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September 15th, 2004

North Greenland ice core reveals gradual, abrupt climate swings

A new, undisturbed Greenland ice deep-core record going back 123,000 years shows the Eemian period prior to the last glacial period was slightly warmer than the present day before it gradually cooled and sent Earth into an extended deep freeze.

The nearly two-mile long ice core, drilled over eight years as part of the North Greenland Ice Core Project (NGRIP), contains annual layers of ice compressed from 123,000 years of snowfall. Although two other deep-ice cores were retrieved from Greenland in the 1990s, those show skewed climate records from the beginning of the last interglacial roughly 115,000 years ago because of ice layer "folding" near bedrock.

"We now have a highly detailed ice record from the Northern Hemisphere of how Earth moved from the last interglacial to the glacial period," said University of Colorado at Boulder Professor James White, a U.S. principal investigator for the international NGRIP effort. "This might help us understand what to be on the lookout for in the future, assuming there is another glacial period waiting in the wings."

Led by project leader Dorthe Dahl-Jensen of the University of Copenhagen's Niels Bohr Institute, NGRIP is an international project with participants in Denmark, Germany, Japan, the United States, Switzerland, France, Sweden, Belgium and Iceland. NGRIP is funded by participating countries, including the National Science Foundation.

The new NGRIP ice core record indicates Eemian-period temperatures, over the polar regions at least, were stable and roughly 9 degrees Fahrenheit warmer than temperatures in the present interglacial. The transition from the Eemian into the most recent glacial period was slow and gradual, marked by general cooling and deterioration of the climate over several thousand years, according to the study.

A paper on the subject will appear in the Sept. 9 issue of Nature.

In late June, the NGRIP team recovered what appear to be plant remnants nearly two miles below the surface between the bottom of the glacial ice and the bedrock. Researchers said the possible organic material recovered from the slushy, reddish ice looks like it may be ancient pine needles, bark or blades of grass.

Thought to date to several million years ago before the Pleistocene smothered Greenland in ice, the material will be analyzed in several laboratories. The research team also is analyzing the reddish basal water recovered from beneath the NGRIP core for indications of microscopic life forms that might have survived, said Dahl-Jensen.

Each yearly record of ice can reveal past temperatures and precipitation levels, the content of ancient atmospheres and even evidence for the timing, direction and magnitude of distant storms, fires and volcanic eruptions. The cores are cylinders of ice four inches in diameter that were brought to the surface in 11.5-foot lengths, said White, a fellow of CU-Boulder's Institute of Arctic and Alpine Research.

The NGRIP ice core also provided evidence of air temperatures jumping 9 degrees Fahrenheit in just five decades roughly 115,000 years ago, just prior to the slide from the Eemian into the glacial period. "This is one of those classic, abrupt changes we have seen before in Greenland ice core records, where temperatures can shift dramatically in less than a human lifetime," said White.

In addition, the NGRIP core shows the air temperature jumping about 18 F in roughly 50 years about 10,500 years ago as Earth was emerging from the last glacial period into the present Holocene interglacial epoch, said CU-Boulder doctoral student Trevor Popp of INSTAAR, chief driller on the 2004 NGRIP effort.

"This jump also was seen in the GISP 2, GRIP and Dye 3 ice core records from Greenland, showing it was not just a wiggle on a graph, but was really a large, rapid and significant climate event," said Popp.

The isotopes in the NGRIP ice and four Greenland ice cores drilled in recent years also indicate while the northern part of the Greenland ice sheet was fairly stable during the Eemian, the ice sheet in southern Greenland may have been dramatically thinner or even nonexistent, said White.

In addition, the researchers also found unexpectedly large temperature differences between the NGRIP core and two other cores drilled 220 miles away. The evidence suggests colder temperatures at the site of the NGRIP drilling in the distant past, perhaps caused by different air masses influencing the climates of two sites.

"If there is a trigger that caused Earth to slide into the last glacial period, it is a slow trigger and there is no smoking gun," said White. "It's curious and remarkable to me that the last glacial period appears to have begun without any real fireworks."

The NGRIP drilling site is located roughly in the middle of Greenland at an elevation of about 9,850 feet. The temperature in the subsurface trenches where ice-core scientists worked is minus 22 degrees Fahrenheit.

Another CU-Boulder graduate student, Annalisa Schilla, also participated in the 2004 NGRIP field season.