Global warming could wreck the world economy as badly as did the world wars or the Great Depression, the British government said in a report issued Monday.
British Treasury chief Gordon Brown, the likely next prime minister who commissioned the report, also said former Vice President Al Gore would become an adviser to the British government on climate change.
Gore's hiring, certain to raise some eyebrows in the Bush administration, is intended to help shape "an economy that is both pro-growth and pro-green," Brown said.
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British political observers think Gore also will give Brown, who has been criticized for lacking Prime Minister Tony Blair's charisma, some much-needed star quality before he replaces Blair as head of the Labor Party government sometime next year.
Announcing the report, Blair said unabated climate change would eventually cost the world between 5 percent and 20 percent of global gross domestic product each year, and called for "bold and decisive action" to cut carbon emissions.
"It is not in doubt that, if the science is right, the consequences for our planet are literally disastrous," Blair said. "This disaster is not set to happen in some science-fiction future many years ahead, but in our lifetime."
The report is expected to increase pressure on the Bush administration — which, like the Clinton administration before it, generally opposed the 1997 Kyoto Protocol climate-change accord — to step up its efforts to fight global warming.
Report author Sir Nicholas Stern, a senior government economist, said that acting now to cut greenhouse-gas emissions would cost about 1 percent of global GDP each year.
"The evidence shows that ignoring climate change will eventually damage economic growth," said Stern's 700-page report, an effort to quantify the economic cost of climate change.
"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 added.
Stern 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."
Blair said the scientific community agrees that the world is warming, and that greenhouse gas emissions are largely to blame.
"Unless we act now ... these consequences, disastrous as they are, will be irreversible," he added.
Stern said the world must shift to a "low-carbon global economy" through measures including taxation, regulation of greenhouse gas emissions and carbon dioxide emission trading.
Brown 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.
The Bush administration argues that joining the Kyoto Protocol would harm the economy of the U.S., which is the biggest emitter of carbon dioxide and other gases blamed for global warming.
Gore signed the protocol in 1998 on behalf of the Clinton administration in a largely symbolic move.
A year earlier, the Senate, which must ratify all international treaties, passed a resolution that it would not approve the Kyoto Protocol until it was rewritten to include emissions-reduction targets for developing as well as industrialized countries.
The Clinton administration never submitted the protocol to the Senate for ratification.
Earlier this year, Blair signed an agreement this year with California Gov. Arnold Schwarzenegger to develop new technologies to combat global warming.
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 Stern report praised states such as California for developing their own objectives and policy frameworks regarding the battle against global warming.
At a news conference, Stern said U.S. cooperation is vital, and Foreign Secretary Margaret Beckett said she would advise the Bush administration that climate change is an "urgent issue that has to be tackled."
Kristen A. Hellmer, deputy director for communications at the White House Council on Environmental Quality, said Bush "has long recognized that climate change is a serious issue, and he has committed the U.S. to advancing and investing in the new technologies to help address this problem."
The United States, she said, "is well on track to meet the president's goal to reduce greenhouse gas intensity of our economy 18 percent by 2012."
Gore's office said that in his unpaid role as an adviser, he would offer Britain's Treasury his thoughts on developments in climate change science, new technologies for cutting emissions and ways of making the needed changes happen quickly.
Vicki Arroyo, director of policy analysis at the Pew Center on Global Climate Change in Arlington, Va., praised the report for trying to measure the cost of action and inaction against global warming.
The report focused on the economic impact of climate change, and did not deal with any new scientific analysis.
"Economic assessments are inexact sciences, but they are used all the time in setting insurance rates and government regulation of highway safety, pollution control and food safety. They are projections of what the risks are and the benefits of averting those risks."
Blair 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.
"Britain is more than playing its part. But it is 2 percent of worldwide emissions. Close down all, all of Britain's emissions and in less than two years just the growth in China's emissions would wipe out the difference," Blair said.
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.
Many major cities could be at risk of flooding from coastal surges, including New York, Miami, London, Tokyo, Shanghai and Buenos Aires, the study said.
Stern's report said "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," it said.
"Using the results from formal economic models, the review estimates that if we don't act, the overall costs and risks of climate change will be equivalent to losing at least 5 percent of global GDP each year, now and forever. If a wider range of risks and impacts is taken into account, the estimates of damage could rise to 20 percent of GDP or more," it said.
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."
Stern said the world must shift to a "low-carbon global economy" through measures including taxation, regulation of greenhouse gas emissions and carbon trading.
In a world where fossil fuels provide about 80 percent of the energy supply, he recommended a halt to deforestation, technology aimed at clean power, heat and transportation and developing ways to capture and store greenhouse gas emissions.
Under the 1997 Kyoto accord, 35 industrialized nations committed to reducing emissions by an average 5 percent below 1990 levels by 2012.
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. It also announced legislation that would set a goal of cutting carbon dioxide emissions by 60 percent from 1990 levels by 2050.
Stern is scheduled to discuss his report next week at the U.N. Climate Change Conference in Kenya.
The Associated Press and the Times of London contributed to this report.