Different features of Earth (such as snow, ice, land, ocean, and clouds) have different albedos—the percentage of solar radiation reflected back into space. For example, land and ocean have low albedos, meaning they absorb more energy than they reflect. Snow, ice, and clouds have high albedos and therefore reflect more energy than they absorb. Overall, Earth's average albedo is about 30 percent; in other words, about 30 percent of incoming solar radiation is reflected back into space, and 70 percent is absorbed.

Earth's radiation budget is a concept that helps us understand how much energy Earth receives from the Sun, and how much energy Earth radiates back to outer space. Simply stated, if more energy than normal is absorbed than reflected, there is an overall warming trend. If more energy than normal is reflected than is absorbed, there is an overall cooling in temperatures. Over the past century, there has been a net warming, which has caused Earth's temperature to increase by about 0.8°C. This is global warming.

An increase in global temperature causes snow and ice to melt, which decreases the extent to which they cover the surface of our planet. This means that there is an overall decrease in Earth's albedo. This decrease in albedo means more energy is absorbed, which causes further warming and in turn causes more melting. This ice-albedo positive feedback loop accelerates change in global temperature and is, therefore, a critical concept to understand when trying to predict global climate change.

Human activities that create pollution influence the energy balance. For example, when we burn coal, oil, wood, and other fuels, the carbon by-product, soot, is released into the atmosphere and eventually deposited back on Earth. The dark particles land on snow and ice, and decrease albedo. Wearing a black shirt (which absorbs radiation) on a sunny day will make you feel warmer than if you wore a white shirt (which reflects radiation). Similarly, the darkened snow and ice absorb more radiation than pure snow and ice. In addition, as the snow and ice melt, the soot embedded in the snow is left behind and becomes more concentrated on the surface, further accelerating warming.




Source: Teachers' Domain, Earth’s Albedo and Global Warming, published January 17, 2008, retrieved on November 4, 2010, http://www.teachersdomain.org/resource/ipy07.sci.ess.watcyc.albedo/