A leading U.S. climate researcher says the world has a 10-year window of opportunity to take decisive action on global warming and avert catastrophe.

NASA scientist James Hansen, widely considered the doyen of American climate researchers, said governments must adopt an alternative scenario to keep carbon dioxide emission growth in check and limit the increase in global temperatures to 1 degree Celsius (1.8 degrees Fahrenheit).

“I think we have a very brief window of opportunity to deal with climate change ... no longer than a decade, at the most,” Hansen said Wednesday at the Climate Change Research Conference in California’s state capital.

If the world continues with a “business as usual” scenario, Hansen said temperatures will rise by 2 to 3 degrees Celsius (3.6 to 7.2 degrees F) and “we will be producing a different planet.”

On that warmer planet, ice sheets would melt quickly, causing a rise in sea levels that would put most of Manhattan under water. The world would see more prolonged droughts and heat waves, powerful hurricanes in new areas and the likely extinction of 50 percent of species.

Clashing with White House

Hansen, who heads NASA’s Goddard Institute for Space Studies, has made waves before by saying that President Bush’s administration tried to silence him and heavily edited his and other scientists’ findings on a warmer world.

He reiterated that the United States “has passed up the opportunity” to influence the world on global warming.

The United States is the largest emitter of greenhouse gases, most notably carbon dioxide. But Bush pulled the country out of the 160-nation Kyoto Protocol in 2001, arguing that the treaty’s mandatory curbs on emissions would harm the economy.

Hansen praised California for taking the “courageous” step of passing legislation on global warming last month that will make it the first U.S. state to place caps on greenhouse gas emissions.

He said the alternative scenario he advocates involves promoting energy efficiency and reducing dependence on carbon burning fuels.

“We cannot burn off all the fossil fuels that are readily available without causing dramatic climate change,” Hansen said. “This is not something that is a theory. We understand the carbon cycle well enough to say that.”

Most scientists believe global warming is due in some measure to the greenhouse effect, which occurs when so-called greenhouse gases are emitted into the atmosphere. These gases trap in Earth’s heat like the glass walls of a greenhouse. Greenhouse gases, especially carbon dioxide, are byproducts of the burning of fossil fuels.

Arctic studies

Hansen spoke as NASA released two studies that found sharp reductions in winter Arctic sea ice.

One of those studies was from Hansen's institute. “It is not too late to save the Arctic, but it requires that we begin to slow carbon dioxide emissions this decade,” Hansen said in a statement.

Scientists and climate models have long predicted a drop in winter sea ice, but it has been slow to happen. Global warming skeptics have pointed to the lack of ice melt as a flaw in global warming theory.

The latest findings are “coming more in line with what we expected to find,” said Mark Serreze, a senior research scientist at the National Snow and Ice Data Center in Boulder, Colo. “We’re starting to see a much more coherent and firm picture occurring.”

“I hate to say we told you so, but we told you so,” he added.

Serreze said only five years ago he was “a fence-sitter” on the issue of whether man-made global warming was happening and a threat, but he said recent evidence in the Arctic has him convinced.

Summer sea ice also has dramatically melted and shrunk over the years, setting a record low last year. This year’s measurements are not as bad, but will be close to the record, Serreze said.

Shrinking Arctic ice means less sunlight gets reflected and more gets absorbed, exacerbating the problem of warming. It also threatens Arctic species, notably polar bears, said Claire Parkinson, a research scientist at the Goddard center.

The polar bear population in Canada’s Hudson Bay has dropped from 1,200 in 1989 to about 950 in 2004, a decline of 22 percent, Parkinson said at the teleconference.

Polar bears typically hunt on Arctic ice, but when ice is depleted, they will forage on land, she said. This has led to more sightings in Inuit settlements, but does not mean that the number of polar bears is increasing.