US: The 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 and 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 the 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 Prof Lonnie G Thompson, a scientist at Ohio State University who has been taking core samples from the ancient ice of glaciers for 23 years.
Prof Thompson, writing with eight other researchers in an article in the US Proceedings of the National Academy of Sciences, said the ice samples showed the climate could 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 suggested.
"There are thresholds in the system," Prof Thompson said. 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."
Prof Thompson's work summarises evidence from around the world and ice core sampling from seven locations in the South American Andes and Asian Himalayas. It extends considerably the reach of a number of scientific findings documenting the historically unusual warming of the earth.
Prof Thompson, whose research has focused on glaciers in the high mountains of the tropics, wrote that the warming was "unprecedented for at least two millennia".
In collaboration with his wife, Ellen Mosley-Thompson, an expert in polar ice sampling, he concluded that the glacial retreat "signals a recent and abrupt change in the earth's climate system".
But the finding likely to cause the most debate is Prof Thompson's conclusion that a swift and sudden cooling of the climate five millennia ago occurred simultaneously with key changes in civilisations. "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 timeframe of the biblical flood."
Prof Thompson said he did not know what caused the abrupt change - one possibility is a "mega La Nina" shift in upper air currents. But the evidence from such diverse sources as Mount Kilimanjaro, African lakes, Greenland and Antarctic ice cores, the Andes and the Alps, pointed to a sudden arrival of cool and often wet conditions, all at about the same time.
That time saw cities form in the Nile Valley and Mesopotamia and the end of a humid period in Africa that "seems to have begun and ended abruptly, within decades to a century".
Gavin Schmidt, a scientist at Nasa's Goddard Institute for Space Studies in New York, applauded Prof Thompson's work but said his conclusions about events 5,200 years ago had many sceptics. "You would have to put that argument as more intriguing rather than definitive," he said.