Those risks are big and small, current and future, according to the report. They will hit rural farms and big cities. Some places will have too much water and some not enough, including drinking water. Other risks mentioned in the report involve food prices and availability and, to a lesser and more qualified extent, some diseases, financial costs and even world peace.

"Things are worse than we had predicted" in 2007, when the group of scientists last issued this type of report, said report co-author Saleemul Huq, director of the International Centre for Climate Change and Development at Independent University in Bangladesh.

"We are going to see more and more impacts, faster and sooner than we had anticipated."

The problems have gotten so bad that the panel had to add a new, greater level of risks. In 2007, the top risk level in one key summary graphic was high and colored blazing red. The latest report adds a level, very high, and colors it deep purple.

You might as well call it a "horrible" risk level, said report co-author Maarten van Aalst, a top official at the International Federation of Red Cross and Red Crescent Societies.

"The horrible is something quite likely, and we won't be able to do anything about it," he said.

The report predicts that the highest level of risk would first hit plants and animals, on land and in the acidifying oceans.

Climate change will worsen problems that society already has, such as poverty, sickness, violence and refugees, according to the report. And on the other end, it will act as a brake, slowing the benefits of a modernizing society, such as regular economic growth and more efficient crop production, it says.

"In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans," the report reads. And if society doesn't change, the future looks even worse. "Increasing magnitudes of warming increase the likelihood of severe, pervasive and irreversible impacts."

While the problems from global warming will hit everyone in some way, their severity won't affect people equally, coming down harder on people who can least afford it, the report says. It will increase the gaps between rich and poor, healthy and sick, young and old and men and women, van Aalst said.

"Read this report and you can't deny the reality: Unless we act dramatically and quickly, science tells us our climate and our way of life are literally in jeopardy. Denial of science is malpractice," U.S. Secretary of State John Kerry said Monday in a press release.

Kerry said, "No single country causes climate change, and no one country can stop it," adding that the White House Climate Action Plan aims to reduce U.S. emissions and increase renewable energy sources ahead of the U.N. Convention on Climate Change.

But the report's authors say this is not a modern-day version of the Four Horsemen of the Apocalypse. Much of the caution comes from more nuanced troubles that grow by degrees and worsen other societal ills. The report also concedes that there are uncertainties in understanding and future climate risks.

The report, the fifth on warming's implications, includes risks to ecosystems, including a thawing Arctic, but it is far more oriented than past versions to what it means to people.

The report also notes that one major area of risk is that with increased warming, incredibly dramatic but extremely 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, which can stay in the atmosphere for a century. Much of these gases in the air came from the United States and other industrial nations. China is now by far the top carbon dioxide polluter, followed by the United States and India.

Unlike past reports, in which the scientists tried to limit examples of extremes to disasters that computer simulations could 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.

Extremely large storms, like 2013's Typhoon Haiyan, 2012's Superstorm Sandy and 2008's ultradeadly Cyclone Nargis, may not have been caused by warming, but their storm surges were worsened by climate change's 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's contribution to creating pockets of poverty and "hot spots of hunger" even in richer countries, increasing inequality between rich and poor.

"Rich people benefit from using all these fossil fuels," University of Sussex economist Richard Tol said. "Poorer people lose out."

Tol, who is in the minority of experts here, had his name removed from the summary because he found it "too alarmist," focusing too much on risk.

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 changes 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 to 7 degrees Fahrenheit (3.5 to 4 degrees Celsius) of warming by 2100 instead of 2 degrees Fahrenheit (1.2 degrees Celsius), which was the international goal.

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."  

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."