A global warming report issued yesterday by the United Nations paints a near-apocalyptic vision of Earth's future: more than a billion people in need of water, extreme food shortages in Africa, a landscape ravaged by floods and millions of species sentenced to extinction.
Despite the harshness of its vision, the report was quickly criticized by scientists who said its findings were watered down at the last minute by government bureaucrats seeking to deflect calls for action.
"The science got hijacked by the political bureaucrats at the late stage of the game," said John Walsh, a climate expert at the University of Alaska who helped write a chapter on the polar regions.
Even in its softened form, the report outlined a range of devastating effects expected to strike all regions of the world and all levels of society. Those without resources to adapt to the changes will suffer the greatest impact, according to the study from the U.N.'s Intergovernmental Panel on Climate Change.
"It's the poorest of the poor in the world, and this includes poor people even in prosperous societies, who are going to be the worst-hit," said Rajendra Pachauri, chairman of the IPCC, which released the report in Brussels yesterday.
The report, the second issued this year by the United Nations, marshaled more than 2,500 scientists to give their best predictions of the consequences of a few degrees increase in temperature. The first report, released in February, characterized global warming as a runaway train that is irreversible but that can be moderated by societal changes.
That report said, with more than 90 percent confidence, that the warming is caused by humans, and its conclusions were widely accepted because of the years of accumulated scientific data supporting it.
The second report was more controversial because it tackled the more uncertain issues of the precise effects of warming and the ability of humans to adapt to it.
"When you put people into the equation, people who can adapt and respond and change their behavior, it adds another layer of complication," said Gary Yohe, an economist at Wesleyan University who helped write the report.
The report, in a sense, is a more focused indictment of the world's biggest polluters - the industrialized nations - and a more specific identification of the victims.
The last-minute negotiations led to deleting timelines for events and scaling back the degree of confidence in some projections. Both actions will ease pressure on industrialized nations to reduce their emission of carbon dioxide and other greenhouse gases that are warming the planet.
Several scientists vowed afterward that they would never participate in the process again because of the political interference.
"Once is enough," said Walsh, who was not present during the negotiations in Brussels but kept abreast of developments with a steady stream of e-mails from colleagues. "I was receiving hourly reports that grew increasingly frustrated."
The report paints a bleak picture of the future, pointing out that the early signs of warming are here.
Spring is arriving earlier, with plants blooming weeks ahead of schedule. In the mountains, the runoff begins earlier in the year, shrinking glaciers in the Alps, the Himalayas and the Andes.
Habitats for plants and animals, both on land and in the oceans, are shifting toward the poles.
Nineteen of the 20 hottest years on record have occurred since 1980, according to previous studies. The report said that more frequent and more intense heat waves are "very likely."
In some places, warming might seem like a good thing at first. Worldwide food production is expected to increase with the first few degrees of temperature increase. For a time, an expanded fertile zone in the higher latitudes could offset losses in the tropics.
But at a certain point, crops everywhere will suffer as drought conditions spread. By midcentury, temperature increase and drying soil will replace tropical forest with savanna in eastern Amazonia, the report predicts.
In North America, snowpack in the West will decline, causing more floods in the winter and reduced flows in the summer, increasing competition for water for agriculture and municipal use.
California agriculture will be decimated by the loss of water for irrigation, experts have previously said.
Water will come more often around the world in its least-welcome forms: storms and floods.
Rising temperature would reconfigure coastlines as the oceans rise and seawater surges onto land. The tiny islands of the South Pacific and the Asian deltas will be overwhelmed by storm surges as sea levels rise.
In the Andes and the Himalayas, melting glaciers will unleash floods and rock avalanches. But within a few decades, as the glaciers and snowpack decline, streams will dwindle, cutting off the main water supply to more than a sixth of the world's population.
Africa will suffer the most extreme effects, with a quarter of a billion people losing most of their water supplies, the report said. Food production will fall by half in many countries, and governments will have to spend 10 percent of their budgets or more to adapt to climate changes, the report said.
At least 30 percent of the world's species will disappear if temperatures rise 3.6 degrees Fahrenheit above the average levels of the 1980s and 1990s, the report said.
"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 a high mountain," said Stephen Schneider of Stanford University, one of the scientists who contributed to the report.
Alan Zarembo and Thomas H. Maugh II write for the Los Angeles Times.