what We Know: Underlying Processes

The balance between energy absorbed by the earth and energy reflected back into space is fundamental in determining how warm or cool the planet becomes. The proportion of radiation reflected away by a surface is called its albedo. Albedo can range between 0 (no reflectance) and 1 (complete reflectance—like a mirror). The earth’s average albedo is .31, which means that, overall, the planet reflects about 31% of incoming solar radiation back into space. But forests and deserts, oceans, clouds, snow, and ice all have different albedos—and changes in these types of ground cover can therefore affect how much solar radiation the earth receives. For example, the albedos of forests lie in the range 0.07–0.15, while deserts have an albedo of around 0.3.

This satellite image shows Antarctica’s Scott Coastline on January 4, 2002. The large, coke-bottle-shaped iceberg in the lower right broke off the Ross Ice Shelf in December 2001. (See “The Collapse of the Larsen B Ice Shelf” on this site for more information.)

The Dry Valleys are one of the few areas of Antarctica not covered by ice. Unlike much of the rest of the earth, the Dry Valleys have cooled over the last 100 years. (See “The Collapse of the Larsen B Ice Shelf” on this site for more information on the implications of this cooling trend.) The albedo of the earth’s surface varies from about 0.1 for the oceans to 0.6–0.9 for ice and clouds, which means that clouds, snow, and ice are good radiation reflectors while liquid water is not. (This is because clouds, snow, and ice have multiple layers that reflect radiation, whereas a body of water reflects only from its surface. A calm ocean is a poor reflector, but when it foams up in the surfline, producing many reflecting surfaces, it becomes white— reflecting most of the light hitting it.) In fact, snow and ice have the highest albedos of any parts of the earth’s surface: Some parts of the Antarctic reflect up to 90% of incoming solar radiation.

Continued global warming will have one obvious effect on the world’s polar ice, sea ice, glaciers, and permanent snow cover: Warmer temperatures will melt some of this frozen water. Melting of land-based ice sheets and glaciers could contribute to sea-level changes. (Melting sea ice would not contribute to rising sea levels: When ice floating in water melts, the level of the water doesn’t change. You can prove this to yourself by watching the ice melt in a glass of water.) (See “Global Glacier Volume Change” on this site for more on melting glacier.)

These photographs, taken in 1928 and 2000, show how South Cascade Glacier in the Washington Cascade Mountains has retreated over time.

Evidences and Uncertainties

Melting ice could change ocean temperatures. This, in turn, could change the course and speed of ocean currents, significantly change the habitats of sea organisms, and affect rainfall by altering the rate of evaporation of seawater.

Increases in sea levels and temperatures are not the only possible outcomes. When ice and snow melt, they generally expose a much darker underlying surface. Dark surfaces absorb more heat (have a lower albedo) than light surfaces. This suggests the possibility that a small amount of melting could lead to a warmer surface, which could melt more ice, warming the surface still further—initiating the positive feedback loop of a “runaway” warming trend. There is some evidence of such an albedo-reducing effect in the Cretaceous Period (120–65 million years ago): Fossil and other evidence suggests that there was little or no snow and ice cover during this time, and global temperatures then were at least 8° to 10°C higher than they are now. (See “Northern Hemisphere Snow and Ice Chart” and “South Pole/Ice Concentration” on this site to see the extent of current snow and ice cover.)

The cryosphere also provides a way to study past climatic conditions. If snow falls in a region of the earth where melting rarely occurs, it leaves a layered record as it deposits contemporary molecules and aerosols. As each layer is pushed deeper and deeper under increasing pressure, the snow turns to ice, capturing small bubbles of air. By examining ice cores taken from these areas, we can determine associations between past temperature and carbon dioxide levels. But one of the biggest problems in any ice core study is determining the age-depth relationship. Many different approaches have been used, and it’s now clear that fairly accurate time scales can be developed for at least the last 10,000 years. (See “Climate records from the Vostok Ice Core Covering the Last 420,000 years” on this site to learn more about the Antartica's Vostok ice core.)