Greenhouse gases produced mainly by the burning of fossil fuels are altering the atmosphere in ways that affect earth's climate, and it is likely that they have ''contributed substantially to the observed warming over the last 50 years,'' an international panel of climate scientists has concluded. The panel said temperatures could go higher than previously predicted if emissions are not curtailed.

This represents a significant shift in tone -- from couched to relatively confident -- for the panel of hundreds of scientists, the Intergovernmental Panel on Climate Change, which issued two previous assessments of the research into global warming theory, in 1995 and 1990.

The conclusions are likely to resonate loudly next month when negotiators from most of the world's nations gather in The Hague to work out details of the Kyoto Protocol, a treaty intended to cut releases of carbon dioxide and other greenhouse gases. The 1997 treaty has been signed by more than 150 countries but has not yet been ratified by any industrialized country.

The panel, which operates under the auspices of the World Meteorological Organization and the United Nations Environment Program, is spelling out its new findings in a climate assessment it began working on three years ago and which fills 1,000 pages. A summary of its findings was sent this week to governments around the world for a last round of comments before the assessment is completed at a meeting in January in Shanghai. Given the significance of issue and the disagreements over how to deal with it, there are likely to be changes before the summary and the 14 chapters of research underlying it go to print sometime next year, several scientists involved in the project said.

A copy of the summary was obtained by The New York Times from someone who was eager to have the findings disseminated before the meetings in The Hague.

Many panel members said that the summary represents the closest thing to a consensus possible in science, which is generally driven more by questioning and challenges than by esprit de corps.

In interviews, several members of the panel declined to discuss details of the report or the summary, saying they were not yet in their final form. But they said recent advances in the study of climate change led them to see with greater clarity the role of people in climate change.

For example, they pointed to additional temperature data gathered in the last few years, which have been substantially warmer than any similar string of years in many centuries; to improvements in computer models designed to project future trends; and to better understanding of the influence of other climate-influencing emissions, like particles of sulfates that can cool the earth by reflecting sunlight back into space.

Meanwhile, they said in interviews and in the summary, evidence of increasing warming has shown up in retreating glaciers, thinning polar sea ice, retreating snow packs, warmer nights, and elsewhere.

''More and more people working in atmospheric science or on climate or ecology have had to come to grips with the fact that climate change is affecting what they're looking at,'' said Dr. Kevin E. Trenberth, the head of the climate analysis division of the National Center for Atmospheric Research in Boulder, Colo., and a lead author of the panel's summary. ''There is increasing evidence from many sources that the signal of human influence on climate has emerged from natural variability, sometime around 1980.''

The report's language is far more constrained than that, reflecting a delicate consensus that was reached only after months of debate and several rounds of comments by hundreds of scientists and government climate experts, Dr. Trenberth said.

One of its most striking findings is its conclusion that the upper range of warming over the next 100 years could be even higher than it estimated in 1995, in a worst case raising the average global temperature 11 degrees Fahrenheit from where it was in 1990. By comparison, average temperatures today are only 9 degrees Fahrenheit warmer than they were at the end of the last Ice Age.