Glaciers

While the effects of glaciers—scouring, till deposits, and rebound—can tell us where they have been in the past. Scientists continue to debate the reasons why ice ages occur, but the consensus view is that several factors interact to produce them: (1) placement by continental drift of large land masses near the poles, on which glaciers can form; (2) uplift of continental plates by plate-tectonic forces, with subsequent changes in global circulations of air and water; (3) reductions in the amount of carbon dioxide in the atmosphere, with diminished greenhouse effect; and (4) long-term oscillations in the shape of the Earth's orbit and the tilt of the its poles.

Present-day glaciers are providing clues to recent and future changes in climate. Satellite radar and aircraft-mounted laser altimetry systems have recently been used to measure contemporary glaciers with great accuracy; the data show that many glaciers are retreating, reflecting an overall global warming trend. The glaciers in the Alps in Europe have lost an estimated one-third to one-half of their ice in the last century, while Alaskan glaciers losing ice thickness at an average rate of about 6 ft (2 m) per year, retreating at rates of 2 mi (3.2 km) in 20 years. By glacial standards, this is a hasty retreat. The U.S. National Academy of Sciences has predicted that, if global temperatures rise from 1.5–5°F (0.75–2.5°C) over the next century as a result of the greenhouse effect, significant portions of the Earth's ice cover could melt. This would result in flooding of every continent's coastlines. Indeed, sea level is already rising. Global average sea level has been rising at about .12 in (3 mm) per year for the last decade, and this rate is expected to accelerate. Alaskan glaciers—which contain for about 13% of the world's glacier area but whose melting accounts for about half of observed sea-level rise—have been thinning twice on average as fast over the last five years as during the preceding 40. In Peru, glacial melting is occurring at exponentially increasing speed; the present rate is 33 times the rate between 1963 and 1978. There is little doubt that global climate change caused by human agricultural and industrial activity is contributing strongly to these effects; data from Antarctic ice cores have shown a direct correlation between warming and cooling trends and the amount of the two major greenhouse gases, carbon dioxide and methane, in the atmosphere. These same cores show significant increases in both gases in the past 200 years. Today, thanks to human activity, atmospheric carbon dioxide is at its highest level in at least 420,000 years.

Glaciers may offer clues about the possibility of life on other planets. In Switzerland, bacteria have been found living under the ice sheets. If microbes can thrive in the dark, cold environment under glaciers, the vast ice sheets that blanket Jupiter's moon Europa and which underlie the soil of Mars may have their own microscopic residents.