An ExxonMobil spokesman dismissed the report as "an attempt to connect unrelated facts, draw inaccurate conclusions and mislead the audience with a fiction about ExxonMobil's true positions."

The union, a nonprofit group based in Cambridge, Massachusetts, said ExxonMobil, the world's biggest publicly traded corporation, had succeeded in parlaying a relatively modest investment into unwarranted public doubt on findings that have been overwhelmingly endorsed by mainstream science.

ExxonMobil did this by using the same methods used for decades by the US tobacco industry, the report said, including:

-- raising doubts about even the most undisputed science;

-- funding a variety of front organizations to create the appearance of a broad platform;

-- recruiting a number of vocal climate change contrarians;

-- portraying its opposition to action as a quest for "sound science" rather than business self-interest;

-- using its access to the Bush administration to shape federal communications and policies on global warming.

TOBACCO TACTICS

US tobacco companies used these tactics for decades to hide the hazards of smoking, and were found liable in federal court last year for violating racketeering laws.

Global warming has been blamed for stronger hurricanes, more wildfires and worse droughts. While there have been cycles of warming and cooling throughout Earth's history, the last 30 years have seen a steep warming trend which most scientists say is due to emission of so-called greenhouse gases by the burning of fossil fuels in vehicles, factories and power plants.

ExxonMobil has funded legitimate scientific studies on climate change, the watchdog report said, but noted it has also spent approximately US$16 million between 1998 and 2005 on 43 organizations that have cast doubt on the reality of human-caused global warming.

The report said these have ranged from US$30,000 for the group Africa Fighting Malaria, which argues on its Web site against urgent action on climate change, to US$1.6 million to the American Enterprise Institute, a pro-business think tank in Washington.

James McCarthy, professor of biological oceanography and director of the Museum of Comparative Zoology at Harvard University, noted a 2005 statement issued by the US National Academy of Sciences and 10 science academies from other countries, affirming that "climate change is now sufficiently clear to justify nations taking prompt action."

"This report reveals for the first time the degree to which efforts to exaggerate uncertainty in climate science produce non scientific reports designed to cast doubt on published scientific climate studies have been orchestrated by ExxonMobil," McCarthy said at the news conference.

Company spokesman Dave Gardner said in an e-mail that the company acknowledged the burning of fossil fuels is a major source of greenhouse gas emissions.

He said ExxonMobil supports various public policy groups but said financial support does not mean it has control over the groups' positions.