Albedo is another factor that has had an influence on the spread and retreat of ice sheets and glaciers. When large areas of Earth are covered by ice and snow, large amounts of insolation are reflected off the ice and snow and returned to space. This is a "high" albedo. When large amounts of the Sun's energy are returned to space instead of being used to heat Earth, it lowers the temperatures at Earth's surface. In a high-albedo situation, a positive feedback scenario can be put into motion; the cooler the area becomes, the more snowfall occurs and more ice builds up and spreads.
Conversely, if ice melts, and darker surfaces—either of land or ocean—become exposed, it can lower albedo and begin to heat up the surface of Earth. In this case, as the area heats up, more ice and snow is melted. This chain of events can continue until all the ice melts.
According to NASA, one concern scientists have today is that of ice collecting deposits of soot or other dark pollution particles on it. When this happens, the dark deposits on the ice absorb the incoming sunlight and heat up the area, melting the ice. This has caused widespread melting in several areas in the Arctic. Scientists at NASA are concerned that this could upset the delicate balance of nature in the polar regions. According to Dr. James Hansen, a climate change expert at NASA's GISS, "This provides a positive feedback—as glaciers and ice sheets melt, they tend to get even dirtier." Hansen has discovered that soot's effect on snow albedo may be contributing to trends where thinning Arctic sea ice and melting glaciers and permafrost may be contributing to earlier springs in the Northern Hemisphere. "Black carbon reduces the amount of energy reflected by snow back into space, thus heating the snow surface more than if there were no black carbon," Hansen comments.
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