very cold air. Meanwhile the west coast was caught on the other side of the trough in a heat wave and drought.
To put our cold winter in perspective, according to the National Oceanic and Atmospheric Administration web site, during the last 365 days across all weather stations in the US there were 12 lowest temperature ever records set, while in the same period there were 76 highest ever records set.
Dr. Jennifer Francis is a Research Professor at Rutgers University who specializes in Arctic climate change and Arctic-global climate linkages with about 40 peer-reviewed publications on these topics. In this presentation Dr. Francis explains that these changes in the jet stream are not something that suddenly started this year; they have been observed for the last 30 years, and the changes track quite closely with the warming Arctic and the corresponding decline in Arctic sea ice.
Most scientists are reluctant to pin any particular weather event on climate change. Instead they speak of higher probabilities and frequencies and severity of events. Is the evidence of this change in patterns strong enough to allow scientists to say that these new patterns are a direct result of climate change caused by human activity? Dr. Francis is reluctant to say yes, but she did say “We can’t say that these are extremes are because of climate change but we can say that this kind of pattern is becoming more likely because of climate change.”
Here are links to some of the web sites refered to above or related to the topic .