Four former heads of the US Environmental Protection Agency who served under Republican presidents urged lawmakers Wednesday to stop bickering over whether climate change is real, and start finding solutions.

Global warming is an increasingly polarizing issue in American politics, with most Republicans questioning the science behind it and most Democrats calling for stricter pollution limits.

In the absence of legislation to curb fossil fuel burning, President Barack Obama earlier this month urged the EPA to set carbon pollution standards for power plants that would slash carbon emissions 30 percent by 2030.

Obama's proposal, his most ambitious yet against climate change, also called for increasing global cooperation to curb pollution and for US financial incentives for renewable energy.

But the regulations "will harm our fragile American economy," Republican Senator John Barrasso told the Senate Committee on Environment and Public Works.

"Thousands of people will lose their jobs," he added, describing the measures as "all pain and little gain" toward reducing global temperatures.

- 'Scare tactics' -

Barrasso and other lawmakers from both parties spent the entire first hour of the 2.5-hour hearing making their own opening statements.

They debated the legitimacy of climate science and traded warnings over the cost of action versus inaction.

California Democrat Barbara Boxer said critics call her a "job killer" each time she backs a clean environment initiative.

"These scare tactics, they have been tried before, and they are just not real," said Boxer, who chairs the committee.

The four former EPA administrators who testified at the hearing served under presidents Richard Nixon, Ronald Reagan, George H.W. Bush and his son, George W. Bush.

As a group, the quartet penned an op-ed in The New York Times last year that said there was no longer any credible debate over whether humans were causing climate change.

At the hearing, they urged lawmakers to put aside their differences.

- Constructive response -

"The two parties were able to rally around a common purpose in the early days of environmental policy making," said Christine Todd Whitman, a former New Jersey governor who served as EPA chief under George W. Bush from 2001 to 2003.

"It is urgent that they do so again."

However, some Republican senators, including David Vitter of Louisiana, expressed doubt about the 97 percent scientific consensus on climate change.

"I am frustrated again quite frankly at some of the cartoonish nature of the assertions," said Vitter as Boxer left the hearing early, walking behind his seat toward the exit.

Consensus on climate change legislation has long eluded lawmakers in the nation blamed for being the second largest world polluter after China.

An April report from the United Nations Intergovernmental Panel on Climate Change said the US must enact laws that would make a major dent in pollution, such as a carbon tax that would force big oil and coal polluters to pay extra.

But such proposals are opposed by Republicans, who hold a majority in the House of Representatives.

William Reilly, who led the EPA under George H.W. Bush from 1989 to the end of 1992, praised Obama's moves and urged lawmakers to follow suit.

"While the president has taken many important steps, a full and constructive response is needed from Congress," Reilly said.

The first-ever administrator of the EPA, founded in 1970, said the United States has a responsibility to lead the rest of the world.

"We like to speak of American exceptionalism," said William Ruckelshaus, who served from 1970 to 1973 under Nixon and again from 1983 to 1985 under Reagan.

"If we want to be truly exceptional, then we should begin the difficult task of leading the world away from the unacceptable effects of our increasing appetites for fossil fuels before it is too late."

Ex-EPA chief Lee Thomas stressed that many of the solutions were known, including better energy efficiency and more reliance on low-emission energy production.

A poll out Wednesday by The Wall Street Journal and NBC News found that most Americans support Obama on climate change.

More than six in 10 of the 1,000 Americans surveyed said climate change action was needed, and 57 percent favored greenhouse gas emission cuts even if this triggered higher energy bills.