WASHINGTON (AFP) - The White House called Tuesday for urgent action to combat climate change, saying in a major report that human-caused warming is already having a serious impact across the United States.

A four-year study by leading scientists warns of the risks of rising sea levels, droughts, fires and pest outbreaks if the world does not tackle the repercussions of greenhouse gas emissions.

"There are things we can do about it, but it's only going to happen if the American people and people around the world take the challenges seriously," President Barack Obama told US network CBS.

"We'll end up saving money and lives in the long term." Mr Obama's assistant on science and technology, John Holdren, said the report - authored by hundreds of scientists from both the private and public sectors - showed that climate change is "not a distant threat."

In fact, the National Climate Assessment marks the "loudest and clearest alarm bell to date," he told reporters.

Mr Obama has repeatedly vowed to make climate change a priority, promising during his victorious 2008 election campaign to make the United States - one of the world's bigger polluters - a leader in addressing the problem.

But he has failed to persuade Congress to take significant action, with industry-friendly lawmakers staunchly opposed to any restrictions on pollution. Mr Obama instead is moving forward with actions on his own, such as tightening standards for carbon emissions by power plants.

"We're going to have to do more, and that shouldn't be a bipartisan issue," he conceded to CBS.

The assessment warned of drought in the most populous US state of California, prairie fires in Oklahoma and rising ocean levels on the East Coast, particularly in Florida. Sea-level rise is also eating away at low-lying areas such as in Mississippi.

The report - accompanied by a user-friendly website at www.globalchange.gov - cited a locally sponsored study as saying that coastal areas in Alabama, Louisiana, Mississippi and Texas could face annual losses of US$23 billion (S$28.6 billion) by 2030, with about half of those costs related to climate change.

The impact of global warming is unevenly distributed across US territory, with spectacular effects in Alaska, which researchers said has warmed twice as fast as the rest of the country.

"Arctic summer sea ice is receding faster than previously projected and is expected to virtually disappear before mid-century," the report said.

"This is altering marine ecosystems and leading to greater ship access, offshore development opportunity and increased community vulnerability to coastal erosion." It warned that rising permafrost temperatures would cause drier landscapes, more wildfire, changes to wildlife habitat and greater infrastructure maintenance costs.

Roads and other facilities vital to the US economy are also under threat from rising water levels or more intense tropical storms, the report said.

It said that State Highway 1 in Louisiana - the only road linking New Orleans to the strategic oil hub of Port Fourchon - is sinking as sea levels rise.

A 90-day shutdown of the highway due to floods or storms would cost the nation some US$7.8 billion, it warned.

The study said that evidence of the planet's warming was "unambiguous" and that the scale of climate change over the past half-century can only be explained by human activity, in particular the burning of fossil fuels such as coal, oil and gas.

Last month, the Intergovernmental Panel on Climate Change - a Nobel Prize-winning scientific group led by the United Nations - said the world had a 15-year window to come up with affordable action to avoid the worst effects of climate change.

But Republican lawmakers, who control the House of Representatives, quickly denounced the study. Senator Mitch McConnell, the Republican leader in the Senate, said that the US priority should be on economic hardship.

Mr Obama will "get loud cheers from liberal elites - from the kind of people who leave a giant carbon footprint and then lecture everybody else about low-flow toilets," said Mr McConnell, whose state of Kentucky is a major coal producer.

"Even if we were to enact the kind of national energy regulations the president seems to want so badly, it would be unlikely to meaningfully impact global emissions anyway unless other major industrial nations do the same."

China has surpassed the US as the largest carbon emitter. It has invested heavily in solar and other renewable energy to reduce the intensity of its carbon pollution but has said it is unrealistic for it to curb its emissions in absolute terms.

Senator Ed Markey, a member of Mr Obama's Democratic Party active on environmental issues, said the report showed that climate change has become a "clear and present danger" that requires action.