NASA's Dr. James Hansen has proved to be both a global warming lightning rod and an outspoken advocate of openness within NASA. Photograph by Forrest M. Mims III.

"Dr Hansen was among the first to see the looming threat of climate change and to sound the alarm," said James Leape, WWF International's director general. "For more than two decades he has made huge contributions to scientific understanding of climate change and to raising awareness among decision makers and the public."

Accepting the award, which includes a gold medal and a Rolex watch, Prof Hansen said: "The rude scientific awakening to the threat of climate change has been sudden and profound. Only in recent years has the extent and immediacy of the threat become clear."

He said this had resulted in a big gap between the understanding of climate scientists and that of the public and policy makers. Dr Hansen is director of Nasa's Goddard Institute for Space Studies in New York.

"I am concerned about the burden that we will leave for our children and grandchildren, if we do not take a leadership role in addressing global warming," he went on.

Apart from the moral burden to act, he speculated that communities affected would seek compensation from polluting nations. "Will not people driven from their land seek reparation from countries most responsible? Science does not leave the opportunity for us to claim ignorance of the consequences of greenhouse gas emissions."

He also criticised adverts funded partly by ExxonMobil designed apparently to promote CO2. The tag line "They call it pollution, we call it life" would "bamboozle" the public, he said. "It is time to stop patronising that company," he added.

Guardian Unlimited © Guardian News and Media Limited 2006

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