The specialist, Dr. Robert T. Watson, chief scientist of the World Bank, is highly regarded as an atmospheric chemist by many climate experts. He has held the unpaid position of chairman of the Intergovernmental Panel on Climate Change since the fall of 1996. Now his term is expiring and the State Department has chosen not to renominate him to head the panel, which is run under the auspices of the United Nations and the World Meteorological Organization.

Dr. Watson is an outspoken advocate of the idea that human actions — mainly burning oil and coal — are contributing to global warming and must be changed to avert environmental upheavals.

Last night, a State Department official said the administration was leaning toward endorsing a scientist from India, which along with other developing countries has been eager for a stronger role in the climate assessments.

But many influential climate experts say they have written to the department supporting Dr. Watson.

One of those letters was sent last month by Dr. Ralph J. Cicerone, an atmospheric scientist who is chancellor of the University of California, Irvine, and chairman of a National Academy of Sciences panel that assessed the international panel's climate analyses last year at the behest of the White House.

In an e-mail message sent to the State Department, Dr. Cicerone urged the administration not to withdraw its support for Dr. Watson and, if it did, at least to replace him with another atmospheric scientist.

Otherwise, "such a change would greatly reduce the emphasis on science in I.P.C.C.," he said, referring to the climate panel. He also said it would be "very, very difficult to find anyone better than Watson."

But energy industry lobbyists and some Republican elected officials have criticized Dr. Watson as biased and focused on building a scientific argument to justify cutting the use of coal and oil. In a letter to the White House a year ago, for example, Dr. Arthur G. Randol III, senior environmental adviser for ExxonMobil, said Dr. Watson used leaks of drafts of his panel's climate reports to further his "personal agenda."

"Can Watson be replaced now at the request of the U.S.?" read the letter. A copy was given to The New York Times by the Natural Resources Defense Council, a private environmental group. Dr. Randol did not respond yesterday to requests for comment. But White House officials said his letter had no bearing on decisions about the panel.

The only other significant candidate nominated for panel chairman is Dr. Rajendra K. Pachauri, an Indian engineer and economist who is now one of five vice chairmen. He is highly regarded, but many scientists said his lack of grounding in atmospheric science made him an unsuitable choice.

Nevertheless several lobbyists for energy companies and auto manufacturers are scheduled to meet with senior State Department officials this afternoon, when they are expected to press the administration to endorse Dr. Pachauri.

One of the lobbyists said that in a two-man race, it was necessary for industry to make a choice — and that the choice should not be Dr. Watson.

The panel's assessments of climate change underpinned negotiations leading to two climate treaties, the latest of them the Kyoto Protocol, which calls for cuts in emissions of heat-trapping gases. President Bush rejected it a year ago.

The panel's findings have been criticized as overly dire by energy industry officials and a few scientists. But many other experts have endorsed them, including the panel convened by the National Academy of Sciences.

Campaigners at private environmental groups yesterday attacked the efforts to replace Dr. Watson.

Some climate panel scientists said that other countries were planning to push for Dr. Watson to remain, and that it might be possible to craft a compromise in which the two scientists served as co-chairmen.

In an interview, Dr. Watson said the most important thing was to keep the panel from becoming divided into factions. "We've always worked well by consensus," he said. "I would hope it does not come down to a divisive vote."