Winter Storms

Climate change is fueling an increase in the intensity and snowfall of winter storms. The atmosphere now holds more moisture, and that in turns drives heavier than normal precipitation, including heavier snowfall in the appropriate conditions.1

Heavy snowfall and snowstorm frequency have increased in many northern parts of the United States.2 The heavier-than-normal snowfalls recently observed in the Midwest and Northeast United States are consistent with climate model projections. In contrast, the South and lower Midwest saw reduced snowstorm frequency during the last century.3 Overall snow cover has decreased in the Northern Hemisphere, due in part to higher temperatures that shorten the time snow spends on the ground.

Snowstorms Shift Northward in the Northern Hemisphere

The regional pattern of fewer snowstorms in the southern United States and more in the North corresponds to a similar northward shift of cold-season storms in the entire Northern Hemisphere over the past 50 years. Mid-latitude storms have decreased in frequency (e.g., in the United States overall) while high-latitude storm activity has increased (e.g., in Canada).4 It is likely that human influence contributed to these changes.5

Rapidly Warming Arctic Drives Recent Surprising Cold

In the past few years, unusually warm air in the Arctic has driven winter storm tracks south into the United States, reflecting the complex and sometimes counteracting ways that climate change may affect local weather extremes. In these events, cold-air usually penned in the Arctic by winds known as the polar vortex, broke out and reached the U.S. and Europe due to an erosion of the vortex, an erosion that may have been driven by an abnormally warm Arctic.6

Despite the outbreak of Arctic air, the winters of 2010 and 2011 experienced by the U.S. and Europe were only slightly colder than the average of the winters experienced from 1951-1980, many of which were much colder that the 2010 and 2011 winters. The remaining eight of the last 10 winters experienced by the U.S. were all warmer than the 1951-1980 average for the U.S.7

“Snowmageddon,” United States, February 2010

A combination of changes in El Niño and the Arctic Oscillation (which is closely related to the North Atlantic Oscillation) has been identified as the immediate driver of the famously heavy snowfall experienced by the mid-Atlantic states in the United States during the winter of 2010.8 Global warming may have played a part in this remarkable event by contributing in two ways to the record negative phase of Arctic Oscillation that helped to erode the polar vortex and permitted a cold-air outbreak south to the United States. Evidence suggests that the negative phase of the Arctic Oscillation was driven in part by warm air (air warmed by the dramatic seasonal loss of Arctic sea ice)9 as well as by changes in snow cover over Eurasia driven by climate change.10  This event is part of an emerging trend in which a warming climate may paradoxically bring colder, snowier winters to northern Europe and the eastern United States.11

Lake-effect Snowfall near the Great Lakes, February 2007

In the Midwest, lake-effect snowfall has increased along and near the southern and eastern shores of the Great Lakes since 1950. Lake-effect snow is produced by the strong flow of cold air across large areas of relatively warmer ice-free water. As the climate has warmed, ice coverage on the Great Lakes has fallen. This has created conditions conducive to greater evaporation of moisture and thus heavier snowstorms. Among recent extreme lake-effect snow events was a February 2007 10-day storm total of over 10 feet of snow in western New York state.12

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(Full List of References)

References

  1. Trenberth 2011
  2. Karl et al. 2009
  3. Karl et al. 2009
  4. Karl et al. 2009
  5. Gutowski et al. 2008
  6. Overland 2010, NOAA 2010a, and NOAA 2010b
  7. Hansen 2011
  8. Seager et al. 2010
  9. Overland 2010, NOAA 2010a, and NOAA 2010b
  10. Cohen et al. 2010
  11. Overland 2010, NOAA 2010a, NOAA 2010b, Cohen et al. 2010, and Vavrus et al 2006.
  12. Karl et al. 2009