"Whether we like it or not, it was extremely effective propaganda," said Timothy Ball, an environmental consultant and former climatology professor at the University of Winnipeg.

"It was appropriate that he got an Oscar from the land of make-believe," he joked.

The gathering was sponsored by the Heartland Institute, a non-profit libertarian organization that studies environmental and other issues "from a free-market perspective" and argues that "property rights and markets do a better job than government bureaucracies."

Attendees watched a movie, "A Climate of Fear," by conservative TV commentator Glenn Beck, who charged that anyone who opposes the view that carbon dioxide and other greenhouse gases are causing the earth to warm up, are branded "heretics or Nazis."

"Al Gore's version of climate change has no longer become science. It's dogma. And if you question it, you are a heretic," Beck said in the film.

The conference challenged a strong majority of world scientific opinion that has concluded that greenhouse gases are contributing to global warming.

This view has been backed by bodies such as the US National Academy of Sciences, American Meteorological Society, American Geophysical Union, the American Association for the Advancement of Science and the Intergovernmental Panel on Climate Change.

"Our imperfect understanding of the causes and consequences of climate change means the science is far from settled," said Fred Singer, of the Science and Environmental Policy Project.

"Proposed efforts to mitigate climate change by reducing greenhouse gas emissions are premature and misguided. Any attempt to influence global temperatures by reducing such emissions would be both futile and expensive," he said.

Lord Monckton, who advised former British Prime Minister Margaret Thatcher, blamed "the international left," for promoting that global warming is dangerous.

"It's the media wanting a scare story," he said.