Scientist who refused to be silenced

Two things about Jim Hansen have remained near-constant for the past 30 years. He has demonstrated an unwavering commitment to the science of global warming, issuing warning after warning about the consequences for the planet if we do nothing to stop the creation of greenhouse gases. And he has run up against a wall of resistance from politicians and policy-makers who simply don't want to believe what they hear from him.

Under the current Bush administration, he has accused political appointees in the public affairs department at NASA, the space agency where he works, of deliberately blocking his access to the media and censoring the published fruits of his research. When the Reagan administration took office in 1981, they responded to his warnings about climate change by paring his staff to the bone and cutting his funding.

Hansen is the very picture of reasonableness: a 65-year-old gentleman from Iowa who insists on presenting scientific fact without political spin. That has made him a menace in the eyes of the energy industry. His first testimony before Congress in 1988 drew considerable ire because he said he was only "99 per cent certain" of some of his claims - falling short of the absolute certainty expected of academic scientists.

Hansen, in turn, accused his adversaries of misrepresenting his testimony - whether it is congressional staffers seeking to minimise the impact of global warming or Michael Crichton, the novelist and global warming sceptic, picking holes in his claims in his novel State of Fear. "Some 'greenhouse sceptics'," he wrote in 2004, "subvert the scientific process, ceasing to act as objective scientists, rather presenting only one side, as if they were lawyers hired to defend a viewpoint."

According to Hansen: "Human-made forces, especially greenhouse gases, soot and other small particles, now exceed natural forces, and the world has begun to warm at a rate predicted by climate models."