Dec. 14 (Bloomberg) -- Irreversible effects of global climate change such as arctic ice melt and lost water supplies may still be averted if the world stops using coal-fired power plants, a leading NASA scientist said.

James Hansen, director of NASA's Goddard Institute for Space Studies, said the planet is at a ``tipping point'' that could lead to rising sea levels, severe droughts and floods, and reduced fresh water supplies if world leaders don't act to reduce emissions such as carbon dioxide. He criticized the fossil fuel industry for resisting efforts to stop warming.

``It's just been taken as a God-given fact that we're going to burn all these fossil fuels and let the CO2 into the atmosphere, and you can't do that if you're going to keep this planet resembling the one that we've had the last 10,000 years,'' Hansen said.

A ``point of no return'' is avoidable with a moratorium on coal-fired power plants that don't capture carbon emissions, and by boosting energy efficiency, appropriate agricultural practices and a cap on carbon-dioxide emissions, Hansen said at the annual American Geophysical Union conference in San Francisco yesterday.

``Getting the CO2 levels to go down is more feasible than you might think,'' Hansen said, adding that energy conservation is the most effective short-term approach to reducing carbon emissions. Unless we do so, we don't have a chance of avoiding a point of no return, he said. ``I don't think that's sunk in yet.''

Points of No Return

Hansen drew a distinction between climate tipping points, which he defined as when no more ``forcing'' is required for ``large rapid climate change, with large climate impacts.'' A climate tipping point may be reversed with sufficient cooling of the atmosphere, he said, over decades or more. A point of no return would be when effects on the climate are unstoppable and irreversible on a ``practical'' time scale.

Coal is the largest source of electricity in China and the U.S., the world's two biggest energy consumers and emitters of greenhouse gases. China burns coal to generate 78 percent of its power, and the U.S. gets about 50 percent of its power from coal.

Since humans have to figure out how to live without fossil fuels someday anyhow, Hansen said, ``why not sooner?''

The Intergovernmental Panel on Climate Change, set up by the United Nations Environment Program and the World Meteorological Organization, has reported that greenhouse gas emissions from power plants, transportation and factories contribute to global warming. The IPCC comprises hundreds of scientists from around the world plus representatives of member countries' governments, according to the scientific body's Web site.

Ice Melt

World leaders have been gathered on the Indonesian island of Bali for the past two weeks to negotiate a global warming treaty to replace the Kyoto Protocol, which runs out in 2012. The talks have faltered because the U.S. is resisting mandatory reductions in greenhouse-gas emissions.

Researchers speaking at the geophysicists' conference in San Francisco said the world doesn't have much time to act. In 2007, the minimum year-round ice cover in the arctic was a record 38 percent less than average over several decades -- and 27 percent less than the previous low in 2005, said Josefino Comiso, a senior NASA research scientist for polar oceanography. The 2007 drop was more than the decline in the previous 26 years, he said.

A sustained cooling of the arctic would be required in both winter and summer for the ice melt to be reversed. Current research shows this may not be possible and a tipping point may have already been reached, he said.

Slow Build, Rapid Melt

Scientists have learned that while ice sheets build slowly, they are unstable and can shrink rapidly, said Richard Alley, a professor at the Earth and Environmental Systems Institute at Pennsylvania State University. While it may take centuries or more to lose an ice sheet, within decades we may reach a tipping point that commits us to long-term changes, he said.

An estimated worst-case scenario over ``many centuries'' could cause a 20-meter rise in sea levels, leading to the loss of Greenland, West Antarctica and coastal East Antarctica, he said.

``We have had thus far a small warming,'' Alley said. ``The warming that will be coming is very large compared to what we've seen.'' If the small warming had a notable effect on ice levels, ``what of the large warming coming?''

To contact the reporter on this story: Adam Satariano in San Francisco at asatariano1@bloomberg.net; Jeanmarie Todd in San Francisco at jetodd@bloomberg.net

Last Updated: December 14, 2007 00:35 EST