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?
The NRC committee emphasizes that it isn't trying to alarm people. But it does want to inject a sense of urgency into discussions of climate change to encourage research. Currently, scientists do not understand what drives drastic changes, which in turn means they cannot simulate or forecast them.
Yet researchers are already aware that some degree of change seems to be under way. For example, the Arctic is warming. Air circulation around the North Pole region has changed. It is bringing warmer, wetter winters to northern Europe, Siberia, and parts of North America. Also, a changed wind pattern is moving young Arctic ice out of the ocean faster. That means it doesn't linger as long and build up its former thickness.
Dorothy Peteet from the NASA Goddard Institute for Space Science in New York - an NRC committee member - notes the Arctic is a good region to explore climate questions. There could well be a climate switch, which might be as simple as permafrost melting. Among other things, that could release methane, a powerful greenhouse gas.
The NRC committee members realize they are sounding a warning that is cloaked in uncertainty. Therefore, they urge what they call a "no regrets" policy: Take no action based on vague fear. Do take actions that will be beneficial, whatever happens. These include measures to curb global warming.
Continue to work to ensure adequate resources of clean air and water. Build resiliency into economic systems. And support the extensive research needed to understand how the climate system works.
Alley says scientists need to face up to a certain amount of inevitable uncertainty, since natural processes may throw climate switches in random ways that are impossible to forecast. To help people learn to anticipate surprises, he says, "We need to build uncertainty into our models of climate change."