James Hansen

Hansen testifies before Congress about climate change in 2008. (Photo: U.S. House of Representatives)

James Hansen, the longtime NASA scientist and early voice of concern on climate change, is stepping down from the agency this week. But rather than retiring, he's leaving the federal government to spend more time on his parallel career: environmental activism.

Hansen has spent 46 years at NASA, most recently as head of the Goddard Institute for Space Studies in New York. He began his career in physics and astronomy, initially studying the planet Venus, but over time his interest drifted back to Earth — specifically to dramatic changes unfolding in the atmosphere. He famously warned Congress about global warming in 1988, long before such concerns were considered mainstream, and he has been a leading figure in both climate science and climate activism ever since.

But as Hansen tells the New York Times, which first reported his retirement, those two roles can cause friction. "As a government employee, you can't testify against the government," he points out. Hansen typically takes vacation time from NASA to join protests and rallies, but he'll be much freer to challenge Uncle Sam's response to climate change from the outside. In an email to the Washington Post, he says he decided to step down "so that I can spend full time on science, drawing attention to the implications for young people, and making clear what science says needs to be done."

The dual roles of researcher and rabble rouser have often bedeviled Hansen, giving him scientific credentials in the activist community but also giving critics a chance to question his objectivity. And as the Times notes, "even some of his allies consider him prone to rhetorical excess and to occasional scientific error." At the same time, though, he has frequently been correct and ahead of the curve in forecasting the perils of climate change, creating an aura of climatic clairvoyance. "Jim has a real track record of being right before you can actually prove he's right with statistics," a colleague tells the Times.

Lately, Hansen has focused his activism on fighting the proposed Keystone XL oil pipeline, a 1,700-mile conduit that would link Canada's oil sands to refineries in Texas. He has become increasingly involved with grassroots groups like Bill McKibben's 350.org, often joining large student protests in which he and other activists are arrested or cited by police. But in addition to rallying against pipelines, coal mines and other individual fossil-fuel projects, he's also involved in bigger-picture policy debates, from cap and trade to nuclear power. He reportedly hasn't ruled out creating a climate institute or taking an academic appointment, but he says he'll initially work from his farm in Pennsylvania.

"When the history of our time is written, he's going to be one of the giants," McKibben tells the Post, noting that 350.org was named after Hansen's discovery that 350 parts per million is the safe upper limit of carbon dioxide in the atmosphere. "If anyone has ever served his country well, it's Jim Hansen, to work that long in the same shop and to do it under that kind of pressure and scrutiny, and to do it with that kind of faithfulness."

For more about Hansen's research and activism, check out this TED Talk he gave last year:

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