Naomi Oreskes explains with great clarity the driving force behind climate denial - aversion to the political and economic implications of climate change.
UPDATE: Some commenters are asking about evidence that humans are causing global warming. A summary is provided at http://sks.to/evidence and http://sks.to/agw with links to the many peer-reviewed papers that provide empirical evidence for human caused global warming.
Many thanks to the producers of I Can Change Your Mind about Climate for granting permission to repost this footage, which didn't make the final cut of the documentary. More info about the film is available at http://www.abc.net.au/tv/changeyourmind/