On December 19, Washington, D.C. was buried by two feet of snow, setting a new record for snowfall during the entire month of December and paralyzing the city for three days. As my neighbor and I shoveled out from the storm, he stopped for a moment, grinned, and asked, “So what happened to global warming?” Boy was he surprised when I said, “Glad you asked,” and launched into a 15-minute oratory on why global warming might mean more, not less, extreme snowfall for some parts of the world.
Before continuing, I need to reiterate that no single weather event can be attributed to global warming . So the question here is not, “What caused the heavy snowfall on December 19?” The question is, “Is heavy snowfall or unusually cold weather inconsistent with global warming?”
You need two things to create heavy snowfall: moist air and cold air. The two generally don’t occur in the same air mass because cold air can’t hold much moisture. So you need two air masses, one that is warm and moist and one that is cold and dry, to collide with each other. That is exactly what happened over the Mid-Atlantic region on December 19.
This condition is not only consistent with global warming, but it can be expected to occur more frequently in certain places as a direct result of global warming. It takes warmth to generate moist air. First, you need warmth to evaporate enough water from lakes or oceans to generate a massive snowstorm. Second, you need warm air to keep the water vapor aloft so that it doesn’t rain out before it finds a cold air mass to collide with. When I asked weatherman Joe Witte  where the moisture that ended up in my snow shovel came from, he said, “Some of the moisture came out of the Gulf of Mexico AND some from the warm Atlantic ocean with the VERY warm (70s!) Gulf Stream along the East coast acting as a hot plate for evaporation of moisture into the cold dry air.”
|(SOURCE: NOAA polar-orbiting satellite data compiled by Rutgers University Coastal Ocean Observation Lab )|
Although the past few weeks have been very cold in the eastern United States, Joe pointed me to NOAA satellite measurements that found sea surface temperatures in the Gulf of Mexico and the Atlantic Ocean to be 1 to 3 °F warmer than normal during the week before the big snowstorm hit (see figures above). There is strong scientific evidence  showing that, on average, the oceans are warmer today than they were a century ago because of human-induced global warming. So the warm ocean temperatures that fed the heavy snowfall are consistent with global warming. In fact, because of global warming, we should expect such conditions to be more common today than in the past and even more common in the future as warming continues.
So where did the cold, dry air come from? Global warming is about changes in long-term averages and not about single events; it does not mean an end to cold weather. Instead, it means that cold weather will become less frequent and hot weather more frequent when averaged over decades. In fact, both of these trends have been observed over the past 50 years in the United States  and globally . So, even with global warming we will have cold winters, just fewer of them. It is also important to remember that a cold winter here doesn’t mean a cold winter everywhere. In fact, many parts of the world , including the Arctic and the tropics, are having an unusually warm winter. The current cold snap is concentrated in the mid-latitudes of the northern hemisphere, and there will always be the potential for cold Arctic air masses to visit the mid-latitudes from time to time.
The current cold snap is related to a known weather pattern called the Arctic Oscillation . When the Arctic Oscillation switches between “positive” and “negative” states, it simply shifts heat between the Arctic and the mid-latitudes. Scientists call this kind of pattern “internal variability,” and it does not change the total amount of heat in the climate system. Internal variability can create strong differences in the weather from year to year and place to place, but these shifts average out to zero net climate change over decades. Only a net change in the total amount of heat in the climate system can change the long-term average climate, and that is the nature of global warming.
When the mid-latitudes get periodic blasts of cold Arctic air, global warming makes it more likely that the cold air from up north will collide with moist, warm air from down south, creating more heavy snowfall events in mid-latitude areas near large bodies of water. A similar phenomenon is affecting the Great Lakes region. Syracuse, New York is one of the snowiest places in the country, but it and other areas around the Great Lakes are getting even snowier ! Because the Great Lakes are getting warmer, they are icing over later and melting earlier than they used to. Without the ice, water can evaporate and enter the atmosphere over the lakes later in the fall and earlier in the spring. When winds blow this moist air over the land where temperatures are lower, we get the famous “lake effect” snow. With more open water during the winter, more lake effect snow is falling.
These are the cold facts of global warming.
Jay Gulledge is Senior Scientist and Program Manager for Science & Impacts