WASHINGTON - Earth won't have to warm up as much as had been thought before causing serious consequences such as more extreme weather and increasing threats to plants and animals, an international team of climate experts reported yesterday.

The Intergovernmental Panel on Climate Change, the premier scientific group studying global warming, estimated that the risk of increased severe weather would rise with a global average temperature increase of 1.8 to 3.6 degrees Fahrenheit above 1990 levels. The National Climatic Data Center currently reports that global temperatures have risen 0.22 of a degree since 1990.

Now, researchers say, "increases in drought, heat waves and floods are projected in many regions and would have adverse impacts, including increased water stress, wildfire frequency and flood risks starting at less than [1.8 degrees] of additional warming above 1990 levels."

Indeed, "it is now more likely than not that human activity has contributed to observed increases in heat waves, intense precipitation events, and the intensity of tropical cyclones," concluded the researchers, led by Joel B. Smith of Stratus Consulting Inc., in Boulder, Colo.

Other researchers, they noted, have suggested that "the likelihood of the 2003 heat wave in Europe, which led to the death of tens of thousands of people, was substantially increased by increased greenhouse gas concentrations."

The new report, in the online edition of Proceedings of the National Academy of Sciences, comes a week after Christopher Field of the Carnegie Institution for Science told the annual meeting of the American Association for the Advancement of Science that humans are now adding carbon to the atmosphere even faster than in the 1990s.

Carbon dioxide and other gases added to the air by industrial and other activities have been blamed for rising temperatures, increasing worries about possible major changes in weather and climate. Carbon emissions have been growing at 3.5 percent per year since 2000, up sharply from the 0.9 percent per year in the 1990s, Field said.

The new study found evidence of greater vulnerability to climate change for specific populations, such as the poor and elderly, not only in developing but also developed countries.

Co-authors of the report include Stephen H. Schneider of Stanford University, Michael Oppenheimer of Princeton University, and researchers in India, Germany, Canada, Zimbabwe, Australia, Bangladesh, Cuba, and Belgium.