Greenland Ice

From: Benny J Peiser <B.J.PEISER@livjm.ac.uk
To: cambridge-conference@livjm.ac.uk <cambridge-conference@livjm.ac.uk
Date: 25. marec 1998 01:57
Subject: CC DIGEST 24/03/98

Third Millennium BC Ctastrophe and Civilization Collapse
From: Bob Kobres <bkobres@uga.edu
Discover Magazine - March 1998
Empires in the Dust, by Karen Wright

Some 4,000 years ago, a number of mighty Bronze Age cultures crumbled. Were they done in by political strife and societal unrest? Or by a change in the climate? ...

I've got some figures I can show you. figures always help," says paleoclimatologist Peter de Menocal, swiveling his chair from reporter to computer in his office at Columbia University's Lamont-Doherty Earth Observatory, just north of New York City. On the monitor, de Menocal pulls up a graph derived from the research project known as GISP2 (for Greenland Ice Sheet Project 2). GISP2 scientists, he explains, use chemical signals in ice cores to reconstruct past climates. There are two kinds of naturally occurring oxygen atoms, heavy and light, and they accumulate in ice sheets in predictable ratios that vary with prevailing temperatures. In a cool climate, for example, heavy oxygen isotopes are less easily evaporated out of the ocean and transported as snow or rain to northern landmasses like Greenland. In a warm climate, however, more heavy oxygen isotopes will be evaporated, and more deposited in the Greenland ice sheets.

By tracking oxygen-isotope ratios within the ice cores, the GISP2 graph reflects temperatures over Greenland for the past 15,000 years. Near the bottom of the graph, a black line squiggle wildly until 11,700 years ago, when the last ice age ended and the current warm era, the Holocene, began. The line then climbs steadily for a few thousand years, wavering only modestly, until 7,000 years before the present. From then until now, global temperatures appear relatively stable - "then until now" comprising, of course, the entire span of human civilization. ...