With a massive East Coast snowstorm forecast for Friday, you may be wondering if the extreme conditions are a sign of climate change.
The answer is complicated. As National Center for Weather Administration director Thomas Karl recently explained, scientists now have the ability to show that some specific weather events - such as temperature extremes - are linked to increases in greenhouse gas emissions linked to human activities. But he said that it's been more difficult to zero in and link other specific weather events to climate change, and this week's big snowfall would fit into that category.
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However, climate scientists say the long-term trend is for climate change to alter weather patterns and cause more severe snowstorms, even as winters become milder overall.
According to a 2014 federal report on climate change impacts in the U.S., since 1950 there's evidence of an increase in both frequency and intensity of winter storms, even as overall annual snowfall has decreased over much of the nation.
You may also be wondering exactly how global warming leads to heavy snowstorms. According to NOAA's Climate.gov website, the explanation seems to be that rising surface temperatures are causing increased evaporation, which in turn puts more water in the atmosphere.
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NOAA says the boost in moisture and increased precipitation can be especially significant for coastal winter storms such as Nor'easters. You might remember that one of those buried Boston under heavy snow in mid-February of 2015.
As this 2014 Slate article by meteorologist Eric Holthaus explains, global warming also puts heat into the atmosphere that weakens the polar vortex, the swirl of high-altitude winds around the north that confines cold Arctic air. That, in turn, can allow cold air masses to slip southward, creating brutal cold weather.
According to a 2009 study, the energy that global warming puts into the atmosphere also causes more extreme shifts of the North Atlantic Oscillation, which basically is the difference between atmospheric pressure in northern Iceland compared to the Azores, a group of islands in the Atlantic. A big change in the NAO can affect the direction and strength of the jet stream, which influences temperature and precipitation patterns across the eastern part of North America, according to Weather.com.
That website also reports that both a weakening of the wind barrier around the Arctic and a shift in the NAO are happening right now, and the combination is driving the expected snowpocalyse this weekend.