Cores are also taken from the ice sheets covering Greenland and Antarctica (see the sidebar "Vostok, GISP, and GRIP on page 41). Polar ice sheets are made from compacted snow—the ice is not made by freezing water, the way you make ice cubes. It is never warm enough for snow to melt in either Greenland or Antarctica, so each year's snow lies on top of the snow that fell in previous years. The weight of all that snow compresses the lower layers, packing the snowflakes tightly together. Cores drilled from the ice retain marks, rather like tree rings, that indicate each year's snowfall. They are not so clear or reliable as tree rings, so it is difficult to date samples to the year, but there are approximate dates for ice-core samples up to 200,000 years old.
Ice cores contain dust that fell with the snow. Strong winds raised the dust over the continents and carried it all the way to the ice sheet. Rain would have washed it from the air to the ground before it reached the ice sheet, so the amount of dust present in an ice-core sample is related to the amount of precipitation in higher latitudes at the time when the sample formed. Samples that contain a large amount of dust are made from snow that fell when the world climate was dry. If climates were dry, less water must have been evaporating from the surface and then condensing to form clouds. Less evaporation suggests the climate was colder. So large amounts of dust mean the climate was cold, and if the ice contains little or no dust, the climate was warm and wet.
The ice also contains biological material, such as spores and pollen grains, and volcanic particles and gases. All of these "impurities" in the ice help scientists to compile an overall picture of conditions in the remote past.
Vostok, GISP, and GRIP
Vostok is the name (the word means "east") of a elevation of 11,401 feet (3,475 m), on the surface of Russian research station in Antarctica, at the geo- the East Antarctic ice sheet. The station was opened magnetic South Pole, 78.46° S, 106.87° E, and at an on December 16, 1957. Work began in 1980 on
Summit, Greenland. Summit is the highest point on the Greenland ice sheet. It is the site of the GISP and GRIP projects.
drilling through the ice sheet at a point near the station that is 11,444 feet (3,488 m) above sea level. A core of ice was removed from the borehole. In 1985 drilling reached a depth of 7,225 feet (2,202 m). It was impossible to drill this hole deeper, but drilling of a second hole began in 1984. In 1989, this became a joint Russian-French-U.S. project. In 1990 the hole (and core) reached a final depth of 8,353 feet (2,546 m). A third hole, started in 1990, reached 11,887 feet (3,623 m) in 1998.
The Vostok ice cores contain a record of climate that goes back about 420,000 years. So far, analysis has revealed the record over the last 200,000 years.
The Greenland Ice Sheet Project (GISP) is a U.S. program, sponsored by the National Science Foundation (NSF), to retrieve ice cores from the Greenland ice sheet. The first core reached bedrock at a depth of about 9,843 feet (3,000 m), and in 1988 the NSF Office of Polar Programs authorized the drilling of a second hole, GISP2. This was completed on July 1, 1993, after the drill had penetrated five feet (1.55 m) into the underlying bedrock. The ice core was 10,018.34 feet (3,053.44 m) long. Ice at the base of the core is about 200,000 years old, and analysis of the GISP2 core has yielded a detailed record of climate over more than 110,000 years.
The Greenland Ice Core Project (GRIP) is a European program organized through the European Science Foundation and funded by the European Union and Belgium, Denmark, France, Germany, Iceland, Italy, Switzerland, and the United Kingdom. Drilling began in January 1989, and on August 12, 1992, it reached bedrock at 9,938 feet (3,029 m), where the ice is about 200,000 years old.
GISP2 and GRIP are located close to Summit, the highest point on the Greenland ice sheet (chosen to provide the longest cores), at 72.6° N 38.5° W. The map shows its location.
Freshly fallen snow lies loosely, with many air spaces between snowflakes and grains. When the snow is compacted into ice, some of these air spaces remain as tiny bubbles in the ice. It is possible to extract the air from these bubbles by melting the ice under carefully controlled conditions. Analysis of the air reveals its composition. Climate scientists are especially interested in the amount of carbon dioxide and methane it contains. These are "greenhouse" gases that affect air temperature.
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