Ice Albedo Feedback

ALBEDo Is AN estimate of the reflectivity of a surface. The sun produces solar radiation in wavelengths that can be reflected off of many of the Earth's surfaces, some more so than others. The albedo of Earth is estimated as the ratio of the outgoing solar radiation (250-2500 nm.), and the incoming solar radiation. A surface that is perfectly reflective (all incoming light is reflected back again) would have an albedo of 100 percent reflection, or, as a ratio, 1.00. A surface that has no reflectivity would have an albedo of zero percent reflection, and a ratio of zero. On Earth, open water, such as the oceans, has an albedo of 0.05, whereas newly fallen snow has an albedo of 0.90. Ice and snow have an average albedo of 0.80. In the polar regions, this means that approximately 80 percent of the sun's rays are reflected back into the atmosphere and only 20 percent of the ray's transfer energy into melting the snow and ice. As the Earth warms, the amount of snow and ice cover decreases.

As snow and ice are very reflective, this works to keep the polar and high-altitude regions cooler. If, as in the polar regions, particularly the Arctic, approximately 80 percent of the solar radiation is reflected back to the atmosphere, the ice and snow do not melt completely and the permafrost that is located directly beneath the ice and snow is never warmed. If the permafrost remains frozen, the ice and snow on the ground are maintained and the air remains cool. This maintenance of the ice and snow keeps the albedo of the region fairly constant, reflecting most of the solar rays back to the atmosphere, and the ice and snow remain even longer. Scientists believe that this feedback loop that maintains the ice and snow has contributed to ice ages in the Earth's past.

The opposite scenario can also be imagined. If the air temperature at the Earth's surface increases, the ice and snow will melt more quickly. If there is less ice and snow cover at the poles and in the high altitude regions, the albedo will decrease (soil and vegetation are less reflective than ice and snow). If the albedo in a region decreases, more solar radiation will be absorbed by the Earth, which will further increase the surface temperature, melt more of the ice and snow, and perpetually warm the Earth. Scientists believe that global warming drives this ice-albedo feedback loop.

As global warming continues to increase the air temperature at the Earth's surface, continual decreases in the amount of ice and snow covering large areas and the polar regions and high altitudes are expected. If this occurs, the ice and snow may permanently disappear from areas that were previously under snow for the much, if not all of the year. If these areas are exposed, and, specifically, the permafrost is allowed to warm, melting of the permafrost layer will occur. If the permafrost layer were to melt, water drainage into the soil layers would increase. If this occurs, the peat layer beneath the permafrost that never decomposed due to a lack of water, will have vast amounts of water and warmer temperatures, suggesting fast rates of decomposition of the peat. Peat stores carbon, and when it is decomposed, the carbon is released as carbon dioxide (CO2) to the atmosphere. Scientists believe that CO2 release is contributing significantly to global warming. If the peat decomposed and released more CO2 to the atmosphere, even higher temperatures at the Earth's surface would, in turn, melt more ice and snow, and concomitantly release even more CO2 through further decomposition of the peat. The cycle would continue until no region would be permanently covered in ice or snow.

Without the high albedo to reflect the sun's rays, expansive melting of ice and snow may occur. The contribution of melting ice to seawater would have a multitude of effects on global climate. For instance, if only the western Antarctic ice sheet alone melted, there would be a 16-20 ft. (5-6 m.) increase in mean sea level. However, if the entire Antarctic ice sheet were to melt, over the next thousands of years there would be a 184-ft. (56-m.) increase in sea level, causing catastrophic scenarios. Beyond sea-level rise, experts are concerned that the influx of freshwater to the oceans with the melting of the Antarctic Ice Sheet will significantly alter the salinity and temperature of the oceans, causing unpredictable effects on the ecosystems in this region. With decreased salinity, the evaporation rate of the oceans will change and the living organisms that are dependent on a stable aquatic environment may not survive. Changes in the ice-albedo feedback loop will likely have these and countless unpredicted effects on the Earth.

SEE ALSO: Climatic Data, Ice Observations; Glaciers, Retreating; Glaciology; Greenland Ice Sheet; Little Ice Age; Radiation, Ultraviolet; Sea Level, Rising.

BIBLIOGRAPHY. R.B. Alley, The Two Mile Time Machine (Princeton University Press, 2000); C.R. Bentley, R.H. Thomas, and Isabella Velicogna, Ice Sheets (United Nations Environmental Programme, 2007).

Sandra Connelly Rochester Institute of Technology

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