Be Prepared: Sudden Shifts in Climate Coming ; Data from Ancient Tree Rings and Ice Cores Suggest Earth May Soon Experience Intense Periods of Drought and Cold

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If you're concerned about forecasts of long-term global warming, you might be worried about the wrong thing.

The US National Academy of Sciences warns that sudden, unexpected climate change - on a scale that could cause widespread drought or plunge Earth into a deep freeze - pose a more immediate danger.

The evidence? Embedded in ancient tree rings and ice cores are signs that quick, drastic change is a fundamental characteristic of Earth's climate. These data show that the climate can switch abruptly from one mode - such as an ice age - to another, such as a milder interglacial period, climatologists say.

Humans have no remembered experience of such sudden, far- reaching shifts. If one were to occur in the near future, human civilization could be vastly ill-equipped to adjust.

The Academy's National Research Council (NRC) organized a study to assess this knowledge, which has come to a head over the past five years. The NRC report was distributed here at a meeting of the American Geophysical Union.

In a sense, the report notes, humanity has been living in the meteorological equivalent of a fool's paradise. Agriculture and other aspects of civilization have developed during a period of relatively benign climate.

The workings of this climate system can be likened to a light fixture that is controlled by both a dimmer and an on-off switch, says Richard Alley of Pennsylvania State University in University Park, chair of the NRC study. You can continuously change the light level by turning the dimmer dial. But nothing may happen until you push hard enough to throw the switch. Then the lights abruptly go out.

"It's clear that climate has both dimmer dials and switches," says Mr. Alley.

In the case of global warming, for example, is the heat-trapping effect of greenhouse gases such as carbon dioxide twiddling a dimmer dial - and is it also pushing on a switch that might suddenly flip the climate? …