Jim Hansen may be the most respected climate scientist in the world. He’s been director of NASA’s premier climate research center, the Goddard Institute for Space Studies (GISS), for 25 years and a member of the National Academy of Sciences (NAS) for 10. And he more or less single-handedly turned global warming into an international issue one sweltering June day in 1988, when he told a group of reporters in a hearing room, just after testifying to a Senate committee, “It’s time to stop waffling so much and say that the greenhouse effect is here and is affecting our climate now.”

It took the rest of the scientific community about eight years to catch up with him on that point. He was ahead of the pack in 1988, and he remains so. He’s been accurately predicting the progress of global warming for 25 years. And as the science grows ever more solid, owing in no small part to his own work, Hansen’s predictions about an issue some see as the greatest threat civilization has ever faced are becoming ominously precise.

An attempt by the Bush administration to silence him early this year also helped turn global warming into one of the biggest news stories of 2006. It began on December 6, 2005, when Hansen declared in a talk at the American Geophysical Union in San Francisco that if our rate of fossil fuel burning continues to grow, we will eventually transform Earth into “a different planet.” He presented an analysis showing that existing technologies can significantly cut greenhouse emissions, and suggested that a global solution requires leadership by the United States.

On December 15, he and three colleagues posted a routine monthly analysis on the GISS website, summarizing data from thousands of weather stations around the globe. It showed that 2005 was coming in as the warmest year since the mid-1800s. He was interviewed about this by ABC News.

According to NASA memorandums provided by Hansen, senior political appointees at NASA headquarters in Washington quickly called career public-affairs officers at the agency and directed them to give headquarters advance notice of Hansen’s speaking schedule, his “data releases,” and his attendance at scientific meetings. The career officers also understood from the phone calls that the posting of all content on the GISS website, including scientific data sets, would now require headquarters approval; that no NASA employees or contractors could grant media interviews without approval; and that public-affairs officers had the right to stand in for scientists in all interviews. Hansen emphasizes that the political appointees made sure to leave no paper trail. But by throwing off this muzzle, Hansen propelled himself – and global warming – into the headlines. The story broke on the front page of the New York Times ; Hansen appeared on NPR and 60 Minutes , too.

Through it all, he remained productive scientifically. One week, he submitted a paper to the Proceedings of the National Academy of Sciences ; the next, he presented an invited talk at the NAS’s annual meeting; the next, he filed a brief in U.S. District Court in California, as an expert witness for the state in a suit brought by automobile manufacturers hoping to strike down a 2004 regulation by the California Air Resources Board that would eventually reduce greenhouse-gas emissions from vehicles sold in the state by about a third.

Hansen now starts off most public appearances by stating that he speaks as a private citizen, not a public employee, that his opinions are those of a climate scientist with more than 30 years’ experience, not of a government policymaker. Indeed, he asked to be interviewed not at his institute, on the campus of Columbia University, but at the small apartment he keeps nearby.

Over lunch in a tasteful but spartan living room on the top floor of a building that affords magnificent southwest views of the Hudson River and the western half of Manhattan, he sits in jeans and an untucked blue-checked shirt, without shoes, sipping his fourth or fifth coffee of the day.

He says he’s been muzzled before – during the Reagan and first Bush administrations – but that in more than three decades as a government employee, he has seen nothing to equal the recent clampdown. He is angry, but he expresses his anger calmly.