The fresh efforts to quiet him, Dr. Hansen said, began in a series of calls after a lecture he gave on Dec. 6 at the annual meeting of the American Geophysical Union in San Francisco. In the talk, he said that significant emission cuts could be achieved with existing technologies, particularly in the case of motor vehicles, and that without leadership by the United States, climate change would eventually leave the earth "a different planet."

The administration's policy is to use voluntary measures to slow, but not reverse, the growth of emissions.

After that speech and the release of data by Dr. Hansen on Dec. 15 showing that 2005 was probably the warmest year in at least a century, officials at the headquarters of the space agency repeatedly phoned public affairs officers, who relayed the warning to Dr. Hansen that there would be "dire consequences" if such statements continued, those officers and Dr. Hansen said in interviews.

Among the restrictions, according to Dr. Hansen and an internal draft memorandum he provided to The Times, was that his supervisors could stand in for him in any news media interviews.

Mr. Acosta said the calls and meetings with Goddard press officers were not to introduce restrictions, but to review existing rules. He said Dr. Hansen had continued to speak frequently with the news media.

But Dr. Hansen and some of his colleagues said interviews were canceled as a result.

In one call, George Deutsch, a recently appointed public affairs officer at NASA headquarters, rejected a request from a producer at National Public Radio to interview Dr. Hansen, said Leslie McCarthy, a public affairs officer responsible for the Goddard Institute.

Citing handwritten notes taken during the conversation, Ms. McCarthy said Mr. Deutsch called N.P.R. "the most liberal" media outlet in the country. She said that in that call and others, Mr. Deutsch said his job was "to make the president look good" and that as a White House appointee that might be Mr. Deutsch's priority.