A ‘Once in a Generation’ Storm. What’s the Role of Climate Change?
The winter storm that ravaged much of the United States and Canada through Christmas was expected to be bad, and it was. Forecasters billed it as a event even before ice began coating the steep streets of Seattle, white-out conditions spread from the Plains to the Midwest and more than four feet of snow was dumped on Buffalo in a storm that two dozen people. The links between climate change and much extreme weather are becoming increasingly clear. In a warming planet, heat waves are hotter, droughts are prolonged, summer downpours are more severe. But when it comes to extreme winter weather like the recent storm, the links are less clear, and often the subject of vigorous scientific debate. Winters, like the other seasons, have been getting warmer in the United States and elsewhere. An analysis by Climate Central, an independent research and communications group, found that by more than 3 degrees Fahrenheit (a little less than 2 degrees Celsius) over the past half-century. In most places winter is the fastest-warming season, and the warming is most pronounced in northern areas like the Great Lakes and the Northeast. Those are average temperatures, however. Cold snaps still occur, and the one that began a few days before Christmas was extreme. Meteorologists say it was caused by the , a mass of cold Arctic air that moved southward into Canada and the United States. That air had become very cold sitting over the North Pole at a time of year when no sunlight reaches the region and the Arctic Ocean is frozen over. When it reached into Canada and the United States, the cold air, intruding between masses of warmer air, caused temperatures and air pressure to plunge drastically. The falling temperatures triggered a deep freeze in many locations, and the pressure changes generated strong winds. The winds combined with the clash of air masses into what meteorologists call , a rotating, fierce storm that can bring heavy snow or rain and cause tidal surges in coastal areas. What causes the polar vortex to make southward excursions like this, however, is hotly debated. Some scientists say that rapid warming of the Arctic it has is responsible. As the Arctic warms, they say, the temperature difference between the pole and the tropics lessens. That leads to weakening of the high-altitude winds called the polar jet stream, which becomes more sinuous, or wavier, and allows the polar vortex to spread southward. But many scientists say that while there is some evidence for the idea that Arctic warming weakens the jet stream, there is less to support the concept that this makes for a wavier pattern that allows for the polar vortex to wander. And at least has found that a short-term trend of a wavier jet stream and polar vortex excursions in the 1990s and 2000s, which prompted the development of the idea that Arctic warming affected the polar vortex, has not continued. Research on the subject is continuing. Weve figured out a few strands of this question, said Steve Vavrus, a climate scientist at the University of Wisconsin who was an author of a that reviewed current understanding of the subject in a paper several years ago. I think well continue to do so in coming years as more and more people research it. Outside of the Arctic, warmer winters have many effects that are more certain. Higher winter temperatures in the Western United States, for example, have increased the survivability of tree-destroying insects like pine beetles, contributing to large scale die-offs in forests. Warming also means more precipitation falls as rain, rather than snow, reducing winter snowpack that in many areas is critical for water supplies. When it is cold enough for snow rather than rain, however as it was in many places during the recent storm more snow can fall. Thats because air can hold more moisture when its warmer. Its the cold-weather version of why rainfall is becoming more extreme in many parts of the world. While its difficult to know whether this storm, overall, was snowier than it might have been without human-caused climate change, in some areas snow totals were off the charts. Buffalo received more than 22 inches of snow on Friday, a record, the local National Weather Service office said. Buffalo is in a part of the country thats subject to lake effect snow, which happens when cold, dry air blows across water (in Buffalos case, Lake Erie), picking up moisture that is then dumped as snow when it reaches land. Lake-effect snow occurs when the water is unfrozen. And, generally, the greater the temperature difference between the air and water, the greater the effect. Lake-effect snow is expected to increase as the planet warms, because lakes will be warmer in winter and will remain unfrozen for longer. At some point, however, average winter temperatures in some areas could rise to the point where snow will diminish, and be replaced by lake-effect rain. specializes in the science of climate change and its impacts. He has been writing about science for The Times for more than 20 years and has traveled to the Arctic and Antarctica.