Maybe everyone who contributed to Channel 4's program The Great Global Warming Swindle is a liar, but they're sure good at it. And, they have impressive job titles. Everyone who contributed came across as credible. Whether they were on the level or distorting the truth I can't say.
The safe bet is to side with the IPCC and my gut instinct is to side with the others. I don't buy the alarmism of the "Inconvenient Truth" brigade, mostly because the Earth has been warmer in the past. That doesn't mean that I don't accept that the Earth may well be warming or that there is some possibility that man is responsible for that, I just don't think we're facing the end of the civilized world.