But the report's authors say this is not a modern day version of the Four Horsemen of the Apocalypse. Much of what they warn of are more nuanced troubles that grow by degrees and worsen other societal ills. The report also concedes that there are uncertainties in understanding and predicting future climate risks.
The report, the fifth on warming's impacts, includes risks to the ecosystems of the Earth, including a thawing Arctic, but it is far more oriented to what it means to people than past versions.
The report also notes that one major area of risk is that with increased warming, incredibly dramatic but ultra-rare single major climate events, sometimes called tipping points, become more possible with huge consequences for the globe. These are events like the melting of the Greenland ice sheet, which would take more than 1,000 years.
"I can't think of a better word for what it means to society than the word 'risk,'" said Virginia Burkett of the U.S. Geological Survey, one of the study's main authors. She calls global warming "maybe one of the greatest known risks we face."
Global warming is triggered by heat-trapping gases, such as carbon dioxide, that stay in the atmosphere for a century. Much of the gases still in the air and trapping heat came from the United States and other industrial nations. China is now by far the No. 1 carbon dioxide polluter, followed by the United States and India.
Unlike in past reports, where the scientists tried to limit examples of extremes to disasters that computer simulations can attribute partly to man-made warming, this version broadens what it looks at because it includes the larger issues of risk and vulnerability, van Aalst said.
Freaky storms like 2013's Typhoon Haiyan, 2012's Superstorm Sandy and 2008's ultra-deadly Cyclone Nargis may not have been caused by warming, but their fatal storm surges were augmented by climate change's ever rising seas, he said.
And in the cases of the big storms like Haiyan, Sandy and Hurricane Katrina in 2005, the poor were the most vulnerable, Oppenheimer and van Aalst said. The report talks about climate change helping create new pockets of poverty and "hotspots of hunger" even in richer countries, increasing inequality between rich and poor.
Report co-author Maggie Opondo of the University of Nairobi said that especially in places like Africa, climate change and extreme events mean "people are going to become more vulnerable to sinking deeper into poverty." And other study authors talked about the fairness issue with climate change.
"Rich people benefit from using all these fossil fuels," University of Sussex economist Richard Tol said. "Poorer people lose out."
Huq said he had hope because richer nations and people are being hit more, and "when it hits the rich, then it's a problem" and people start acting on it.
Part of the report talks about what can be done: reducing carbon pollution and adapting to and preparing for changing climates with smarter development.
The report echoes an earlier U.N. climate science panel that said if greenhouse gases continue to rise, the world is looking at another about 6 or 7 degrees Fahrenheit (3.5 or 4 degrees Celsius) of warming by 2100 instead of the international goal of not allowing temperatures to rise more than 2 degrees Fahrenheit (1.2 degrees Celsius). The difference between those two outcomes, Princeton's Oppenheimer said, "is the difference between driving on an icy road at 30 mph versus 90 mph. It's risky at 30, but deadly at 90."
Tol, who is in the minority of experts here, had his name removed from the summary because he found it "too alarmist," harping too much on risk.
But the panel vice chairman, Jean-Pascal van Ypersele, said that's not quite right: "We are pointing for reasons for alarm ... It's because the facts and the science and the data show that there are reasons to be alarmed. It's not because we're alarmist."
The report is based on more than 12,000 peer reviewed scientific studies. Michel Jarraud, secretary general of the World Meteorological Organization, a co-sponsor of the climate panel, said this report was "the most solid evidence you can get in any scientific discipline."
Michael Mann, a climate scientist at Pennsylvania State University who wasn't part of this report, said he found the report "very conservative" because it is based on only peer reviewed studies and has to be approved unanimously.
The Obama White House hailed the report as comprehensive and authoritative. John Holdren, the presidential science adviser, said in a written statement that "the report underscores the need for immediate action in order to avoid the most severe impacts of climate change." Environmental groups echoed that sentiment.
Pachauri said the U.N. panel doesn't tell governments what they should do, but said Holdren is right that "it is a call for action." Without reductions in emissions, he said, impacts from warming "could get out of control."
There is still time to adapt to some of the coming changes and reduce heat-trapping emissions, so it's not all bad, said study co-author Patricia Romero-Lankao of the National Center for Atmospheric Research in Colorado.
"We have a closing window of opportunity," she said. "We do have choices. We need to act now."
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