US climate scientist James Hansen won a $100,000 environmental prize for decades of work trying to alert politicians to what he called an unsolved emergency of global warming.

Dr Hansen, born in 1941, will visit Oslo in June to collect the Sophie Prize, set up in 1997 by Norwegian Jostein Gaarder, the author of the 1991 best-selling novel and teenagers' guide to philosophy Sophie's World.

"Dr Hansen has played a key role for the development of our understanding of human-induced climate change," the prize citation said.

Dr Hansen, director of the Nasa Goddard Institute for Space Studies since 1981, testified to the US Congress as long ago as 1988 about the risks of global warming from human activities led by the burning of fossil fuels.

"We really have an emergency," Dr Hansen said in a video link with the prize panel in Oslo about feared climate changes such as a thaw of ice sheets on Greenland or Antarctica or a loss of species of animals and plants in a warming world.

"The US is not taking a path which is going to solve the problem," he said, adding that other nations were also doing too little. Legislation to cap US greenhouse gas emissions is stalled in the Senate.