In 1993, a team headed by Alley found that the accumulation of snow in Greenland had doubled sharply, in possibly one to three years, as the Younger Dryas gave way to warmer temperatures. In sub-Arctic latitudes like Greenland, more snow and ice accumulate in warm periods, when there is more moisture in the atmosphere, than in cold ones. Last October, a team headed by Taylor reported that it had analyzed another Greenland core segment and discovered that most of the transition from the deep freeze of the Younger Dryas to the warmth of the last 10,000 years, called the Holocene period, had come in two quick temperature jumps, each of nearly 10 degrees and each lasting less than a decade, within a 40-year transition period. Severinghaus and his colleagues have made a similar discovery, and, in addition, have found evidence that the climatic change signified by the ice corings extended beyond Greenland to the wider world. The discoveries were reported in the Jan. 8 issue of the journal Nature.
Scientists have a number of ways to detect climatic changes preserved in ancient ice.
Severinghaus used yet another technique, one especially suited to detecting abrupt change. The technique analyzes the behavior of relatively lighter and heavier isotopes of nitrogen. In a stratum of ice representing a time period when the temperature changed sharply, the lighter forms migrate to the top while heavier ones gravitate to the bottom. Applying this analysis to the Younger Dryas-Holocene transition, the Severinghaus team discovered that the transition had begun with a sharp rise of about 9 to 18 degrees Fahrenheit, on the way to an increase of about 27 degrees. A computer analysis of the nitrogen isotopes' behavior suggested that the initial jump had taken place in less than a decade.