Snow affects and is affected by Earth’s climate. By reflecting sunlight, it helps keep the Earth at a temperature capable of supporting life as we know it. It is an important source of water for humans and ecosystems, and a lack of water from snow can cause droughts and predispose dry areas to wildland fires.
Scientists have been keeping records of snow extent, volume, and water content for almost 100 years in the United States, and weather stations all over the world have collected weather data including overall precipitation and snowfall for an even longer period of time. Because climate is the pattern of weather in an area over a long period of time, having this type of baseline information is critical for detecting changes in climate on global, regional, and local scales.
Based on long-term measurements and observations, scientists have observed changes in snow patterns in recent years, and these changes can have effects far beyond the locations on Earth in which they occur. Some of these changes are summarized below.
Decreasing snow extent
During the past 30 years, annual snow extent, which means the total amount of the Earth’s surface that is covered by snow, has been decreasing (Brown, 2000; Derksen and Brown, 2012). One result of reduced snow extent is a decrease in the Earth’s albedo, which measures how much incoming radiation from the sun is reflected from or absorbed by the Earth’s surface. With less of the Earth’s surface covered by snow, less radiation is reflected and more is absorbed, causing the planet to become warmer.
Decreasing snow amount
In recent decades, scientists have observed a reduction in the total volume of snow that the Earth receives (Sturm et al., 2015). The total volume of snow factors in snow extent and how much snow there is at the locations where it occurs. Among things, a decrease in snow amount means that there is less snow available for recreation and the tourist industries that depend on it, and it also means that there is less snow available to provide water for agriculture, electricity, human consumption, and sustaining natural ecosystems.
Earlier spring snow melt
Snow has been melting earlier in the spring in recent years (Brown, 2000; Clow, 2010; Easterling et al., 2016; Kunkel et al., 2009-b; Liston and Hiemstra, 2011). With snow covering the ground for a shorter period of time each year, the overall albedo of the Earth decreases, causing the planet to become warmer. Earlier snow melt also means that less water will be available during the summer months for ecosystems and human use downstream.
More precipitation as rain
Some locations are receiving more winter precipitation as rain versus snow now than they did in the past (McCabe et al. 2007, Ye, 2008). In cases where rain falls on snow and then freezes, causing icy layers to form in the snowpack, this can make it more difficult for animals such as caribou to get to small plants and lichen upon which they rely in winter. Because ice and dense snow provide less insulation than fluffier snow, more rain intermixed with snow can reduce the ability of snow to insulate low-growing plants and animals during the cold winter months.
More precipitation arriving as rain than snow can cause an increase in river temperatures, with detrimental effects to cold freshwater species such as salmon. Warmer rivers flow into the ocean and raise ocean temperatures in turn, influencing water currents and contributing to declines in coral reefs.
More precipitation arriving as rain also increases the potential for flooding and drought. The snowpack stores water, and snow melt is usually gradual, with meltwater running into streams and rivers over a period of weeks and months in the spring. In contrast, rain increases the amount of water in streams and rivers over a relatively short period of time – hours or days, which could result in downstream flooding in the short term. In the longer term, more rain and less snow could result in summer droughts, because reservoirs that store water downstream aren’t big enough to store a large amount of water that arrives all at once (as can happen when it rains a lot). If there is too much water at one time for a dam to contain, water must be released even if it cannot be used by people at that time, and therefore, it is not available for people’s needs.