Sometimes the truth is just so inconvenient, people choose subconsciously to reject it. Climate change is a very good example of this. If the claims of scientists are true, then something has to be done - and whatever the something is will be horribly expensive, economically disadvantagious, personally inconvenient for millions of people and politically difficult in a time when any form of regulation meets with popular resistance. Far easier simply to deny anything is wrong, and thus remove the need to do anything. It isn't even something people realise they are doing.
There are several reasons that people are skeptical of global warming:
1. The current global warming evangelists are the equivalent of a Christian televangelist who gets caught with hookers and blow. If you believe that carbon is killing the planet, then don't buy giant mansions and yachts and have Global Warming conferences in Cancun. Live in modest houses and teleconference.
2. Environmentalists should go out of their way in supporting every alternate energy source, including nuclear. However, instead of working on answers they are always presenting roadblocks--even in technologies like wind and solar. http://solarpowernews.org/environmentalists-mojave-desert/ If you are serious about global warming, you will take some risks on desert animals to save all the rest of them.
3. Climate science seems like a bit circular---All scientists believe in AGW, but to be accepted as a scientist you need to believe in AGW. And it isn't a "hard" science in that you can experiment and see the results because, well, if AGW is occurring you can't wait till everyone is dead. On the other hand, we are aware of significant climate change in relatively recent human history (Medieval Warm Period, Little Ice Age, etc) that are not related to humans. And AGW isn't a new idea--Edward Gibbon blamed deforestation for Germany's warming in HISTORY OF THE DECLINE AND FALL OF THE ROMAN EMPIRE.
Personally, when I hear the science is settled, everyone agrees 100%, and if you don't believe you are like a holocaust denier I get quite skeptical because NOTHING in science is that clear cut. It feels like all this pressure to agree and submit is because someone is hiding something.
4. Based on above, AGW seems to many like a profit scam and a means to control people. It is a little too convenient that some of the evangelists are getting very rich off of AGW and also a convenient way of keeping people "in their place". Gore and Bono can fly private jets, because they are important. I can't, because I'm not. The lesser classes can't have too much or otherwise the planet with burst into flames. The rich, however, never need to change--they just way their indulgences and go on their merry way.
I personally actually *believe* that we need to be carbon neutral, and am angry that the very people who should be moving society in that direction are some of the biggest obstacles to change. If environmentalists can't tell a bunch of obscenely rich and powerful people in Nantucket to suck it up and allow a wind farm to be built to help save the planet, how can you ask millions to reduce their entire standard of living to do the same?