A new British government study considered the most comprehensive review on the economic impact of global warming says “staying the course” will have dire consequences for human social and economic activity. At the same time, it says the most catastrophic consequences can be averted if concerted international action is taken now.

The 16-month study, released this week, was led by Sir Nicholas Stern, a former chief economist at the World Bank who now heads Britain’s Government Economic Service. It predicts that without prompt action, global warming could have effects comparable to those of the world wars and Great Depression of the 20th century, leaving hundreds of millions of people hungry and thirsty, increasingly threatened by diseases and displaced by rising sea levels and drought.

“There is still time to avoid the worst impacts of climate change, if we act now and act internationally,” Stern said as the report was released. “But the task is urgent. Delaying action, even by a decade or two, will take us into dangerous territory,” he added. “We must not let this window of opportunity close.”

In a statement released together with the report, its framers said global warming’s effects will be worldwide, but the poorest countries will suffer most. They pointed out that the cost of doing nothing could be as high as 20 percent of the world’s gross domestic product each year, but avoiding the worst impacts of climate change would be vastly cheaper, about 1 percent of global GDP.

The report emphasizes the urgency of a global policy response, with a common international understanding of goals and a strong framework for cooperation.

As the world’s biggest emitter of greenhouse gases, the United States has a special responsibility in averting this looming disaster. But the Bush administration has adamantly rejected earlier warnings about climate change, and has refused to ratify the Kyoto Protocol, the world’s first international agreement requiring cuts in global warming emissions.

The Stern report offers yet another reason for Americans to vote to end the extreme right’s domination of Congress, and to retake the White House from Republican control in 2008.