Glaciers and Ice Sheets
Ice sheets and glaciers form the largest component of perennial ice on Earth. Over 75% of the world's fresh water is presently locked up in these frozen reservoirs.
A Glacier is any large mass of perennial ice that originates on land by the recrystallization of snow or other forms of solid precipitation and that shows evidence of past or present flow. A glacier occupying an extensive tract of relatively level land and exhibiting flow from the center outward is commonly called an ice sheet. Glaciers form when snow accumulates on a patch of land over tens to hundreds of years. The snow eventually becomes so thick that it collapses under its own weight and forms dense glacial ice. When enough of the ice is compacted together it succumbs to gravity and begins to flow downhill or spread out across flat lands. What makes glaciers unique is their ability to move. Due to sheer mass, glaciers flow like very slow rivers.
More than 90 percent of the 33 million cubic kilometers of glacier ice in the world is locked up in the gigantic Greenland and Antarctic ice sheets.
Ice shelves occur when ice sheets extend over the sea, floating on the water. In thickness they range from a few hundred meters to over 1000 meters. Ice shelves surround nearly all of the Antarctic continent. Retreating ice shelves may provide indications of climate change.
Ice caps are miniature ice sheets. An ice cap covers less than 50,000 square kilometers. They form primarily in polar and sub-polar regions that are relatively flat and high in elevation.
"We have long predicted that the first signs of changes caused by global warming would appear at the few fragile, high-altitude ice caps and glaciers within the tropics," the band extending from 30 degrees North to 30 degrees South. "These findings confirm those predictions," Thompson said.
For Kilimanjaro, four-fifths of the vast ice field that covered the top of the highest mountain in Africa has disappeared in the last 80 years. "At this rate, all of the ice will be gone between the years 2010 and 2020. "And that is probably a conservative estimate," he said.
African officials worry that the loss of the ice cap atop Kilimanjiro will be devastating to the thriving tourist trade that brings thousands of people to the mountain each year and fuels the country's economy. But for Quelccaya in Peru - and similar ice caps and glaciers in the Andes - the loss represents a much greater threat than lost tourism dollars.
"The loss of these frozen reservoirs threaten water resources for hydroelectric power production in the region, and for crop irrigation and municipal water supplies," he said. The ice in the high-altitude glaciers represents a "bank account" of sorts to feed their power needs. With the melting ice caps, streams have grown and the government is building new dams and hydroelectric plants.
Thompson said that other researchers have documented similar ice losses. An ice cap on Mount Kenya has shrunk by 40 percent since 1963. Two glaciers atop mountains in New Guinea are disappearing and should be gone in a decade. And in Venezuela in 1972, there were six such glaciers - now there are only two left and they will have melted in the next 10 years.
What's happening to these glaciers?
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