Dr. Holdren: If you've been hearing that extreme cold spells like the one that we're having in the United States now disprove global warming, don't believe it. The fact is that no single weather episode can either prove or disprove global climate change. Climate is the pattern of weather that we observe geographically and over the seasons, and it's described in terms of averages, variations, and probabilities. But a growing body of evidence suggests that the kind of extreme cold being experienced by much of the United States as we speak is a pattern that we can expect to see with increasing frequency as global warming continues. And the reason is this: in the warming world that we're experiencing, the far north, the Arctic, is warming roughly twice as rapidly as the mid-latitudes, such as the United States. That means that the temperature difference between the Arctic and the mid-latitudes is shrinking, and that temperature difference is what drives what is called the circumpolar vortex, which is the great counterclockwise-swirling mass of cold air that hovers over the Arctic.
As the temperature difference between the Arctic and the mid-latitudes declines, the polar vortex weakens, and it becomes wavier. The waviness means that there can be increased, larger excursions of cold air southward — that is, into the mid-latitudes — and, in the other phase of the wave, increased excursions of relatively warmer mid-latitude air into the far north. Computer models tell us that there are many different factors influencing these patterns. And, as in all science, there will be continuing debate about exactly what is happening. But I believe the odds are that we can expect, as a result of global warming, to see more of this pattern of extreme cold in the mid-latitudes and some extreme warm in the far north..