Earth has already crossed a number of climate change "tipping points" at which today's levels of greenhouse gases will cause additional large and rapid changes, a leading climate scientist said yesterday.

But it's not too late to avoid much of the damage by curbing the use of fossil fuels such as oil and coal, climatologist James Hansen added during a presentation at the American Geophysical Union fall meeting in San Francisco.

Such fuels are responsible for most of human-caused emissions of carbon dioxide (CO2), which are widely believed to be driving global warming.

Today's level of CO2 in the atmophere is enough to cause Arctic sea ice cover and massive ice sheets such as in Greenland to eventually melt away, said Hansen, of the NASA Goddard Institute for Space Studies in New York City.

Climate zones such as the tropics and temperate regions will continue to shift, and the oceans will become more acidic, endangering much marine life, he added. (Related: "Climate Change Pushing Tropics Farther, Faster" [December 3, 2007].)

"I think in most of these cases, we have already reached the tipping point," Hansen said.

But there's still hope if people soon change how they use energy.

"In my opinion, we have not passed the point of no return, so that it's still possible to avoid the impacts," Hansen said

"The problem is that it's just been taken as a God-given fact that we're going to burn all of these fossil fuels and let the CO2 in the atmosphere," he added.

"You just can't do that if you're going to keep this planet resembling the one that we've had for the last 10,000 years."

Chilly News

Melting ice and subsequent increases in ocean levels were among the most cited examples of reaching or nearing a tipping point.

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SOURCES AND RELATED WEB SITES American Geophysical Union NASA Goddard Institute for Space Studies NASA Goddard Space Flight Center: Josefino Comiso Pennsylvania State University: Richard Alley

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