By Kathryn Westcott
BBC news website
As the Kyoto protocol on climate change comes into effect this week, the world's biggest emitter of greenhouse gases, the US, remains on the sidelines.
But the pressure on President George W Bush to re-engage with the international community to curb global warming may build in his second term.
The US is responsible for nearly a quarter of global greenhouse gas emissions
The US is responsible for nearly a quarter of carbon dioxide emissions worldwide, but President Bush refuses to budge on his opposition to mandatory restrictions.
However, this is not the end of the story.
The mood in Congress is becoming more favourable towards emissions cuts. At state level, action is already being taken. And an increasing number of large US firms are announcing plans to rein in energy consumption.
In Congress, Republican Senator John McCain and Democratic Senator Joe Lieberman are about to reintroduce their Climate Stewardship Bill which narrowly failed to pass in the Senate two years ago.
The legislation would establish a federal ceiling on carbon dioxide emissions and create an emissions trading system that would help companies meet the targets imposed.
Senator McCain expects to fail again this time round, but is optimistic of succeeding before long.
At a news conference last week he and Senator Lieberman shared the podium with a bipartisan group from both houses, and stressed that concern over global warming was growing and that it crossed party lines and geographic regions.
One convert is a senior Republican senator from Nebraska, Chuck Hagel, who fought to keep the US from joining the Kyoto protocol but is now planning to introduce himself what he describes as "comprehensive climate change legislation".
Nigel Purvis, an environment scholar at the Brookings Institution in Washington, says that on the issue of climate change, "the heart of the Republican party is moving".
"The president is not completely in charge of his party. There is an evolution, with the president, whose bread is buttered with the fossil fuel industry, on one side," Mr Purvis told the BBC news website.
On the other side, are some of the party's most popular figures, such as Senator McCain and California governor Arnold Schwarzenegger, stars of the Republican convention in 2004.
In California, Mr Schwarzenegger has promised to defend legislation signed by his Democratic predecessor, Gray Davis, that would require carmakers to cut harmful emissions from cars and trucks sold in the state by about 30%, from 2016.
Other states have indicated they may follow suit, if the measure survives a legal challenge mounted by the carmakers last month.
Environmental issues are high on the agenda in California but not in the Midwest or south
A number of states are already involved in regional initiatives on climate change.
Mr Schwarzenegger and the governors of Washington and Oregon have jointly backed a series of recommendations to reduce global warming pollution.
And in the north-east, nine states, six of them with Republican governors, are working together to develop an emissions trading scheme for carbon dioxide emissions from power plants. A number of other states are observers in the initiative.
Judi Greenwald, Director of Innovative Solutions from the Pew Center on Global Climate Change, says there are various ways these initiatives could evolve.
"It could be that this just continues and we wind up with policy from the bottom up as opposed to from the top down," she says.
"Or, it could be that enough of this work goes on at state and regional level that national policymakers may feel they want to do something more co-ordinated."
Push from businesses
At the same time, many states in the Midwest and south, have not made any moves to tackle climate change.
The issue barely registered in last autumn's presidential elections, and an annual Gallup Poll carried out just under a year ago reported that global warming was "a bit of a yawn" to most Americans. The poll did, however, find that 51% of people did find it worth worrying about.
Professor Stephen Schneider of Stanford University predicts it will actually be the business community that will push this administration or, more likely, the next one into international action.
A number of multinationals, particularly those with operations that will come under the Kyoto protocol are in favour of action, he says.
"Reinsurance industries are strongly in favour of policy action, as are an increasing number of energy companies like BP, Shell, Chevron and the hi-tech sector."
He sees businesses pushing for change because of commercial opportunities in the growing market for clean and efficient technologies. And he says there are advantages to joining an emissions trading system earlier rather than later.
The Bush administration is unlikely to have a change of heart over global warming soon
"If the US engages on an international level in say five or 10 years, under an ideologically different administration, much of the 'low hanging fruit' - the lowest cost opportunities to cut carbon dioxide abroad via trading mechanisms - will have already been bought up by Japanese and European firms."
It could actually prove more expensive, he argues, for the US to "sit on the sidelines... while its political machinery plays catch-up to the rest of the world".
Some companies are already setting voluntary limits.
A number, including DuPont, American Electric Power, Ford and Motorola have formed the Chicago Climate Exchange, which allows member companies to buy and sell emission "credits" to help them meet self-imposed cuts.
UK Prime Minister Tony Blair says he will press the administration to re-engage with other countries on climate change, during the UK's G8 presidency.
But some commentators say that Mr Blair would be better off looking to the next generation of US political leaders.
Peter Frumhoff of the Union of Concerned Scientists foresees public pressure for action on climate change steadily building on the White House - but he does not envisage an early change of heart by the Bush administration.
"Mandatory cuts are inevitable," says the Brookings Institution's Nigel Purvis. "The trend is good - but the pace will be slow."