WASHINGTON – Climate scientists released a grim portrait Monday of the likely effects of global warming on the United States and Canada.

More droughts, floods, heat waves, infectious diseases and extinctions are possible for two of the most prosperous countries on the planet, according to the North American section of the latest report by the Intergovernmental Panel on Climate Change. Poor countries in tropical Africa and Asia will be hit even harder by the effects of higher temperatures and rising sea levels.

In the U.S., the Southwest, California, Alaska, and the Atlantic and Gulf coasts are the most seriously threatened, the authors of the new report said at a Washington news conference.

“The impacts are faster and larger than we ever anticipated,” said Anthony Janetos, a climate researcher at the Pacific Northwest National Laboratory. “Things are happening much more quickly than the science community imagined. It’s profoundly concerning.”

Another author, Kristi Ebi, an epidemiologist in Alexandria, Va., said matters were even worse than they appeared in the panel’s report, which is based on data previously published in peer-reviewed scientific journals.

“The scientific literature is behind what people on the ground are seeing,” Ebi said. She cited, for example, eyewitness accounts of accelerated thawing of Alaskan tundra and melting of Himalayan glaciers.

“It’s not just global warming, it’s local warming. It’s happening where we live,” Larry Schweiger, the president of the National Wildlife Federation, an environmental action group, said in a statement.

Also on Monday, 11 retired U.S. generals and admirals said that worldwide water shortages caused by global warming were a “serious threat to America’s national security” and might drag the U.S. into fights over shortages of natural resources.

The United Nations Security Council prepared to hold its first discussion of global warming Tuesday in New York.

A 23-page summary of the full 1,000-plus-page report on the worldwide impact of global warming by the Intergovernmental Panel on Climate Change was released in Brussels, Belgium, on April 6. Detailed chapters devoted to specific regions of the world are being published separately.

The North American section, Chapter 14, describes what’s happening now and what the authors think will occur as the world heats up. Here are some of the developments it forecasts as “likely” or “very likely”:

• By 2039, average temperatures across North America will rise by 1.8 to 5.4 degrees Fahrenheit. By 2100, the warming will be 3.6 to 5.4 degrees in the west, south and east, but more than 9 degrees in the far north.

• Less rain will fall in the Southwest, but more will fall in the rest of the continent. The chances of extreme precipitation and flooding will increase.

• Trends in hurricane frequency and intensity are uncertain, but there will be more intense mid-latitude storms and extreme wave heights.

• Shrinking western mountain snowpacks will melt earlier, causing spring floods and drier summers. The Columbia and Colorado rivers will be especially vulnerable. Southwestern states will battle for water.

• Water levels in the Great Lakes will drop, affecting ship navigation and fishing, and exposing buried pollutants.

• Warmer lake and river water will threaten fish and spread pollution. Surface temperatures in Midwestern lakes could reach 86 degrees.

• Growing seasons will lengthen for most of the 21st century. Forests will increase by 10 percent to 20 percent. As much as one-third of plant and animal species may be doomed to extinction.

• Sea levels will rise 9 to 18 inches by 2100 along U.S. coasts, higher in Canada and Alaska. Up to 21 percent of coastal wetlands in the mid-Atlantic region will be lost. Higher seas and accompanying storm surges will harm transportation along the Gulf and Atlantic coasts.

-Severe heat waves and more pollution will threaten human health. Chicago can expect 25 percent more heat waves annually. Los Angeles will experience four to eight times as many heat-wave days. Severe hay fever, asthma and other lung diseases will mount.

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