The appointment, coming less than a week after Ms Clinton assumed her job, signalled to US allies how urgently President Barack Obama's administration takes the threat after Mr Bush played it down.

The state department official confirmed reports that Ms Clinton, wife of the former President who has listed fighting climate change a top priority, had chosen Mr Stern and that she would formally unveil the appointment later Monday.

Mr Stern is a "former Clinton White House official with experience at Kyoto and Buenos Aries climate Change negotiations," the official told AFP on the condition of anonymity.

"He has been active on the environmental front for some time," the official added.

Lawyer and environmental expert at the Washington think tank Centre for American Progress, Mr Stern served as an adviser for Ms Clinton from 1993 to 1998.

He played a key role in the Kyoto Protocol negotiations from 1997 to 1999, before becoming an adviser to the secretary of the treasury from 1999 to 2001.

Mr Obama was meanwhile set Monday to make his first moves to reverse Bush administration climate policies, with two measures designed to prod automakers into making more fuel efficient vehicles. Mr Obama was set to require the Environmental Protection Agency to reconsider whether to grant California a waiver to regulate car exhaust emissions blamed for contributing to global warming, a White House aide said.

Mr Bush had blocked efforts by California and a dozen other states to impose their own limits on carbon dioxide gas emissions.

During her Senate confirmation hearing on January 13, Ms Clinton said that Mr Barack Obama, who was inaugurated President a week later, will lead "a global and coordinated response" toward combating climate change.

Climate change is "an unambiguous security threat," she said. "At the extreme it threatens our very existence but well before that point it could well incite new wars of an old kind over basic resources like food, water and arable land," she said. -AFP