SIDEBAR 1-7 Ice Core Reuse
Ice cores are cylindrical sub-surface samples of glacier ice. These samples have been collected from the Antarctic and Greenland ice sheets since the early 1960s. Most U.S. cores are housed at the National Ice Core Laboratory at the Denver Federal Center in Lakewood, Colorado (see Sidebar 2-11). An important characteristic of ice cores is that they contain old air (Alley, 2000)—air trapped when the ice formed many years earlier. The deeper the origin of the core, the older the air. Near the base of the Antarctic Ice Sheet, at depths of more than 4 kilometers (2.49 miles), the trapped air bubbles are older than 400,000 years.
This old air is currently of great societal and scientific interest because it carries a record of past levels of atmospheric CO2. For example, a central piece of information in the global-warming debate is the comparative magnitude of pre-industrial atmospheric CO2 levels and modern values. Looking further back in time, the variation of CO2 through glacial cycles (each cycle lasting about 100,000 years) gives clues to driving forces behind global climate change, and whether or not industrialization has affected any of these driving forces.
Techniques to measure CO2 from bubbles within ice cores were developed in the early 1980s—a decade after the original long cores were collected at Camp Century, Greenland (1963–1966), and Byrd Station, Antarctica (1968). Fortunately, the cores were preserved in anticipation of improved analytical techniques. The results revealed atmospheric CO2 levels for the last 30,000 to 50,000 years for the first time. Levels of CO2 in northern and southern hemispheres during the last glaciation were shown to be roughly half the modern values. Large changes in the biosphere were likely responsible for the substantially reduced CO2 levels during the last glaciation (Bradley, 1985). Such large changes in a gas so important in the global energy balance have profound implications for hypotheses of climate change.
Since the initial discovery of atmospheric CO2 levels during the last glacial period, the measurement of CO2 concentration has become routine on new ice cores. In the case of the Vostok Core from Antarctica, the record of CO2 has now been extracted back to 420,000 years before present. In general, modern ice cores provide a substantial amount of paleoclimatic information. The existence of the National Ice Core Laboratory (NICL) is a reflection of the value now placed on ice-core data and the understanding that potential discoveries await in existing cores. This facility is jointly operated by the USGS and NSF’s Office of Polar Programs. Occasionally, NICL offers old cores for destructive analysis if, for example, duplicate cores exist. This practice allows scientists to develop new techniques, such as those to analyze CO2 levels, without fear of wasting unique or expensive samples. Ice-core science thus progresses even from cores that have no further life at the repository.
Scientist cleaning a piece of ice core in the cold clean room. SOURCE: Geoffrey Hargreaves, NICL.
Close-up of an ice core from the Greenland Ice Sheet Project. After an ice core is cleaned, it is sawed in half lengthwise to reveal features like these seen here. The bands are formed by individual years of snow accumulation; the core in this photograph contains a sampling of 4 or 5 years of ancient atmospheric conditions. The core was collected from a depth of 1,850 meters (6,070 feet). The age of ice at that depth is approximately 16,750 years. The width of the ice in the picture is 5.2 inches. SOURCE: Geoffrey Hargreaves, NICL.