In the interview and in subsequent e-mails, Dr. Hansen made it clear that his new independence would allow him to take steps he could not have taken as a government employee. He plans to lobby European leaders — who are among the most concerned about climate change — to impose a tax on oil derived from tar sands. Its extraction results in greater greenhouse emissions than conventional oil.

Dr. Hansen’s activism of recent years dismayed some of his scientific colleagues, who felt that it backfired by allowing climate skeptics to question his objectivity. But others expressed admiration for his willingness to risk his career for his convictions.

Initially, Dr. Hansen plans to work out of a converted barn on his farm in Pennsylvania. He has not ruled out setting up a small institute or taking an academic appointment.

He said he would continue publishing scientific papers, but he will no longer command the computer time and other NASA resources that allowed him to track the earth’s rising temperatures and forecast the long-run implications.

Dr. Hansen, raised in small-town Iowa, began his career studying Venus, not the earth. But as concern arose in the 1970s about the effects of human emissions of greenhouse gases, he switched gears, publishing pioneering scientific papers.

His initial estimate of the earth’s sensitivity to greenhouse gases was somewhat on the high side, later work showed. But he was among the first scientists to identify the many ways the planet is likely to respond to rising temperatures and to show how those effects would reinforce one another to produce immense changes in the climate and environment, including a sea level rise that could ultimately flood many of the world’s major cities.

“He’s done the most important science on the most important question that there ever was,” said Bill McKibben, a climate activist who has worked closely with Dr. Hansen.