The relationship between albedo, solar radiation, and Earth's surface features is important for understanding global temperature changes. Photo courtesy of doraemon/Flickr.
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 (typically about 10 to 40 percent is reflected back into space) and absorb more energy than they reflect, while snow, ice, and clouds have high albedos (typically 70 to 90 percent) and 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. Depending on the balance, Earth may be experiencing a net warming or a net cooling. 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, which then decreases 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.
Aerosols (tiny particles in the air) also alter the amount of radiation absorbed and reflected by the atmosphere. Some aerosols, such as soot, absorb radiation; they have a warming effect. However, light-colored aerosols, such as sulfates, increase the amount of solar radiation that is reflected; they have a cooling effect. Currently there is a partial balance between dark and light aerosols, but their overall effect is similar to what happens after a large volcanic eruption: particles in the air reduce the amount of solar energy that reaches Earth's surface. This effect, known as global dimming, appears to have been masking the full impact of global warming. In the 1990s, as efforts to decrease the amount of air pollution reduced the particulates in the air, studies showed an increase in solar radiation. Ironically, reduced particulate emissions may cause Earth's temperature to rise faster than many earlier models predicted.
The interactive activity on the next page, adapted from NASA and the USGS illustrates the concept of albedo. It explains how the balance between the amount of solar radiation reflected and absorbed by Earth's surface plays an important role in regulating global temperature. You will learn about how Earth's materials (such as snow, ice, and water) differ in their ability to reflect and absorb the sun's energy. You will see how melting polar ice creates a positive feedback loop that accelerates global warming, and you will investigate how the presence of pollution lowers the albedo of ice and further increases melting. Additionally, you will observe the decline in Arctic sea ice cover from 1979-2007 and the effect of melting ice on sea levels.
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/
Last modified: Wednesday, 11 April 2012, 7:28 AM