Ancient ice reveals sudden climate shift.

Ancient ice reveals sudden climate shift

From deep within Greenland's ice cap, investigators have retrieved signs that the North Atlantic region took fewer than 20 years to shift from glacial conditions to warmer ones at the end of the last ice age. The discovery underscores the possibility that a similarly dramatic climate swing lies just around the greenhouse corner.

Previous studies suggested that the ice age's final cold spell, called the Younger Dryas, ended abruptly around 10,700 years ago. The refined analysis of a deep ice core from south Greenland reveals the extreme speed of the transition to a milder, less stormy climate.

As it emerged from the ice age, the North Atlantic bounced between several warm and cold times. The Younger Dryas period, which started around 11,500 years ago according to ice core dates, marked the last time this region slipped back into glacial conditions. The system of ocean currents that warms Europe ceased operating, and sea ice covered much of the North Atlantic during this period.

Greenland's ice cap, which builds layer by layer each year, provides a climatic record of such events. By comparing deuterium and oxygen isotope levels in the ice cores, researchers can estimate the extent of the ancient sea ice. These data show that the ice retreated quickly to the north, in less than two decades. At the same time, dust concentrations in the ice cores dropped by a factor of three, indicating a weakening in the storms that carried North American dust to Greenland, say W. Dansgaard of the University of Copenhagen in Denmark and his colleagues from the United States and Iceland.

While the sea ice retreated and for 30 years afterward, Greenland warned by a substantial 7[deg.]C, which was half the total temperature difference between peak glacial and interglacial times for this area, Dansgaard and his co-workers report in the June 15 NATURE.

It is unclear what triggered the rapid changes that ended the Younger Dryas, says ice core chemist David A. Peel from the British Antarctic Survey in Cambridge. This year, U.S. and European researchers began drilling two new cores in central Greenland that will help answer these questions.

An understanding of the Younger Dryas climate changes may provide warnings for the future. Scientists use computer models to estimate how increasing greenhouse gases will heat the globe. Yet, says Peel, "the current global models are totally inadequate to deal with rapid climate fluctuations. They can't account for the shifts that we're seeing at the end of the Younger Dryas."