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Climate Change

Climate Change Indicators in the United States

Key Points
  • When averaged over the entire year, snow covered an average of 3.25 million square miles of North America during the period from 1972 to 2013 (see Figure 1).
  • The extent of snow cover has varied from year to year. The average area covered by snow has ranged from 3.0 million to 3.6 million square miles, with the minimum value occurring in 1998 and the maximum in 1978 (see Figure 1).
  • Between 1972 and 2013, the average extent of North American snow cover decreased at a rate of about 3,500 square miles per year. The average area covered by snow during the most recent decade (2004–2013) was 3.21 million square miles, which is about 4 percent smaller than the average extent during the first 10 years of measurement (1972–1981)—a difference of 120,000 square miles, or approximately an area the size of New Mexico (see Figure 1).
  • Decreases in snow cover have largely occurred in spring and summer, whereas fall and winter snow cover have remained fairly steady over the time period studied (see Figure 2). Spring and summer snow cover can have a particularly important influence on water supplies.

Snow cover refers to the amount of land covered by snow at any given time. Naturally, it is influenced by the amount of precipitation that falls as snow. Air temperature also plays a role because it determines whether precipitation falls as snow or rain, and it affects the rate at which snow on the ground will melt. As temperature and precipitation patterns change, so can the overall area covered by snow.

Snow cover is not just something that is affected by climate change; it also exerts an influence on climate. Because snow is white, it only absorbs a small portion of the sunlight that hits it (10 to 20 percent in the case of fresh snow), and it reflects the rest back to space. In contrast, darker surfaces such as bare ground and open water absorb the majority of the energy they receive and heat up more quickly. In this way, the overall amount of snow cover affects patterns of heating and cooling over the Earth's surface. More snow means more energy reflects back to space, resulting in cooling, while less snow cover means more energy is absorbed at the Earth's surface, resulting in warming.

On a more local scale, snow cover is important for many plants and animals. For example, some plants rely on a protective blanket of snow to insulate them from sub-freezing winter temperatures. Humans and ecosystems also rely on snowmelt to replenish streams and ground water.

About the Indicator

This indicator tracks the total area covered by snow across all of North America (not including Greenland) since 1972. It is based on maps generated by analyzing satellite images collected by the National Oceanic and Atmospheric Administration. The indicator was created by analyzing each weekly map to determine the extent of snow cover, then averaging the weekly observations together to get a value for each year. Average snow cover was also calculated for each season: spring (defined as March–May), summer (June–August), fall (September–November), and winter (December–February).

Indicator Notes

Although satellite-based snow cover maps are available starting in the mid-1960s, some of the early years are missing data from several weeks during the summer, which would lead to an inaccurate annual average. Thus, the indicator is restricted to 1972 and later, with all years having a full set of data.

Data Sources

The data for this indicator were provided by the Rutgers University Global Snow Lab, which posts data online at: http://climate.rutgers.edu/
. The data are based on measurements collected by the National Oceanic and Atmospheric Administration's National Environmental Satellite, Data, and Information Service at: www.nesdis.noaa.gov.

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