US lawmakers this week took up the issue of climate change and how to address it after President Barack Obama made an impassioned plea for action on fighting global warming.

But even as the Senate and House of Representatives convened experts Wednesday who presented scientific bases for global warming, some scientists and political experts voiced skepticism over the need to combat climate change and the likelihood of passing any such legislation this year.

In his first address to the US Congress since taking office five weeks ago, Obama late Tuesday called on legislators to forge "a market-based cap on carbon pollution," similar to systems adopted in Europe which create financial incentives for non-polluters.

He also highlighted the need to "truly transform our economy, protect our security, and save our planet from the ravages of climate change."

According to Michael Levi, an expert at the Council on Foreign Relations, even if Obama's commitment to such a system were embraced by all players in the contested battle over carbon gases, passage of a law requiring such a system this year would be difficult.

"The president appeared to be careful to ask for a bill this Congress, not for a bill this year," Levi said, noting that there is a key United Nations climate conference in Copenhagen set for December.

"I know there are members in the House who would like to move a bill this year... but the job in the Senate will be considerably more difficult," he said.

Levi said several energy initiatives in the works are getting lukewarm reception from some lawmakers, particularly those representing coal-producing states averse to punishing restrictions on their energy production.

"It would be very embarrassing for the president to try and fail in advance of Copenhagen," where Washington is expected to aggressively take up the fight on global warming after inaction by the previous administration, Levi told AFP.

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

Democrat Barbara Boxer of California, chair of the Senate's environment committee, said Wednesday's hearing would "answer that call" made by Obama the night before.

"We must be guided by the best available science as we address the challenge of global warming," she said.

But in a stark display of the polarization of US opinions about the reality of climate change and the debate over whether it needs to be fought, the committee's number two member, Republican James Inhofe of Oklahoma, took the opposite view.

"Contrary to what the media and the UN have promoted, there is a growing body of scientific studies and scientists who are openly rebelling against the so-called consensus" that use of fossil fuels is causing climate change, Inhofe said.

Obama administration officials have already floated the prospect of a carbon emissions tax, and hinted that any new climate legislation would likely lead to higher energy costs even as Americans grapple with a harsh economic recession.

Inhofe, warning of the extraordinary financial costs of reaching "pie-in-the-sky" reduction levels, decried Obama's proposed cap-and-trade system as a 6.7-trillion-dollar "climate bailout."

Some researchers at the Senate hearing, among them India's Rajendra Pachauri who chairs the UN's Intergovernmental Panel on Climate Change, cited earlier climate models and recent studies in reiterating the need to reduce carbon emissions as quickly as possible in order to reverse global warming.

"Delayed emission reductions significantly constrain opportunities to achieve lower stabilization levels and increase the risk of more severe climate change impacts," Pachauri warned.

One of the four scientists who testified, physics professor William Happer of Princeton University, openly challenged the IPCC's conclusions and told the committee that the increase in carbon gases "is not a cause for alarm and will be good for mankind."