Within two or three decades, there could be one and a half billion people without enough water, according to a new report on the impacts of global warming.
Such droughts would produce "refugee crises like we've never seen," as one of the study's lead authors told ABC News.
Scientists working on the "Impacts, Adaptation and Vulnerability" report have been telling ABC News for months that its findings, once public, would be alarming. The report is being prepared by the IPCC, the Intergovernmental Panel on Climate Change, made up of 2,500 experts in the field.
Scientists are hammering out that report's final wording in Brussels and are due to announce it on Friday.
But its basic findings, they say, won't change.
Drafts of the IPCC report depict a world already changed dramatically in the past 35 years by manmade global warming, with increasing drought, heavy precipitation and flooding.
It also says humankind is in for much worse in the next few decades.
The IPCC scientists are finalizing one chart that projects how, with each degree of future warming, Earth's natural life-support systems break down more and more.
It predicts mountain glaciers and snow-pack melting away around the world, faster than scientists thought possible only 20 years ago.
Ancient civilizations that have depended for millenia on fresh water from melted ice and snow are facing unprecedented crises, according to the two reports so far released.
For example, Himalayan glaciers and snow-pack which feed the Ganges and Brahmaputra rivers and which support hundreds of millions of people, are fast disappearing in the rising heat.
"What you can end up with is people homeless in places like Bangladesh having to move to places like India and China -- which will not be particularly welcoming to refugees," Peter Schwartz, a future systems analyst, told ABC News.
"You will have a refugee crisis like we've never seen," he said.
Emergencies, Not Slow and Steady
Schwartz, who heads the Global Business Network, directed the new study, "Impacts of Climate Change," commissioned for the U.S. defense and intelligence community.
He explained that some of the worst of upheavals feared from global warming are likely to arrive not in a slow and steady manner, but as sudden emergencies, because the temperatures themselves will not rise steadily.
Instead, the rise will be a series of spikes, and when temperatures spike in regions already stressed, the result will be disaster.
For example, said Schwartz, a record heat wave could hit an area already suffering from long-running drought.
"When you think of places like Bangladesh, Haiti, parts of Central America, where they already have over-stretched societies," he said, "and they get hit with severe droughts or severe storms, they're going to experience massive disruptions."
Ecosystems around the world are also affected by the rising temperature, according to the draft report.
It shows more and more species -- in a vast array of ecosystems worldwide -- to be facing extinction.
In regions already drying out, especially close to the equator, even more crops would fail within the coming decades, and more forests and jungles become desiccated, according to the study.
Temperate latitudes nearer the poles are already seeing more and more heavy downpours and floods, expected to get worse.
Some regions further from the equator, says the study, will probably see increased crop yields due to the warming -- but only for a while, until they get even hotter.
Too Late to Prevent 50 Years of Warming
Scientists generally agree that the world's average temperature will rise about 2 degrees Fahrenheit in the next 50 years, no matter what humanity does.
But many calculate that if humankind somehow agrees drastically to curb greenhouse gas emission in the immediate future -- within the next 10 or 15 years -- there's a good chance that global temperatures could level off after that.
However, say many scientists, they probably couldn't begin to fall for a few centuries because carbon dioxide and other greenhouse gases stay in the atmosphere a very long time.
Action Wanted on Two Fronts, Many Scientists Say
Most climate scientists ABC News has talked to say humanity now faces a massive, unprecedented challenge on two fronts.
It must figure out how to prevent temperatures from continuing to rise after 2050, which requires immediate action now. And since there is probably no way to avoid a two-degree rise over the next 50 years, governments must also figure out how to adapt to the warmer world that is coming.
Failing to stop the rise after 2050, scientists say, will be catastrophic.
Both studies make clear that it is the poorest countries -- those with the least money to help them adapt to the rising temperature, and who produce by far the least greenhouse gases -- that will be hit hardest.
Less affected, due to their wealth, are the U.S., Europe and China -- which produce, by far, most of the man-made greenhouse gasses emissions.