Climate change will make drought and flooding events like those that have battered the United States and other countries in 2011 more frequent, forcing nations to rethink the way they cope with disasters, according to a new report the U.N. Intergovernmental Panel on Climate Change issued Friday.

The report — the culmination of a two-year process involving 100 scientists and policy experts — suggests that researchers are far more confident about the prospect of more intense heat waves and heavy downpours than they are about how global warming is affecting hurricanes and tornadoes. But the new analysis also speaks to a broader trend: The world is facing a new reality of more extreme weather, and policymakers and business alike are beginning to adjust.

Gerald Meehl, a senior scientist at the National Center for Atmospheric Research and one of the report’s reviewers, said it highlights why climate change means more than just a gradual rise in the global temperature reading.

“The fact is, a small change in average temperature can have a big impact on extremes,” Meehl said in an interview. “It’s pretty straightforward. As average temperatures go up, it’s fairly obvious that heat extremes go up and [the number of] low extremes go down.”

Meehl co-authored a 2009 study showing that during the past decade the number of record highs in the United States outnumbered the record lows by an average of 2 to 1; historically, the two have been roughly even. Two Australian researchers last year found a similar trend between 1997 and 2009.

Christopher Field, one of the leaders of the U.N. climate panel, said its members teamed up with disaster experts around the world to answer three central questions: “What do we know about the changes in climate extremes that have already occurred and are expected to occur? What are the consequences of these changes? And what can you do about it?”

The report says there is at least a 66 percent chance that climate extremes have changed as a result of greenhouse gas emissions caused by human activities, including from coal-fired power plants and fuel burned through transportation. It notes that “economic losses from weather- and climate-related disasters are increasing,” though they can fluctuate from year to year. The overall economic and insured losses are greater in industrialized nations, while in poor countries extreme weather events cause more deaths and represent a greater proportion of the gross domestic product.

Connie Hedegaard, the European commissioner for climate action, said in an interview Thursday that policymakers cannot afford to ignore the sort of scientific findings summarized in the U.N. panel’s new report. “The science is not getting more uncertain — it’s actually getting more and more certain,” she said. “It’s getting in line with what people intuitively feel.”

This year has already set a record in terms of billion-dollar disasters for the United States, according to the National Climatic Data Center, with at least 10 disasters approaching a total of $50 billion.

“I’ve been a meteorologist for 30 years, and I’ve never seen a year like 2011 in terms of extreme weather events,” Jeff Masters, who co-founded the Web site Weather Underground, said in a conference call with reporters that was organized by the Union of Concerned Scientists.

In its report, the U.N. panel says it has “low confidence” in drawing scientific conclusions yet from any observed changes in the increased number of tropical storms worldwide.

Kerry Emanuel, an atmospheric-science professor at MIT who studies hurricane intensity and frequency, said researchers need to look more closely at whether intense storms are making landfall more frequently, since that is what threatens society the most.

“It’s sort of an irony that the things that matter the most to people are the things climate scientists have the least handle on, by our own admission,” Emanuel said in an interview.

The report calls on governments to adopt “low-regrets measures” that will offer societal benefits even if climate change does not cause as much damage as many researchers predict.

Many public and private institutions across the globe are already adjusting to the idea that extreme weather has become the new normal. Thomas Wilson, chairman and chief executive of Allstate, told a group of Wall Street analysts this spring that his company cannot afford to ignore the increased frequency of climate disasters in recent years.

Sharlene Leurig, senior manager of the insurance program at the investors group Ceres, said the fact that 2011 ranks as the most expensive year in the insurance industry’s history means consumers will have to pay more to get insurance.

And groups such as the Climate Corp., which offers full-season weather insurance for farmers in more than two dozen states, now rely on climate modeling to help determine their rates. The firm’s chief executive, David Friedberg, said that, because the weather has become so unpredictable, more corn and soybean farmers are willing to supplement federal crop insurance by paying between $30 and $40 an acre.

“What we see is really the acute pain experienced by farmers because they’re suffering from more floods and more droughts than they’ve ever experienced,” Friedberg said, adding that this year North Dakota farmers contended with two separate deluges and extreme heat.

In poorer regions of the world, such as Africa, countries are hoping to use technology and pooled resources to respond. The African Union has created the African Risk Capacity program, or ARC, which will use satellite weather surveillance and software developed by the U.N. World Food Program to calculate and direct disaster assistance as soon as a severe drought hits.

Princeton University’s Michael Oppenheimer, one of the lead author’s of the panel’s report, said the group will issue another chapter in February detailing how some countries have begun to adapt to the increased risk of climate-related disasters. Bangladesh has reduced its cyclone-related fatalities by adopting an early-warning system and building concrete shelters for those living on the coast, he noted.

“We don’t have to sit there like punching dummies and take this on the chin,” Oppenheimer said. “The good-news part of this is these things work.”

Still, Oppenheimer said, most nations are not doing enough.

“Governments are behind the eight ball on this,” he said. “They’re just not doing what they should be doing to protect people.”