WASHINGTON -- In his worn navy windbreaker, 63-year-old climatologist James E. Hansen looks more like the Iowa farm native that he is than a rebel -- but he's both.

Hansen, a lifelong government employee who heads NASA's Goddard Institute for Space Studies in New York, has inspired both anger and awe in the nation's scientific and political communities since denouncing the Bush administration's policy on climate change last year.

Speaking in the swing state of Iowa days before the presidential election, Hansen accused a senior administration official of trying to block him from discussing the dangerous effects of global warming.

In the University of Iowa speech, Hansen recounted how NASA Administrator Sean O'Keefe told him in a 2003 meeting that he shouldn't talk "about dangerous anthropogenic interference" -- humans' influence on the atmosphere -- "because we do not know enough or have enough evidence for what would constitute dangerous anthropogenic interference."

But Hansen said scientists know enough to conclude that the world has reached a danger point, and that efforts to get the word out are being blocked by the administration.

"In my more than three decades in government, I have never seen anything approaching the degree to which information flow from scientists to the public has been screened and controlled as it has now," Hansen said. Senior administration officials deny Hansen's charges: O'Keefe spokesman Glenn Mahone said the administrator doesn't "recall ever having the conversation" on climate change that Hansen described, adding that O'Keefe "has encouraged open dialogue and open conversation about those issues."

But Hansen, who has worked for NASA since he was 25, has continued to chide the administration for not moving swiftly enough to address global warming.

"As the evidence gathers, you would hope they would be flexible," Hansen said in the slow, measured tones he has retained from his years growing up on an Iowa farm. "We have to deal with this. You can't ignore it."

The ongoing sparring match between Hansen and his superiors underscores a broader tension between President Bush's top policy advisers and many senior U.S. scientists, who have blasted the administration's approach to environmental questions in recent months. Nearly 50 Nobel laureates endorsed Sen. John F. Kerry, D-Mass., for president; this year, the Union of Concerned Scientists has collected more than 6,000 scientists' signatures on a letter questioning how the president applies research to policymaking.