Introduction: Background: Ice
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 is. The fraction 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).
Earth’s average albedo is 0.31, which means that, overall, the planet reflects about 31% of incoming solar radiation back into space. But forests, deserts, oceans, clouds, snow, and ice all have different albedos—so changes in these types of ground cover can affect how much solar radiation the Earth absorbs. For example, the albedos of various types 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 Breakup of the Larsen B Ice Shelf to see how an ice shelf disintegrates.)
The albedo of Earth’s surface varies from about 0.1 for the oceans to 0.6–0.9 for ice and clouds—meaning 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. Snow and ice have the highest albedos of any parts of Earth’s surface: Some parts of Antarctica reflect up to 90% of incoming solar radiation.
The Dry Valleys are one of the few areas of Antarctica not covered by ice.
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. The melting of ice sheets and glaciers on land is already contributing to sea-level rise. (Melting sea ice does 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.)
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 cause ocean temperatures to rise. This, in turn, could change the course and speed of ocean currents, alter the habitats of ocean organisms, and increase rainfall by boosting the rate of seawater evaporation.
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—thus initiating a runaway positive feedback loop of global warming. There's 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.
Ice also provides a way to study past climate conditions. If snow falls in a region of the Earth where melting rarely occurs, like Antarctica, 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.