Friday, January 14, 2011

If the globe is warming, why is it snowing so much?

Georgia and Florida hardly ever get snow, but they got dumped on this week. 49 out of the 50 states, in fact, had snow. What's going on?

Simple answer on the global scale: warmer air means more evaporation from oceans, lakes and rivers. More evaporation means there's more water in the air — so more precipitation (including snow) is to be expected. Snow is just frozen rain.

On local and regional scales, though, the real question isn't "why is it snowing?" but "why is it cold?" Strangely enough, here too the best answer may be "because of global warming."

Here's a very clear explanation by Judah Cohen in the New York Times:
...the overall warming of the atmosphere is actually creating cold-weather extremes. Last winter, too, was exceptionally snowy and cold across the Eastern United States and Eurasia, as were seven of the previous nine winters.

For a more detailed explanation, we must turn our attention to the snow in Siberia.

Annual cycles like El NiƱo/Southern Oscillation, solar variability and global ocean currents cannot account for recent winter cooling. And though it is well documented that the earth’s frozen areas are in retreat, evidence of thinning Arctic sea ice does not explain why the world’s major cities are having colder winters.

But one phenomenon that may be significant is the way in which seasonal snow cover has continued to increase even as other frozen areas are shrinking. In the past two decades, snow cover has expanded across the high latitudes of the Northern Hemisphere, especially in Siberia, just north of a series of exceptionally high mountain ranges, including the Himalayas, the Tien Shan and the Altai.

The high topography of Asia influences the atmosphere in profound ways. The jet stream, a river of fast-flowing air five to seven miles above sea level, bends around Asia’s mountains in a wavelike pattern, much as water in a stream flows around a rock or boulder. The energy from these atmospheric waves, like the energy from a sound wave, propagates both horizontally and vertically.

As global temperatures have warmed and as Arctic sea ice has melted over the past two and a half decades, more moisture has become available to fall as snow over the continents. So the snow cover across Siberia in the fall has steadily increased.

The sun’s energy reflects off the bright white snow and escapes back out to space. As a result, the temperature cools. When snow cover is more abundant in Siberia, it creates an unusually large dome of cold air next to the mountains, and this amplifies the standing waves in the atmosphere, just as a bigger rock in a stream increases the size of the waves of water flowing by.

The increased wave energy in the air spreads both horizontally, around the Northern Hemisphere, and vertically, up into the stratosphere and down toward the earth’s surface. In response, the jet stream, instead of flowing predominantly west to east as usual, meanders more north and south. In winter, this change in flow sends warm air north from the subtropical oceans into Alaska and Greenland, but it also pushes cold air south from the Arctic on the east side of the Rockies. Meanwhile, across Eurasia, cold air from Siberia spills south into East Asia and even southwestward into Europe.

That is why the Eastern United States, Northern Europe and East Asia have experienced extraordinarily snowy and cold winters since the turn of this century. Most forecasts have failed to predict these colder winters, however, because the primary drivers in their models are the oceans, which have been warming even as winters have grown chillier. They have ignored the snow in Siberia.
...It’s all a snow job by nature. The reality is, we’re freezing not in spite of climate change but because of it.
Full article is here.


  1. Dear Professor,
    That last line, very well said. Can we now presume that both the Summer and Winter will reach extremes in the coming years?

  2. Extremes, definitely, in the coming years (plural). That doesn’t mean that any given single year will see more extreme events than we have now.

    Another simple but generally accurate way to see it: "warming" means there's more energy in the system (that's what temperature is - a measure of heat energy). More energy means more motion in the atmosphere as well as more evaporation. That translates into greater extremes than we see today.