James E. Hansen, the climate scientist who issued the clearest warning of the 20th century about the dangers of global warming, will retire from NASA this week, giving himself more freedom to pursue political and legal efforts to limit greenhouse gases.

His departure, after a 46-year career at the space agency’s Goddard Institute for Space Studies in Manhattan, will deprive federally sponsored climate research of its best-known public figure.

At the same time, retirement will allow Dr. Hansen to press his cause in court. He plans to take a more active role in lawsuits challenging the federal and state governments over their failure to limit emissions, for instance, as well as in fighting the development in Canada of a particularly dirty form of oil extracted from tar sands.

“As a government employee, you can’t testify against the government,” he said in an interview.

Dr. Hansen had already become an activist in recent years, taking vacation time from NASA to appear at climate protests and allowing himself to be arrested or cited a half-dozen times.

But those activities, going well beyond the usual role of government scientists, had raised eyebrows at NASA headquarters in Washington. “It was becoming clear that there were people in NASA who would be much happier if the ‘sideshow’ would exit,” Dr. Hansen said in an e-mail.

At 72, he said, he feels a moral obligation to step up his activism in his remaining years.

“If we burn even a substantial fraction of the fossil fuels, we guarantee there’s going to be unstoppable changes” in the climate of the earth, he said. “We’re going to leave a situation for young people and future generations that they may have no way to deal with.”

His departure, on Wednesday, will end a career of nearly half a century working not just for a single agency but also in a single building, on the edge of the Columbia University campus.

From that perch, seven floors above the diner made famous by “Seinfeld,” Dr. Hansen battled the White House, testified dozens of times in Congress, commanded some of the world’s most powerful computers and pleaded with ordinary citizens to grasp the basics of a complex science.

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