Correction: An earlier version of this story incorrectly stated the age of James E. Hansen. He is 72, not 75. This version has been corrected.

James E. Hansen, the nation’s most outspoken scientist on global warming, is stepping down from his federal post to engage full time in activism on the issue.

Hansen, who heads NASA’s Goddard Institute of Space Studies in New York City, spent 46 years working at the agency.

After warning Congress in 1988 that climate change posed a serious threat to the planet, he has spent much of the past quarter-century trying to persuade policymakers to take bold action to curb global carbon emissions.

In an e-mail Monday, Hansen wrote that 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 New York Times first reported the news of Hansen’s retirement Monday night.

Climate activist Bill McKibben, co-founder of, said Han­sen, 72, decided to step down so he could engage in lawsuits and protests full time. Han­sen was arrested in February in Washington in a protest against the Keystone XL pipeline plan. He participated in the demonstration on a day off from his government job.

Hansen has frequently tangled with his superiors in the federal government, especially during the George W. Bush administration. At one point, political appointees barred Han­sen from talking directly to the press.

“When the history of our time is written, he’s going to be one of the giants,” McKibben said in an interview. “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.”

McKibben sent an e-mail to his group’s supporters Monday night calling Hansen the “patron saint” of his organization, urging them to honor the atmospheric researcher by lobbying against the pipeline aimed at transporting crude oil from Canada’s oil sands to the U.S. Gulf Coast.

“Here’s what I hope you’ll do: honor Jim’s lifetime of work by making a public comment to the State Department about Keystone XL and tell them to reject the pipeline,” he wrote in the e-mail.

Gavin Schmidt, deputy chief of the Goddard Institute, wrote in an e-mail that Hansen “has been at the forefront of almost every conceptual advance in climate in science over 40 years.”

He noted that on the issues that have come to define the scientific debate, from the impact of black carbon and greenhouse gases beyond carbon dioxide to constructing climate models and gauging climate sensitivity, “the stuff that Jim wrote 20 years ago has set the tone for the whole field [and the] predictions he made have generally worked out very favorably (for him, not really the planet).”