In its 2001 assessment, its third, the Intergovernmental Panel on Climate Change estimated that in the next hundred years sea level would rise globally by at least a few inches and perhaps as much as three feet, a catastrophe for low-lying coastal areas and island nations.

In Paris today the panel will issue its fourth assessment, and people familiar with its deliberations say it will moderate its gloom on sea level rise, lowering its worst-case estimate.

In theory that is good news, because rising seas bring erosion and flooding to coastal areas. But a lower estimate has not been uniformly cheered.

In letters to and conversations with panel members, and in scientific journals, several climate experts said the estimate was almost certainly wrong because the panel was leaving out a growing body of data on melting glaciers and inland ice sheets, which are major contributors to sea level rise.

Those experts say that unless the finding is modified, the panel �?widely cited as an authoritative voice on climate change �?risks condemning itself to irrelevance.

Climate experts have “a great deal of confidence�?in observations that sea level rise is accelerating, said Laury Miller, an oceanographer at the National Oceanographic and Atmospheric Administration who was a reviewer for part of the coming report.

Good satellite measurements date only from the last decade or so, he said, so it is hard to draw firm long-term conclusions from them. Also, he said, computer models of how glaciers and ice sheets melt cannot account for much of the observed melting, even though “presumably it is going into the ocean.�?

But so far at least, he said, “the observed sea level rise has been tracking the upper range�?of the 2001 estimate. “It’s pretty unequivocal,�?he said.

Michael C. MacCracken, who led the Office of Climate Change in the Clinton administration and who was also a reviewer for some of the new assessment, said he could understand why scientists on the panel might be uneasy about relying too much on models. But in that event, he said, they should make it known that their estimates did not include factors like ice sheet movement and collapse, which appear to be accelerating.

In a letter to panel members on Jan. 21, Dr. MacCracken said lowering the worst-case sea level estimate would “result in a serious misimpression being conveyed to policy makers and the public.�?In fact, he said, most American experts have felt that the estimate was already too optimistic.

Other experts said the panel might have missed some important new developments, because it set a December 2005 cutoff date for submission of scientific papers and other data.

Since then, researchers have reported that Greenland’s ice sheet is melting faster than had been thought, that Antarctica is feeding more melt water into the oceans than had been predicted and that the melting of glaciers around the world is accelerating rapidly.

In a brief report in today’s issue of the journal Science, an array of leading climate researchers said recent findings “raise concern that the climate system, in particular sea level, may be responding more quickly than climate models indicate.�?

But in an interview last week, Susan Solomon, a climate expert at the National Oceanographic and Atmospheric Administration and a leader of one of the climate panel’s working groups, said researchers were invited several times last year to comment on the group’s work. It received thousands of comments, she said.

Drew Shindell, a climate expert at the NASA Goddard Institute for Space Studies, said at a House of Representatives hearing on climate science on Tuesday that part of the problem was the difficulty of making firm scientific statements about a field in which research was moving fast.

Dr. Shindell, who emphasized that he was speaking as an individual, said, “The melting of Greenland has been accelerating so incredibly rapidly that the I.P.C.C. report will already be out of date in predicting sea level rise, which will probably be much worse than is predicted in the I.P.C.C. report.�?

James McCarthy, a climate expert at Harvard who was a leader in the 2001 assessment, noted in an e-mail message that the panel’s report could be changed until the moment it was made public. Nevertheless, he said he worried that unless its discussion of sea level rise was altered, the panel would so underestimate the problem that it would look “foolishly cautious and maybe even irrelevant�?on the issue.

But one prominent critic of mainstream climate science, S. Fred Singer, a retired physicist, is already seizing on the report as evidence that people like former Vice President Al Gore who argue that human activity is changing the earth’s climate are now the contrarians