The home of global capitalism can be found just outside Dallas. Set in the midst of a sprawling industrial park, it is a huge, squat pink stone edifice, with a sloping black roof like a rustic villa, but a villa made for giants. It is the headquarters of Exxon Mobil, and it houses a plush management suite that is known across the energy industry as the "God Pod", with the reverence befitting a corporation which last month emerged as the most profitable in the history of human endeavour. It is no exaggeration to say the decisions made here in Irving, in this high temple of private enterprise, will shape the future of the planet. Exxon Mobil, which trades in Europe as Esso, does not believe in the certainty of global warming - it casts doubt on evidence that industrial emissions of greenhouse gases are raising temperatures. And not only is it sceptical, it has conducted an aggressive and expensive public relations operation to challenge scientific orthodoxy on the subject, as part of its battle to halt international efforts to put an expensive cap on the smokestacks. Now a new Republican government, elected with the help of $1.2m from Exxon Mobil, has abandoned the centrepiece of those international efforts, the Kyoto treaty on global warming. The Bush administration, staffed from the president down by former oil executives, has also ruled out plans to limit US emissions of carbon dioxide in the foreseeable future.

The exact link between campaign contributions and the subsequent acts of an administration can only be guessed at. But Exxon's critics argue that the behemoth's assertive embrace of any scientific evidence against global warming - however anecdotal or dubious in origin - has lent it a credibility it does not deserve. It has also given President George Bush "cover" for his rejection of Kyoto. The mood in Irving in the new Bush era is confident, even jovial. But it is equally clear that its executives have been put on their guard against complacency. Exxon Mobil (born of a mega- merger in 1999) has, to say the least, an image problem. In Australia, the first ever conference of the world's green parties yesterday agreed to launch a boycott against Exxon and other US oil companies. They want to "send a message" to the companies on the role they allegedly played in getting Mr Bush elected. "We know we have a giant target painted on our chests," said Ken Cohen, Exxon Mobil's head of government relations and public affairs. Consequently, the company has decided to emerge from its customary insularity and mount something resembling a charm offensive. And that is presumably why the outer gates of the God Pod were opened last week, and two of the corporation's vice presidents were deployed to explain why Exxon Mobil remains dubious about global warming and how it is nevertheless cleaning up its act the free-market way. Mr Cohen and Frank Sprow, in charge of safety and environmental health, both insist that Exxon Mobil's position has been misunderstood.

Rather than denying the existence of global warming outright, they argue, Exxon Mobil is simply pointing out the room for error in such an ever- changing and unpredictable phenomenon as climate, and urging caution. "You really can't bring human influence out of the noise of natural variability at this point," Mr Sprow said. "Science is a process of inquiry... I'd like the answer tomorrow afternoon but it may be a decade before the science really gets crisp, because there's so much fundamental information that has to be worked on." Even though the science may not be rock hard, Mr Sprow said, Exxon is working on alternative energy sources, such as low emission fuel cells for cars, and cutting down emissions in its refineries. It spends $12m a year researching means of reducing carbon dioxide emissions, and has so far managed to reduce its own output by 3%. These arguments have not convinced the corporation's enemies in the green camp. It is big enough and controversial enough to have galvanised an entire environmental movement, Campaign Exxon Mobil, devoted to keeping it under surveillance. The campaign's spokesman, Peter Altman, argues that the vaunted $12m in carbon dioxide research is a fairly paltry share of the $17bn net income Exxon Mobil earned last year. Furthermore, he said, whatever beneficial effect that money might have is more than outweighed by the corporation's role in undermining the accepted wisdom that global warming is a real threat. Other oil companies, such as BP and Shell, have crossed the barricades. At its annual general meeting on Thursday, BP will come under pressure from green activists who have laid down formal motions calling on the company to switch more resources to the development of renewable energy sources.

However, Exxon Mobil has kept up the fight on climate change, going out of its way to support maverick sceptics whose conclusions agree with its own. Mr Sprow is urbane and sophisticated. He insisted repeatedly that Exxon is not "in a state of denial" over global warming. However, on two vital issues it is clear that Exxon's position remains unchanged. It does not have faith in the dire warn ings issued this year by the UN-appointed International Panel on Climate Control (IPCC) and it is vigorously opposed to the Kyoto treaty. In its latest assessment of the threat, the IPCC found "new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". The panel, which consulted about 2,000 scientists from 100 countries, predicted that the earth would heat up between 1.4C and 5.8C over the next century, with potentially catastrophic results. The report was presented as a scientific consensus, but Exxon challenges that claim. It points to the role of political appointees on the IPCC in selecting and summarising scientific evidence. The same sort of people were promoting a bureaucratic solution to the problem embodied in Kyoto. Mr Sprow argued that there is little likelihood of Kyoto being implemented by the majority of industrialised countries, and that it would hardly make a significant difference to long-run greenhouse gas emissions even if they did.

For Exxon, these are both reasons to dump the treaty. For Kyoto's supporters, however, they are all reasons to put the treaty (which would require a 7% drop in US emissions between 1990 and 2012) into effect quickly and then move beyond it. By poking spanners into the works, the environmental lobby believes, Exxon is helping delay concerted action to stave off global warming and the chaos it may wreak with the climate. "Exxon is grasping at straws," said Kert [SIC] Davies, the director of Greenpeace's US global warming campaign. "They're looking for everything they can do to reposition the existing knowledge on global warming from fact to theory." Even before its current public relations drive, Exxon has had remarkable success in making its influence felt. But perhaps more importantly, Exxon's executives appear to hold sway over a man who once dreamed of rivalling their success but failed as an oil man and had to settle this year for becoming president of the United States.