Little separates Barack Obama and John McCain on the issue of global warming. That means victory for either of them in the November 4 elections will signal a sea change in the way the United States addresses the issue.

Both agree that climate change results from human activity. Both favor cutting US emissions by 2020 and deepening them by 2050. Both want a cap-and-trade system to achieve this -- something President George W. Bush has bitterly opposed.

"In my opinion, there are more similarities than differences," said Eileen Claussen, president of the Pew Center on Global Climate Change, an independent think tank in Washington.

"Whoever will be in the White House in January, it will be a different world on the climate front," she told AFP. "They all accept the science and both have the same idea on what kind of program to have."

McCain's position on climate change stems from the Climate Stewardship Acts, which co-sponsored with fellow US senator Joseph Lieberman and which Obama -- also a senator -- supported before they failed to get enough votes to clear the Senate.

Notwithstanding their common ground on principles, the rivals do differ on details.

Obama, the Democratic hopeful, wants greenhouse gas emissions to be slashed by 80 percent within 50 years. McCain, in the Republican camp, prefers 60 to 65 percent.

Obama also wants to see 25 percent of US electricity needs met by renewable sources by 2025 -- a target that McCain opposes.

Finally, McCain is more gung-ho on offshore oil drilling -- a feeling shared by his running mate, Alaska Governor Sarah Palin -- and supports the construction of 45 new nuclear power stations by 2030.

Nuclear today accounts for 20 percent of US electricity output. But since the 1978 accident at the Three Mile Island facility in Pennsylvania, no new nuclear power stations have gone up on US soil.

Though not opposed to nuclear energy, Obama has reservations on how to deal with radioactive waste.

When it comes to international cooperation, Claussen sees little difference between Obama and McCain, given how they concur on the need to conclude a global agreement to address climate change.

"Both say we need to have a global agreement," said Claussen, a former undersecretary of state for environmental issues during Bill Clinton's Democratic presidency.

"They are both reasonably realistic about what you might expect or get from the big developing countries like China and India."

Claussen expects that, in the first six months of the new presidency, Obama or McCain will set down either a set of principles or a piece of legislation to be sent to Congress, setting out their ideas for the way forward.

But she doubts the incoming president will have time enough to have a detailed position worked out and negotiated with other nations before the next global climate change conference in Copenhagen at the end of 2009.

She cautioned Europeans against unrealistic expectations when they call for developed nations to bring emissions down 20 to 40 percent from 1990 levels, when the United States is currently 16 percent over that marker.

The Sierra Club, the biggest environmental group in the United States, which endorsed Obama in July, says it is "excited" by the fact that, for the first time, both White House candidates want to act on global warming.

"The question is no longer, 'Should se do something about global warming?'" said its spokesman David Willett. "It has become, 'What exactly are we going to do?' That is progress."

"The general scientific consensus is that to prevent the worst impact of global warming, we need to reduce our emissions of CO2 by 80 percent in the next 50 years -- and that is the target that Obama sets in his plan."

Frank Maisano, from Washington lobbying firm Bracewell Giuliani, which represents major energy producers, said it remains to be seen how lofty goals can be fulfilled in the context of the US economic downtown and the worst financial crisis since the 1930s.

"The question is not to listen to what they say, but what they try to do in their first 100 days or first year in office," he told AFP.