"To truly transform our economy, protect our security, and save our planet from the ravages of climate change, we need to ultimately make clean, renewable energy

the profitable kind of energy," Obama told lawmakers in his maiden speech to Congress.

"So I ask this Congress to send me legislation that places a market-based cap on carbon pollution and drives the production of more renewable energy in America."

The United States is the world's largest emitter of carbon gases, blamed for global warming, yet the previous administration of president George W. Bush walked away from the 1997 Kyoto treaty aimed at battling climate change.

A carbon-trading system sets a cap on the amount of pollutants companies can emit and then forces heavy polluters to buy credits from companies that pollute less -- creating financial incentives to fight global warming.

But any such measures to curb gas emissions had been fiercely rebuffed by the Bush administration, which argued that it would be too costly for companies to implement.

A day after a new report highlighted that the effect of even slight temperature increases may have been underestimated, Obama said energy was a critical issue for the future of the world's top energy consumer.

He pledged it would be one of three key areas -- along with health care and education -- that he would focus on in his first budget to be laid before US lawmakers in the coming weeks.

The president vowed to pump 15 billion dollars a year into developing technologies like wind power and solar energy, vowing to double the country's supply of renewable energy in the next three years.

Obama also praised China for making the largest effort in history to turn its economy energy efficient, and warned that the United States which invented solar technology was falling behind its competitors in Germany and Japan.

"New plug-in hybrids roll of our assembly lines, but they will run on batteries made in Korea," he said sternly.

Senator Barbara Boxer, chairwoman of the Senate Environment Committee, hailed Obama's speech and his call for legislation on capping carbon emissions.

"We will work in partnership with the president, and we will answer his call," she said in a statement.

Energy Secretary Steven Chu has already floated the idea of a carbon emissions tax in an interview last week with The New York Times.

During the US presidential campaign, the notion was kept largely on the back burner as candidates were reluctant to promote the idea of costlier energy at a time when gasoline prices were soaring.

But since Obama's administration took office in January, Congress has been working on setting up a system for swapping greenhouse gas emissions quotas similar to the one used in the European Union.

Chu, a Nobel laureate in physics, said "alternatives could emerge, including a tax on carbon emissions," the Times reported.

But he acknowledged it would be a tough sell to get a law passed in the United States that could lead to higher energy prices.

Gigatonnes of greenhouse gases spew each year into the Earth's atmosphere, acting like an invisible blanket that stores solar heat and changes the climate system.

A study published Monday by the Proceedings of the National Academy of Sciences, updating a 2001 assessment by the UN's Inter-governmental Panel on Climate Change, warned the risks to the planet of even small changes in average temperatures had been underestimated.

Those included risks to threatened systems such as coral reefs or endangered species; and extreme weather events like cyclones, heat waves or droughts.