The group has now issued five major reports since 1990, each of them finding greater certainty that the world is warming and greater likelihood that human activity is the chief cause. The new report finds a 95 to 100 percent chance that most of the warming of recent decades is human-caused, up from the 90 to 100 percent chance cited in the last report, in 2007.

But the new document also acknowledges that climate science still contains uncertainties, including the likely magnitude of the warming for a given level of emissions, the rate at which the ocean will rise, and the likelihood that plants and animals will be driven to extinction. The scientists emphasized, however, that those uncertainties cut in both directions and the only way to limit the risk is to limit emissions.

Climate-skeptic organizations assailed the new report as alarmist even before it was published.

The Heartland Institute, a Chicago organization, issued a document last week saying that any additional global warming would likely be limited to a few tenths of a degree and that this “would not represent a climate crisis.”

One issue much cited by the climate doubters is the slowdown in global warming that has occurred over the past 15 years. The report acknowledged that it was not fully understood, but said such pauses had occurred in the past and the natural variability of climate was a likely explanation.

“People think that global warming means every year is going to be warmer than the year before,” said Gerald A. Meehl, an American scientist who helped write the report. “It’s more like a stair-step kind of thing.”

Climate scientists not involved in writing the new report said the authors had made a series of cautious choices in their assessment of the scientific evidence. Regarding sea level rise, for instance, they gave the first firm estimates ever contained in an intergovernmental panel report, declaring that if emissions continued at a rapid pace, the rise by the end of the 21st century could be as much as three feet. They threw out a string of published papers suggesting a worst-case rise closer to five feet.

Similarly, the authors went out of their way to include recent papers suggesting that the earth might be less sensitive to carbon dioxide emissions than previously thought, even though serious questions have been raised about the validity of those estimates.