State of the Climate: 2011 Snow Cover in Northern Hemisphere
Most of Earth’s land area is concentrated in the Northern Hemisphere, and the area covered by snow each year in the Northern Hemisphere is an important indicator of the state of the climate. Snow influences soil moisture and spring runoff, and in very cold climates, it insulates the roots of plants and trees over the winter, protecting them from repeated destructive freezes.
In 2011, annual snow cover extent over Northern Hemisphere continents (including the Greenland ice sheet) averaged 24.7 million square kilometers, which is 0.3 million square kilometers less than the long-term average, ranking 2011 as having the 17th smallest snow extent on record. The map above shows snow cover anomalies in the Northern Hemisphere 2011 compared to the long term average (1981-2010), based on the number of days in the year where a location was snow covered.
The map shows a somewhat surprising pattern: higher latitudes around the rim of the Arctic Circle were less snow-covered than average, while areas farther south were more snow-covered than average. northern Canada, Alaska, and Russia all sported shades of brown, while the north-central United States, Mongolia, and Kazakhstan were all shaded with blue.
Like mountain glaciers, the annual average snow cover extent during the cold season is shrinking. Although snow extent and depth can naturally vary from place to place and winter to winter, a widespread, consistent decrease over time is a clear sign that the climate is getting warmer.
The graph above shows Northern Hemisphere monthly average snow extents for the past three decades compared the long term average. Most of the highest peaks above the line—times of greater than average snow cover—occurred in the earliest part of the time series; most of the lowest valleys below the line—times of less than average snow extent—have occurred in recent decades.
NOAA map by Dan Pisut, Environmental Visualization Lab, based on data from the Rutgers University Snow Lab. Graph adapted from Figure 2.13 in the BAMS’ State of the Climate in 2011. Caption by Caitlyn Kennedy. Reviewed by Jessica Blunden and Deke Arndt, Climate Monitoring Branch, National Climatic Data Center.
Robinson, D.A.: 2012: [Global climate] Northern Hemisphere continental snow cover extent [in “State of the Climate in 2011”]. Bull. Amer. Meteor. Soc., 93 (7), S21–S22.