Ground ice is found in cavities, voids, pores, and other openings in frozen or freezing ground. It is a feature of a permafrost region, but the two terms are not interchangeable. A permafrost region is defined as an area where the soil and rocks making up the land remain below the freezing point for two or more consecutive years and can include areas with little or no water content. Permafrost regions account for nearly one-quarter of the landmass of the Northern Hemisphere, and some of these areas have been frozen since the Pleistocene ice age.

Ground ice differs from glacier ice as well. Glaciers are created when fallen snow compresses into thick ice masses over long periods of time, and they are capable of riverlike movement, albeit at a pace so slow as to be imperceptible to the human eye. Ground ice is formed once the temperature drops below 0œ Celsius, when most of the moisture in the soil freezes. There are two different types of ground ice: structure-forming ice and pure ice. Structure-forming ice, which holds sediment together, comes in multiple varieties, including ice crystals, intrusive ice, reticulate vein ice, segregated ice, and icy coatings on soil particles. Pure, or massive, ice exists primarily toward the surface and is found as ice wedges, massive icy beds, and pingo cores. Massive ice beds are those with a minimum ice-to-soil ratio of 2.5 to 1.

Ancient ground ice, known as fossil ice, is a valuable source of historical information for geologists, paleontologists, and climatologists. Fossil ice preserves organic material and provides a measurable history of sediment and air quality covering hundreds of thousands of years. Cores drilled from fossil ice are studied to learn about the climates of the distant past.

In 2002, the National Aeronautics and Space Administration reported that the Odyssey orbiter had identified ground ice on Mars, a find that was confirmed by the Phoenix lander in June, 2008. The extreme cold and thin atmosphere on Mars would cause surface water to vaporize as dry ice does on Earth, but ground ice buried under the surface remains frozen and should prove invaluable to studies of the planet's history.

Ground ice and permafrost play crucial roles in the industrial development of a region's energy and mining industries, as well as in the infrastructure needed to support such enterprises. It is possible to build in the presence of ground ice in its structureforming state, so long as its properties are taken into consideration.

The presence of ground ice is closely linked to a region's climate, especially to the temperature at ground level. Its impact varies based on a variety of factors, including drainage, snow cover, soil composition, and vegetation. Ground ice affects both topography and vegetation, as well as a region's response to changes in climate and population.

In regions where the temperature remains close to freezing, a change of only a few degrees can be enough to cause ground ice to start melting. Changes can occur naturally, with normal temperature fluctuations, or as a result of ground clearing through construction or forest fire. When structureforming ground ice melts, the surrounding terrain is significantly weakened, creating slope instabilities and thaw settlement.

Scientists predict that as much as 90 percent of the permafrost in the Northern Hemisphere could melt by the end of the twenty-first century, with most of the thaw taking place in the top 3 meters of terrain. Some regions in Alaska and Siberia have already experienced collapsed infrastructure and increased rock fall in the high elevations. The resulting meltwater would eventually reach the oceans, causing sea levels to rise around the world. Since the 1930's, Arctic water runoff has increased by 7 percent, and it is projected to reach a 28 percent increase by 2100.

Permafrost traps and holds approximately 30 percent of Earth's carbon, sequestering it from the atmosphere. Melting ground ice will release the trapped carbon into the atmosphere, contributing to the greenhouse effect, which in turn will cause even more ice to melt.

 Coastlines in permafrost regions become more vulnerable to erosion as the ice retreats. The Alaskan island of Shishmaref, home to a native population whose ancestors have lived there for more than four thousand years, lost 7 meters of coastline annually between 2001 and 2006.

Bibliography:

1) Alley, Richard B. The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton, N.J.: Princeton University Press, 2000.

2) Davis, Neil. Permafrost: A Guide to Frozen Ground in Transition. Fairbanks: University of Alaska Press, 2001.

3) Gosnell, Mariana. Ice: The Nature, the History, and the Uses of an Astonishing Substance. Chicago: University of Chicago Press, 2007.

4) MacDougall, Doug. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press, 2004.

5) Pielou, E. C. After the Ice Age: The Return of Life to Glaciated North America. Chicago: University of Chicago Press, 1991.

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