Earth s climate is undergoing an abrupt change, ending a cooler period that began with a swift "cold snap" in the tropics 5,200 years ago that coincided with the start of cities, the beginning of calendars and the biblical great flood, a leading expert on glaciers has concluded. The warming around Earth s tropical belt is a signal suggesting that the "climate system has exceeded a critical threshold," which has sent tropical-zone glaciers in full retreat and will melt them completely "in the near future," said Lonnie G. Thompson, a scientist who for 23 years has been taking core samples from the ancient ice of glaciers.

Thompson, writing with eight other researchers in an article published Monday in the Proceedings of the National Academy of Sciences, said the ice samples show that the climate can and did cool quickly, and that a similarly abrupt warming change started about 50 years ago. Humans may not have the luxury of adapting to slow changes, he suggests. "There are thresholds in the system," Thompson said in an interview in his lab at Ohio State University. When they are crossed, "there is the risk of changing the world as we know it to some form in which a lot of people on the planet will be put at risk."

"I think the temperature will continue to rise, the glaciers will continue to melt. Sea levels will continue to rise. I think there is a good indication now that the magnitude of severe storms will rise," he said. Thompsons work summarizes evidence from around the world and ice core sampling from seven locations in the South American Andes and the Asian Himalayas. It considerably extends the reach of a growing number of scientific findings documenting the historically unusual warming of Earth. A top scientific panel last week endorsed an earlier study, by Penn State professor Michael E. Mann, that concluded the recent warming in the Northern Hemisphere is of a scale probably unseen for 400 to 1,000 years.

Thompson, whose research has focused on glaciers in the high mountains of the tropics, writes that the warming there "is unprecedented for at least two millennia." He teamed with his wife, Ellen Mosley-Thompson, an expert in polar ice sampling, and concluded that the glacial retreat "signals a recent and abrupt change in the Earths climate system."

Caspar Amman, a scientist at the National Center for Atmospheric Research in Boulder, Colo., said Thompson s "perspective of the changes over the past 2,000 years is striking. Something is definitely different towards the end of the 20th century." But the finding likely to cause the most debate is Thompson s conclusion that a swift and sudden cooling of the climate five millennia ago occurred simultaneously with key changes in civilizations.

"It represents a time where, for many parts of the world, people ceased to be hunters and gatherers and formed cities," he said. "Many of the modern calendars began around this time. It would also fall in the general time frame of the biblical flood." Thompson said he does not know what caused the abrupt change -- one possibility is a "mega La Ni?a" shift in upper air currents. But he said the evidence from such diverse sources as Mount Kilimanjaro; African lakes; Greenland and Antarctic ice cores; the Andes and the Alps point to a sudden arrival of cool and often wet conditions, all about the same time.

That time saw cities form in the Nile Valley and Mesopotamia, his paper says, and the end of a humid period in Africa that "seems to have begun and ended abruptly, within decades to a century." In what is now Florida, water levels rose rapidly. In Washington state, glaciers covered whole trees. In the Alps, a mortally wounded hunter nicknamed Otzi was buried quickly by snow and captured within a growing glacier until it melted enough to expose him in 1991.

Theories linking climate change with changes in the history of humans are increasingly popular. The book "The Winds of Change" by Eugene Linden argues that climate shifts accompanied the fall of many civilizations. Gavin Schmidt, a scientist at NASA s Goddard Institute for Space Studies in New York, applauded Thompson s work but said his conclusions about events 5,200 years ago have many skeptics. "You would have to put that argument as more intriguing rather than definitive," Schmidt said. "There are a number of issues in the tropical ice cores that are problematic for dating things 4,000 to 5,000 years ago."

Thompson and other scientists typically drill down to layers of glaciers put down by snow thousands of years ago. The air bubbles caught in those cores are analyzed to determine the atmosphere at the time. Sediment, insects and pollen are further clues to the climate in ancient history.
(Por Doug Struck, Washington Post, 27/06/2006)