The report is the second of four coming this year from the Intergovernmental Panel on Climate Change, a U.N. network of 2,000 scientists.
"This is a glimpse into an apocalyptic future," the Greenpeace environmental group said.
Brussels, Belgium As the world gets hotter by degrees, millions of poor people will suffer from hunger, thirst, floods and disease unless drastic action is taken, scientists and diplomats warned Friday in their bleakest report ever on global warming.
All regions of the world will change, with the risk that nearly a third of the Earth's species will vanish if global temperatures rise just 3.6 degrees above the average temperature in the 1980s and '90s, the new climate report says. Areas that now have too little rain will become drier.
Yet that grim and still preventable future is a toned-down prediction, a compromise brokered in a fierce, around-the-clock debate among scientists and bureaucrats. Officials from some governments, including China and Saudi Arabia, managed to win some weakened wording.
Even so, the final report "will send a very, very clear signal" to governments, said Yvo de Boer, the top climate official for the United Nations, which in 1988 created the authoritative climate change panel that issued the document.
And while some scientists were angered at losing some ground, many praised the report as the strongest warning ever that nations must cut back on greenhouse gas emissions.
The new document tries to explain how global warming is changing life on Earth; a report in February said scientists are highly confident most warming is due to human activity.
The new report notes those who can afford the least get hit the most by global warming.
"Don't be poor in a hot country, don't live in hurricane alley, watch out about being on the coasts or in the Arctic, and it's a bad idea to be on high mountains with glaciers melting," said Stephen Schneider, a Stanford University scientist who was one of the study's authors.