29-06-06 The ongoing global warming is abrupt and unprecedented in recent 5,200 years, a study said.
The observed glacial retreat in the tropical region suggests that the current increase in the global average temperature may now have interrupted the natural progression, and that human activities within a century may have nudged global-scale climate conditions closer to those that prevailed 5,000 years ago, US scientists reported.

The study, led by Lonnie Thompson from the Ohio State University, was published in the June 26 edition of the Proceedings of National Academy of Sciences. The researchers cited three evidences of the world's abrupt warming.

First, isotope traces collected from ice core samples demonstrate that the current warming at high elevations in the mid- to low-latitudes is unprecedented for at least the last two millennia.

Second, the continuing retreat of most mid- to low-latitude glaciers, many having persisted for thousands of years, signals a recent and abrupt change in the Earth's climate system. The ongoing global-scale, rapid retreat of mountain glaciers is not only contributing to global sea-level rise, but also threatening freshwater supplies in many of the world's most populous regions, the researchers warned.
Finally, they said, rooted, soft-bodied wetland plants, now exposed along the margins as the Quelccaya ice cap in Peru retreats, have been radiocarbon dated and, when coupled with other widespread proxy evidence, provide strong evidence for an abrupt climate transition to cooler conditions 5,200 years ago.

This abrupt event was widespread and spatially coherent through much of the tropical region. It also coincided with structural changes in early human civilizations, the researchers noted.
For example, Neolithic settlements in the inner desert of Arabia were abandoned around that time, whereas hierarchical societies formed in the overpopulated Nile Valley and Mesopotamia.

The recent, swift global warming should be a converse to that abrupt cooling, the researchers suggested.
"If this is the case, then Earth's currently retreating glaciers may signal that the climate system has exceeded a critical threshold and that most low-latitude, high-altitude glaciers are likely to disappear in the near future," they wrote.