The amount of snow and ice that covers the Earth’s land and ocean changes with latitude, altitude, and the seasons. Over the last century average snowfall and global ice cover has decreased in many regions due to rising average temperatures caused by the release of greenhouse gases from human activities. The decrease in snow and ice cover is changing ecosystems and affecting our climate system. The extent of snow and ice cover has also varied greatly in the past, from tens of thousands to millions of years ago, due to a variety of Earth system processes. There have been times when there was no ice at the poles, for example, during the reign of the dinosaur Tyrannosaurus rex, to times when there have been extensive ice sheets, for example during the ice ages.
A variety of human activities and Earth system processes affect the extent of snow and ice cover, including:
- Human activities that release greenhouse gases that cause global warming, such as the burning of fossil fuels, agricultural activities, and deforestation. Increasing average global temperatures has caused the rapid melting of glaciers and ice sheets over the last few decades.
- Increased average global temperatures have altered atmospheric and ocean circulation patterns, changing how heat is distributed across the globe, thus changing the volume of snow and ice. For example, warm ocean currents are increasing the rate of melting at the base of glaciers in Antarctica.
- The burning of fossil fuels and fires, either lit deliberately to remove trees to clear land for agricultural activities or by accident, release airborne particles into the atmosphere that settle on snow and ice. These particles are dark in color and absorb more sunlight than the white snow, increasing surface temperatures and thus the rate and magnitude of seasonal snow and ice melt.
- Over much longer time periods — tens of thousands to hundreds of thousands of years —changes in Earth’s spin, tilt, and orbit, periodically increase and decrease the amount of sunlight absorbed by different areas of the Earth’s surface. This influences regional climate and affects the extent of snow cover and ice sheets.
- Over millions of years changes in the carbon cycle, including the rates of weathering and volcanic activity, alter the amount of carbon dioxide and other greenhouse gases in the atmosphere, which affect Earth’s average temperature, and in turn, the extent of ice sheets.
- Over millions of years changes in the distribution of continents alter the extent of ice sheets, allowing them to expand when continents are located near the poles (if it is already cold enough for ice to accumulate).
Changes in snow and ice cover affect various Earth system processes and phenomena, including:
- Increasing or decreasing the amount of sunlight absorbed by different areas of the surface of the Earth, which affects the climate, including regional temperatures and precipitation patterns. Decreasing snow and ice cover decreases the reflection of sunlight from the Earth’s surface (that is, it decreases the Earth’s albedo). This increases the amount of light absorbed by the water and land, which increases regional temperatures. This can further decrease the extent of snow and ice cover, resulting in a feedback loop. Similarly, feedback can lead to dramatic increases in snow and ice cover. For example, sometimes in the Earth’s past, temperatures have decreased causing increased ice cover, which increased the albedo. This increase in albedo resulted in additional cooling and further ice cover.
- Changing global sea level. The melting of ice on land causes sea level to rise, while increases in the amount of ice on land decreases global sea level. For example, during the last ice age sea level was abut 120 meters (almost 400 feet) lower than it is today. The extent of floating sea ice also changes over time, but this does not affect sea level because the volume of water that floating ice displaces is exactly the same as the volume of the sea water contained in the ice.
- Altering density-driven (thermohaline) patterns of ocean circulation. The large scale-pattern of the Earth’s ocean circulation is driven in part by the sinking of dense water. Thus, if large volumes of freshwater, which is less dense than sea water, enters the ocean due to melting caused by global warming, it can weaken the strength of ocean circulation. This, in turn, can limit the amount of oxygen transported to deeper waters, and alter the strength of surface currents that transport heat across the globe.
- Affecting freshwater availability. The melting of snowpack provides water for many ecosystems and human communities in drier summer months. Thus, as seasonal snowpack decreases and glaciers disappear, water supplies will decrease. Additionally, faster snow melt in the spring can result in flooding events, whereas a slower melting of snow and ice allows the water to soak into the ground.
- The life cycles and traits of organisms that have evolved to live in environments with snow and ice. Changes in the local environment can directly alter the conditions organisms need to survive or change the timing of environmental cues that plants and animals use to transition between life stages. For example, seal species give birth and raise their young on sea ice, but this habitat is disappearing as the world warms, resulting in deceases in seal populations. Additionally, if the melting of snow begins earlier in the season it causes problems for flowering plants by causing them to bloom earlier, before their pollinators have migrated into the areas where they live.
- The distribution of ecosystems, which affects where organisms live (species ranges) and how species migrate and interact.
Can you think of additional cause and effect relationships between snow and ice cover and other parts of the Earth system?