While the Earth's temperature is dependent upon the greenhouse-like action of the atmosphere, the amount of energy retained by the Earth is strongly dependant on the albedo of Earth surfaces.
Just as some clouds reflect solar energy into space, so do light-colored land surfaces. Scientists use the term albedo to define the percentage of solar energy reflected back by a surface. This surface albedo effect strongly influences the absorption of sunlight. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate solar energy differently. Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark soil or rock is strongly absorbed, and contributes to significant heating of the Earth's surface and lower atmosphere.
Understanding local, regional, and global albedo effects is critical to predicting global climate change. Light colored ice and snow are very weakly absorptive, reflecting 80-90% of incoming solar energy. Dark-colored land surfaces, are strongly absorptive and contribute to warming, reflecting only 10-20% of the incoming solar energy. If global temperatures increase, snow and ice cover may shrink. The exposed darker surfaces underneath may absorb more solar radiation, causing further warming. The magnitude of the effect is currently a matter of serious scientific study and debate.
How Much Are Glaciers Melting?
Currently glaciers cover about 10% of the Earth's land surface. In most areas of the world, mountain glaciers are melting. Between 1961 and 1998 small glaciers lost an average of 7 meters of ice thickness. Glaciers in mountainous areas near the equator have been particularly hard-hit. According to global climate models, all of the glaciers in Glacier National Park in Montana will be gone by the year 2030.
Snow and ice cover near the north pole is currently decreasing at approximately 0.4% per year. Arctic sea ice has been decreasing at about 2.9% per decade. Since 1974, seven ice shelves, most in Antarctica have retreated by a total of approximately 13,500 square kilometers.
About the Image
The image used in this activity is of retreating mountain glaciers in Bhutan. It is a satellite image taken by the ASTER instrument aboard NASA's Terra satellite. Visible in the image are several glaciers in the Himalayan mountains of Bhutan. The glaciers have been melting over the past few decades, and lakes have formed on the surfaces and near the termini of many of the glaciers. Some of the glaciers are white as the ice is covered with snow pack. Other parts are rocky and have the same color as the surrounding land.
This image and other satellite pictures can be found at the NASA Earth Observatory web site http://earthobservatory.nasa.gov/.
Other pictures of glaciers and other varieties of snow and ice can be found at the National Snow and Ice Data Center web site http://nsidc.org/.