LONDON - Unchecked global warming will devastate the world economy on the scale of the world wars and the Great Depression, a British government report said Monday, as the country launched a bid to convince doubters that environmentalism and economic growth can coincide.

Britain hired former Vice President Al Gore, who has emerged as a powerful environmental spokesman since his defeat in the 2000 presidential election, to advise the government on climate change - a clear indication of Prime Minister Tony Blair's dissatisfaction with current U.S. policy.

Blair, President Bush's top ally in the Iraq war, said unabated climate change would eventually cost the world between 5 percent and 20 percent of global gross domestic product each year. He called for "bold and decisive action" to cut carbon emissions and stem the worst of the temperature rise.

"It is not in doubt that, if the science is right, the consequences for our planet are literally disastrous," he said. "This disaster is not set to happen in some science fiction future many years ahead, but in our lifetime."

The report emphasized that global warming can only be fought with the cooperation of major countries such as the United States and China, and represents a huge contrast to the Bush administration's wait-and-see global warming policies.

Sir Nicholas Stern, the senior government economist who wrote the report, said that acting now to cut greenhouse gas emissions would cost about 1 percent of global GDP each year. He recommended a "low-carbon global economy" through measures including taxation, regulation of greenhouse gas emissions and carbon trading.

"That is manageable," he said. "We can grow and be green."

Bush kept America - by far the biggest emitter of carbon dioxide and other gases blamed for global warming - out of the Kyoto international treaty to reduce greenhouse gases, saying the pact would harm the U.S. economy. The international agreement was reached in Kyoto, Japan, in 1997 and expires in 2012.

Blair made his displeasure with U.S. environmental policy clear when he signed an agreement this year with California Gov. Arnold Schwarzenegger to develop new technologies to combat the problem. The measure imposed the first emissions cap in the United States on utilities, refineries and manufacturing plants in a bid to curb the gases that scientists blame for warming the Earth.

The prime minister and the report also said that no matter what Britain, the United States and Japan do, the battle against global warming cannot succeed without deciding when and how to control the greenhouse gas emissions by such fast-industrializing giants as China and India.

Stern's 700-page report said evidence showed "that ignoring climate change will eventually damage economic growth."

"Our actions over the coming decades could create risks of major disruption to economic and social activity, later in this century and in the next, on a scale similar to those associated with the great wars and the economic depression of the first half of the 20th century," he said.

The report said at current trends average global temperatures will rise by 3.6 to 5.4 degrees within the next 50 years or so, and the earth will experience several degrees more of warming if emissions continue to grow.

It said such warming could have effects such as melting glaciers, rising sea levels, declining crop yields, drinking water shortages, higher death tolls from malnutrition and heat stress, and widespread outbreaks of malaria and dengue fever. Developing countries often would be the hardest hit.

The report acknowledged that its predictions regarding GDP relied on sparse data about high temperatures and developing countries, and placed monetary values on human health and the environment, "which is conceptually, ethically and empirically very difficult."

Treasury Chief Gordon Brown, who is expected to replace Blair as prime minister next year, said Britain would lead the international effort against climate change, establishing "an economy that is both pro-growth and pro-green." He called for Europe to cut its carbon emissions by 30 percent by 2020 and 60 percent by 2050 - and Blair's government on Monday said it would propose a British law to that effect.

Under the 1997 Kyoto accord, 35 industrialized nations committed to reducing emissions by an average 5 percent below 1990 levels by 2012.

But Britain is one of only a handful of industrialized nations whose greenhouse gas emissions have fallen in the last decade and a half, the United Nations said Monday.

The U.N. said Germany's emissions dropped 17 percent between 1990 and 2004, Britain's by 14 percent and France's by almost 1 percent.

Overall, there was a 2.4 percent rise in emissions by 41 industrialized nations from 2000 to 2004, mostly because former Soviet-bloc countries, whose emissions declined in their economic downturn of the 1990s, increased emissions during the recent four-year period by 4.1 percent.

The British government is considering new "green taxes" on cheap airline flights, fuel and high-emission vehicles.