WASHINGTON, US - US SECRETARY of State Hillary Clinton on Monday named a special envoy for climate change and declared the 'time for realism and action is now', in a break with Bush policy on climate change.

Her appointment of Mr Todd Stern, a veteran of the Kyoto Protocol talks, signalled how seriously President Barack Obama's administration sees the threat from climate change after his predecessor George W. Bush played it down.

'So the urgency of the global climate crisis must not be underestimated,' said Mrs Clinton as she stood next to Mr Stern, less than a week after Mr Obama was sworn into office.

'Under President Obama, America will take the lead in addressing this challenge, both by making commitments of our own and engaging other nations to do the same,' Mrs Clinton said.

Mr Stern's appointment sends 'an unequivocal message that the United States will be energetic, focused, strategic and serious about addressing global climate change and the corollary issue of clean energy', Mrs Clinton said.

Mr Stern, a lawyer and environmental expert at the Center for American Progress, was an advisor from 1993 to 1998 to president Bill Clinton, working on the Kyoto and Buenos Aries climate change negotiations.

But Mr Bush angered governments and environmentalists worldwide when he rejected the Kyoto Protocol, which expires in 2012.

Mr Stern, named as the principal adviser on international climate policy and strategy and as the administration's chief climate negotiator, said change was now coming.

'President Obama and Secretary Clinton have left no doubt that a new day is dawning in the US approach to climate change and clean energy,' Mr Stern told a ceremony in the ornate Benjamin Franklin room at the State Department.

'The time for denial, delay and dispute is over. The time for the United States to take up its rightful place at the negotiating table is here,' Mr Stern said.

Environmentalists alleged that the Bush administration ignored and tried to hide looming evidence of global warming and the key role of human activity in climate change.

Mr Obama meanwhile, vowed to lead the world on climate change as he set about shredding Bush administration global warming policies with new domestic measures designed to force the development of fuel-efficient cars.

The new president required the Environmental Protection Agency to reconsider whether to grant California a waiver to regulate car emissions blamed for contributing to global warming.

Former president Bush's administration had blocked efforts by the vast western state and a dozen others to impose their own limits on carbon dioxide gas emissions.

A total of 172 countries and government entities have ratified the Kyoto Protocol, which requires them to meet targeted curbs in their greenhouse-gas emissions by 2012, but exempts developing countries.

That means countries such as China and India are exempt, even though they are on track to outpace the United States as the world's biggest polluter.

Washington has made their exemption a central argument for not signing on to the protocol which was established in 1997.

The Bush administration called Kyoto 'fatally flawed', and said it would cripple the US economy. -- AFP