By Kim Robson:
We hear it every winter from the climate change deniers: “Oh, SURE, how can we have global warming when it’s negative twenty degrees here in Chicago?” This is the main reason why we should stop calling it global warming and start calling it climate change.
All weather is caused by the uneven heating of the earth’s surface. Any pilot will tell you that there’s far more turbulence over plowed fields than over woodlands — the reason is that bare, plowed fields heat up much faster than cool, green forests do. Warm air = unstable air.
As the earth’s temperatures rise, we will see more and more instability in normal weather patterns. This increased instability will lead to stronger and more frequent storms, and less predictability in certain geographical areas. When we see frigid temps in the deep south and bikini weather in Alaska, the cause is instability — specifically, instability of the jet stream.
The jet stream is a powerful current of air that circles the globe, usually near the Arctic Circle, more or less marking the boundary between the colder north and the warmer south. But instability caused by warming temperatures can cause the jet stream to take a dramatic detour, dipping all the way to the southern states. When that happens (and we’ll see it happen more and more), arctic air from Canada comes flowing like a river down to the U.S. And then we see snow as far south as Florida, dangerously bitter cold in the northeast, and warm, dry conditions in the west and northwest. It gets warm in places that should be cold, and cold in places that should be warm. This “polar vortex,” as it’s called, can linger for days on end.
The Yale Forum on Climate Change & the Media, in collaboration with The Grantham Foundation For The Protection of the Environment, produced this short video. In it, Dr. Jeff Masters from Weather Underground explains how and why the jet stream can so drastically affect normal weather patterns in North America.
Another reason these cold spells seem so brutal is that we’re just not used to seeing them anymore. Our short memories and relatively short lifespans mean we forget how things were just a few decades ago. Americans under the age of 28 have never once experienced below average temperatures for more than a month. What seems normal now is actually part of an abnormally long-term warming trend. Long-term normal temperatures would seem unusually cold to people based on their personal experience.
This web comic (XKCD by artist Randall Munroe) perfectly illustrates this point:
Climate scientists are developing informed, scientific opinions about long-term climate change trends by gathering hard data over long periods of time. For instance, even taking recent cold waves into account, December 2013 was still the third-hottest December since 1880, the beginning of weather records.