"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 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.

Hansen, who heads NASA's Goddard Institute for Space Studies, has made waves before by saying that President George W. 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 US 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.