PALEOCLIMATOLOGY

Twin Ice Cores From Greenland Reveal History of Climate Change, More

Locked within two cores of ancient ice is evidence of unprecedented swings in Earth's climate throughout the ages. These icy archives tell us that large, rapid, global change is more the norm for the Earth's climate than is stasis.

by R. Alley, The Pennsylvania State University, University Park; P. Mayewski, University of New Hampshire, Durham; D. Peel, British Antarctic Survey, Cambridge, England; and B. Stauffer, University of Bern, Bern, Switzerland

Two projects conducted from 1989 to 1993 collected parallel ice cores just 30 kilometers apart from the central part of the Greenland ice sheet. Each core is more than 3 kilometers deep and extends back 110,000 years. In short, the ice cores tell a clear story: humans came of age agriculturally and industrially during the most stable climatic regime recorded in the cores. They also indicate that today, Greenland is roughly 20°C warmer than it once was.

    Scientists who have studied the cores agree that the Earth experienced large, rapid, regional-to-global climate oscillations through most of the last 110,000 years, of a scale that agricultural- and industrial-age humans did not face. Though a few of these stadial/interstadial oscillations such as the Younger Dryas event were known for decades, many more were found in the first Greenland deep ice cores, but most of the oscillations occurred in ice from close to bedrock where flow may have disturbed the climatic record. In the new cores, these events are recorded far enough above the bed that ice flow is unlikely to have altered the early section of the 110,000-year climatic record. The almost perfect match back to this date between records from the two cores should dispel any lingering doubt about the climatic origin of the events. These millennial-scale events represent large climate deviations that probably include change in temperature of many degrees Celsius, twofold changes in snow accumulation, large changes in how much wind-blown dust and sea salt were carried by the atmosphere, and large changes in methane concentration. Changes during these events commonly occur over decades or less. Shifts in the patterns of atmospheric circulation could explain the rapidity and magnitude of these events. Most recently, subtle versions of these rapid climate change events were identified through the reconstruction of atmospheric circulation patterns in the Holocene portion of the Greenland ice record. Major climatic change events are also recorded in the isotopic temperature record of the Vostok core from central East Antarctica, but not as clearly as in cores from Greenland.

Ice Cores Challenge Standing Theories

Initial interpretation of the ice cores indicated that the large, rapid climate oscillations that dominate the record of the last 110,000 years also persisted through the previous warm period, the Eemian, which took place about 120,000–130,000 years ago. Both cores also show rapid oscillations in climate during that time period, but with different timing and character. In both cores, there is evidence of ice flow beginning at or slightly above the depth at which difference in their climate records appears—roughly 2800 m, or approximately 110,000 years ago. Ice flow disturbs the climate record by allowing ice from different layers to mix. The amounts of gases in both cores differ from those of the Vostok, Antarctica core, where the Eemian era ice is undisturbed by ice flow. Much remains to be learned about Eemian climate from these cores. Just as they were needed to confirm the rapid oscillations observed in older cores, a core from a site where the Eemian is farther above the bed and thus is less subject to flow disturbance will provide the best information. Scientists are already looking at sites in North Greenland and Antarctica capable of delivering such records.

    Measurements of gas-bubble compositions from Antarctic cores provide the best paleorecords of CO₂ concentrations. Greenlandic records indicate some unexplained "noise"—data that may be added by random processes not related to the true environmental record or just plain high-frequency unexplained data—possibly related to chemical reactions with the more abundant carbonate dust in Greenland ice. Scientists agree that interpreting the record of CO₂ in the Greenland ice is more complex than interpreting it in the Antarctic ice. However, the results do not question earlier findings about the increase of the atmospheric CO₂ concentration at the end of the last glaciation and a steady increase since the beginning of the industrial age.

Ice Cores Lead to Progress in Related Research

Great progress is being made on more basic science as well. The ability to count annual layers in the cores well into the glacial period and probably through 110,000 years will help to answer questions about the timing of the glacial periods and the usefulness of radiocarbon calibrations. The use of volcanic markers (such as dust and certain gases) and atmospheric-oxygen isotope ratios to determine the ages of ice cores and ocean records greatly extends scientists' ability to map climate changes and understand their causes.

    Reconstruction of atmospheric circulation patterns and their changes over time from chemical indicators and dust sources provides new insight into the large, rapid changes documented in the cores. Vigorous work on the air-snow transfer function for chemicals and particulates is clarifying the significance of the paleoclimatic records. Glacier geophysics and flow modeling, coupled with physical and electrical studies of ice cores, are leading to better understanding of the ice cores and ice-sheet behavior, and possible contributions to sea level change. Many studies are underway to help understand the Greenland record in more detail. Scientists expect to use the cores to learn more about changes in atmospheric acids, past extraterrestrial impacts, humankind's influence on the chemistry of the atmosphere, and details of Holocene climate variability.

Source: Eos, May 28, 1996, p. 209.

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