SOTC: Northern Hemisphere Snow
We all associate snowstorms with cold weather, but snow's influence on the weather and climate continues long after the storm ends. Because snow is highly reflective, a vast amount of sunlight that hits the snow is reflected back into space instead of warming the planet. Without snow cover, the ground absorbs about four to six times more of the Sun's energy. The presence or absence of snow controls patterns of heating and cooling over Earth's land surface more than any other single land surface feature.
In many locations in recent decades, temperatures have risen while precipitation levels have remained largely the same. Satellite data have confirmed that average snow cover has decreased, especially in the spring and summer. Where snow cover is disappearing earlier in the spring, the large amounts of energy that would have melted the snow can now directly warm the soil.
In terms of spatial extent, seasonal snow cover is the largest single component of the cryosphere and has a mean winter maximum areal extent of 47 million square kilometers, about 98 percent of which is located in the Northern Hemisphere.
Snow cover is an important climate change variable because of its influence on energy and moisture budgets. Snow cover accounts for the large differences between summer and winter land surface albedo, both annually and inter-annually. Snow may reflect as much as 80 to 90 percent of the incoming solar energy, whereas a snow-free surface such as soil or vegetation may reflect only 10 to 20 percent. A warming trend results in decreased snow cover. With the resulting decrease in reflected energy, absorption of solar radiation increases, adding heat to the system, thereby causing even more snow to melt. This is the classic temperature-albedo feedback mechanism; it is a "positive feedback" because it reinforces itself. Surface temperature is highly dependent on the presence or absence of snow cover, and temperature trends have been linked to changes in snow cover (Groisman et al. 1994, Brown and Robinson 2011, Peng et al. 2013).
In addition to the albedo effect, snow cover represents a significant heat sink during the melt period of the seasonal cycle due to a relatively high latent heat of fusion. As a result, the seasonal snow cover provides a major source of thermal inertia within the total climate system, as it consumes large amounts of energy with little or no fluctuation in temperature as snow crystals melt.
During the past four decades, satellite remote sensing has provided valuable information on hemispheric-scale snow extent. Since 1966, the National Oceanic and Atmospheric Administration (NOAA) has produced weekly snow extent maps for Northern Hemisphere land surfaces using visible-band satellite imagery (Robinson and Frei 2000, Rutgers 2017). Because snow has such a high albedo compared to other surfaces on Earth, snow-covered areas appear much brighter in satellite imagery than most other surface types.
Remote sensing data sets from the microwave portion of the electromagnetic spectrum can also be used to derive snow cover maps, with the added benefit of being able to "see" through clouds. When snow covers the ground, some of the microwave energy emitted by the underlying soil is scattered by the snow grains; therefore, when moving from snow-free to snow-covered land surfaces, a sharp decrease in emissivity indicates the presence of dry snow.
These remote sensing data sets are derived using different types of analyses and separate regions of the electromagnetic spectrum, yet their results are strikingly similar. Both visible and passive microwave data sets show similar patterns of inter-annual variability, and both consistently indicate maximum snow extent that exceeds 40 million square kilometers for the Northern Hemisphere.
In the Northern Hemisphere spring (April through June), snow cover extent is mainly over the Arctic, where snow blankets the ground for up to nine months a year. The timing of springtime snow melt is particularly important in terms of spring river runoff, permafrost thaw, and the length of the growing season. Springtime snow cover extent has historically fluctuated over three- or four-year cycles, but recent observations have shown long-term snow extent declines. A study published in 2012 found an overall drop in snow cover after 1967, with an acceleration of the decline rate after 2003 (Derksen and Brown 2012).
Recent years have shown a decreasing trend in Northern Hemisphere June snow cover extent, with most June extents since 2000 falling below the 1981-2010 average.
Long-term Northern Hemisphere snow trends are somewhat mixed, but overall show decreases in metrics such as snow cover extent, snow water equivalent, and snow depth (Kunkel et al. 2016). Findings include a negative trend in maximum seasonal snow depth between the winter of 1960-1961 and the winter of 2014-2015. The trend is especially strong in North America, and apparent to a lesser extent in European stations discussed in the study. The authors report, "These results are mostly, but not fully, consistent with simple hypotheses for the effects of global warming on snow characteristics."
Snow totals and snow extents measured in 2015 and 2016 fit the larger pattern of declining trends. In the winter of 2015-2016, most parts of the contiguous United States showed below-average seasonal totals. The one exception was the mid-Atlantic, which received most of its snow in a single blizzard in late January 2016 (Robinson 2016a). Throughout the Northern Hemisphere, 2016 snow cover extent ranked 12th lowest out of the 47-year average (Robinson 2016b).
Declining snow extents are consistent with expectations for a warming climate, but another factor may also be at work. Aerosols such as dust and soot accelerate snowmelt by reducing albedo. In the San Juan Mountains of the southwestern United States, disturbed desert dust on the snow surface has affected seasonal snowpack duration. Precipitation was near average on the Colorado Plateau in 2005; the following year, the plateau experienced severe drought, and the resulting dust deposition shortened the snow-cover duration by 18 to 35 days (Painter et al. 2007). Dust loading on the Colorado River Basin snowpack has increased fivefold since the mid-19th century, driven largely by increased land use and abuse, new dirt roadways, and agricultural practices in the region. By accelerating snowmelt, dust deposition has a significant effect on river runoff and water availability for seven U.S. states and parts of Mexico, which depend on water from the Colorado River (Skiles et al. 2015).
Last updated: 23 February 2017