Unusually cold conditions bring out the climate change deniers, who ask how it can be so chilly if the world is warming up. Of course, a few days of inclement weather hardly refute a global trend, but according to one theory, higher temperatures might actually be causing more extreme cold bouts at northern latitudes in winter. The idea, once thought to be pretty out there, has just got more backing.
As the world has warmed, the Arctic Ocean has experienced one of the largest increases in temperature of anywhere on Earth – just as climate models predicted. This has led to a dramatic loss of sea ice, which normally forms an insulating barrier on the top of the ocean. Without this insulation, more heat reaches the atmosphere, causing warming that spans all the way to the stratosphere.
Nevertheless, the stratosphere is very, very cold, particularly over the pole. Normally that does not really affect the rest of the world. “In winter, the freezing Arctic air is normally ‘locked’ by strong circumpolar winds several tens of kilometers high in the atmosphere, known as the stratospheric polar vortex, so that the cold air is confined near the pole,” said Marlene Kretschmer, a graduate student at the Potsdam Institute for Climate Impact Research, in a statement.
However, Kreschmer is the first author of a paper in the Bulletin of the American Meteorological Society, showing that extra warmth rising from the polar ocean has frequently weakened the stratospheric polar vortex. This has allowed blasts of bitterly cold air to sweep down on Europe and Russia, which she argues is the reason why these areas have had so many cold Januaries and Februaries in recent years.
In Western Siberia, it has happened so frequently that winter temperatures have actually decreased, while the rest of the world has warmed. Other northern regions, notably North America, are still experiencing a warming trend, but one punctuated by bouts of exceptionally cold temperatures.
“Several types of weather extremes are on the rise with climate change, and our study adds evidence that this can also include cold spells, which is an unpleasant surprise for these regions,” said co-author Dr Judah Cohen of MIT.
The idea that global climate change can lead to heavier mid-winter snowstorms is well-established as increased moisture in the air means both more rain and more snow. However, more intense bouts of cold are much trickier to confirm, and the Washington Post reports that some climate scientists are skeptical of Kretschmer's conclusions.
The question is further complicated because there are actually two phenomena known as polar vortexes, one in the stratosphere and one in the troposphere (lower atmosphere). These can interact but are separate, and have differing effects.