Opinions vary about how long it will take to reach those "tipping points" and whether attempts to cut planet-warming gases churned out by power plants, vehicles and other human industry can slow, halt or reverse the harmful effects in coming decades. Some suggest it might be cheaper for society to adapt to the changing climate than to roll back the pace of warming.

But in the report, the second of three this year by the Intergovernmental Panel on Climate Change, thousands of climate scientists and representatives of more than 100 nations, including the USA, present in the most stark terms the "key global risks" — serious environmental consequences from the changing climate — that threaten humanity.

"It's time (a report) puts people on the planet into the picture" of global warming, says economist Gary Yohe of Wesleyan University, a lead author of the report.

Concerning the USA, the report will reference numerous scientific studies on the effects of spring arriving weeks earlier, says University of Montana ecologist Steve Running, an author of the chapter on North America. The "big climate signal and impacts" will be in the West, he says. Earlier melting of mountain snow, on which much of the region depends for water, would mean more severe dry spells and droughts that would trigger worse wildfire seasons. Lower stream flows also would threaten fish and wildlife.

Research also has predicted more frequent heat waves, increased rainfall and flooding in northern states, and more severe tropical storms on the Gulf and East coasts.

In its first report in February, the panel, backed by the World Meteorological Organization and conducted under the auspices of the United Nations Environmental Programme, concluded that "unequivocal" evidence shows industrial releases of greenhouse gases have warmed the Earth an average of about 1 degree Fahrenheit in the past century. That makes it "very likely" that temperatures will rise 3 to 7 degrees this century, depending on future emissions.

This week's report, essentially a review and condensation of climate research since 2001, is designed to identify the dangers that the failure to curb emissions of carbon dioxide, methane and other greenhouse gases present for the planet. Just as new buildings in earthquake zones are designed to handle more than everyday shocks, and fire insurance is meant to cover more than burnt toast, politicians and planners want to know worst-case scenarios, says Stanford University climatologist Stephen Schneider.

"In a sense, we are looking at a series of tipping points for humanity and climate," says Richard Moss, director of the U.S. Global Change Research Program Office.

Irreversible effects on plants, animals, farming and weather already are apparent, says biologist Camille Parmesan of the University of Texas in Austin, one of the scientists assigned to review the report. Studies weighed in the report show that warming has eliminated about 70 animal species and affects 59% of wild species surveyed.

"We are seeing plenty of potentially dangerous outcomes where the hotter it gets, the worse it gets," Stanford's Schneider says.

Moss says the roughly 5-degree rise in global average temperatures envisioned in the February report will cause damage that cannot be recovered. He echoes a warning by NASA scientist James Hansen in 2004 that the window for action is only 10 years. The Stern Review, a high-profile report last year by the United Kingdom's chief economist, Nicholas Stern, warns of serious financial threats to agriculture and commerce.

Some scientists question such concerns. Danish statistician Bjorn Lomborg has become a spokesman for the view that trying to repair global warming will cost more money than just working to adapt the world to it. He suggests, for instance, that it would be cheaper to cure and eradicate malaria than to attack the rising temperatures that could expose millions more people to the disease.

Gore makes his case

Last month, Lomborg followed former vice president Al Gore to the microphone in testifying before a House committee. Gore, star of the Oscar-winning documentary on warming, An Inconvenient Truth, called the phenomenon a "planetary emergency." Lomborg countered that Gore "has gotten carried away and has come to show only worst-case scenarios."

Schneider argues that worst-case scenarios are still real threats. A collapse of the Greenland and Antarctic ice sheets, which Lomborg downplayed at the hearing, genuinely worries scientists such as Stefan Rahmstorf of Germany's Potsdam Institute for Climate Impact Research. Rahmstorf has suggested sea levels could rise as much as 4.6 feet worldwide by 2100. Schneider says a simple cost-benefit analysis ignores the reality that poor people in Bangladesh and other low-lying lands would have to bear the brunt of climate change.

In Brussels this week, about 60 lead authors are working with representatives of more than 100 nations to distill, clarify and approve the panel's findings in a short summary for policymakers. The summary is out Friday; the scientific chapters arrive Tuesday.

Environmental and energy analyst Anthony Patt of Boston University, a report co-author, says the report will divide the possible effects of temperature increases this century into three grades: a 3.6-degree rise with warmer winters but few human catastrophes; an up to 7.2-degree rise that wealthy nations could handle but would prove calamitous to poor nations and many species; and an even higher rise, which "would prove difficult for any society to adapt to."

What are the yardsticks?

In grades of scientific certainty, physical effects such as temperature, sea level rise and concentrations of greenhouse gases are most certain, Schneider says. Next come biological ones, such as species extinctions. And the hardest to estimate are human effects, such as disease and hunger.

What the panel's report will not establish is whether vast infestations by pine beetles in the forests of the western USA and Canada are tied to warming, Running says. Although many scientists believe there is a link, he says, research has not focused enough on temperature. "My nose is telling me there's a climate-change signal here, but the papers in print yet aren't doing a strong enough analysis."

Worldwide, thresholds were outlined last year in "Avoiding Dangerous Climate Change," a summary of tipping points for which British Prime Minister Tony Blair wrote the foreword. They include:

•At a 3.6-degree rise, all Indian Ocean coral reefs go extinct, and 97% of the rest around the globe are "bleached" or severely damaged. All Arctic ice disappears.

•At a 5.4-degree increase, half of all nature reserves become unable to conserve native species. The Amazon rainforest disappears.

•At 7.2 degrees or higher, coastal flooding is seven times worse than in 1990. Malaria threatens 330 million more people a year, and hunger jeopardizes 600 million. Australia no longer can grow food.

All of this leaves aside the most extreme risks that Schneider calls the "dark edge of the bell curve": melting of the vast Antarctic ice sheets; shutdown of Atlantic Ocean circulation, which brings warm weather to the United Kingdom; and the release of more greenhouse gases frozen in the Arctic tundra.

Some scientists, such as Penn State's Michael Mann, worry that the panel's reports lag behind the latest science because of a six-month research cutoff before their release, a lifetime in climate study.

Last month, for instance, a report in Geophysical Research Letters found that ocean acidification from increased carbon dioxide is likely to wreak "havoc" for shellfish and coral and disrupt food chains.

A Colorado State University analysis in March said warming will make grazing lands less productive by 2050.

A University of Minnesota team reported that Lake Superior has warmed an average of 4.5 degrees since 1979, about twice the local atmospheric warming.

Because the panel's reports trail such research, they are "always by design … a little conservative," Mann says.

In May, the climate panel picks up where this month's report leaves off: an assessment of ways to counteract and adapt to warming.

"I suspect we're reluctant to think about it because we're worried that if we start, we will have no choice but to think about nothing else," John Lanchester, who reviewed recent works on climate change, wrote in a recent London Review of Books.

"The scientists involved are not just talking in a new way, one unfamiliar to both them and us, but are in effect trying to sell us something," Lanchester says. "And we the public might be undereducated, but we know not to trust entirely someone who is trying to sell us something. … We deeply don't want to believe this story."

But James McCarthy of Harvard, incoming head of the American Association for the Advancement of Science, says the reality of warming is accepted, with regional climate-change trends already playing out as predicted.

The biggest tipping point already may have happened, says John Drexhage of Canada's International Institute for Sustainable Development: Talk of global warming has become routine and accepted for all politicians, not just Al Gore.

By Dan Vergano and Patrick O'Driscoll, USA TODAY