Some of the world's top climate scientists say wind and solar energy won't be enough to head off extreme global warming, and they're asking environmentalists to support the development of safer nuclear power as one way to cut fossil fuel pollution.

The Associated Press was given an advance copy of the open letter that is being sent Sunday to leading environmental groups around the world.

The letter says that with the planet warming and carbon dioxide emissions rising faster than ever, nuclear power needs to be considered.

The letter signers are former top NASA scientist James Hansen, Ken Calderia of the Carnegie Institution, Kerry Emanuel of MIT, and Tom Wigley of the University of Adelaide in Australia.