With the Midwest battered by the worst flooding in 15 years, which has submerged vast areas of fertile farmland and displaced thousands of people, the report said future "heavy downpours are very likely to further increase in frequency and intensity," in North America.

The atmosphere absorbs more water vapor as temperatures rise, raising the likelihood of rain storms and flooding. The report said total precipitation in the continental United States has increased 7 percent over the past century.

Led by the National Oceanographic and Atmospheric Administration, the report was the government's widest assessment yet of how global warming may change the climate in coming decades.

The economic and physical ramifications of the extreme weather events forecast in the report can already be grasped by the flooding in the Midwest, said Richard Moss, vice president for climate change at the World Wildlife Fund.

Moss, who previously headed the CCSP coordination office, "the longer we delay on cutting emissions, the higher the bill will be from these impacts."

The report said higher temperatures from global warming will also increase the likelihood of severe droughts in the US Southwest, parts of Mexico, and the Caribbean. Heat waves are more likely "over most land areas, most pronounced over the northwestern two thirds of North America," it said.

Another government report issued on Friday and led by the Environmental Protection Agency, said some of the effects of climate change could be mitigated by methods like restoring vegetation along streams, increasing the resilience of ecosystems.

President George W. Bush's climate change policy has evolved from skepticism about the science of global warming from greenhouse gases to calling in April for a halt in the growth of carbon emissions growth by 2025.

But that falls short of targets agreed in the Kyoto Protocol, signed by all developed nations except the United States. The world is now trying to form a successor agreement to Kyoto by late 2009. (Reporting by Timothy Gardner, editing by Chris Wilson)