David Evans once believed in global warming, and even advised the Department of Climate Change in Australia.  He has since changed his mind because the empirical evidence has not confirmed the original predictions.  In fact, it has disproven them.  He admits that carbon dioxide emissions are warming the planet, but it is clear that humans are not solely responsible for the warming, and can do little to change it.  He ends the article by saying:

Even if we stopped emitting all carbon dioxide tomorrow, completely shut up shop and went back to the Stone Age, according to the official government climate models it would be cooler in 2050 by about 0.015 degrees. But their models exaggerate 10-fold — in fact our sacrifices would make the planet in 2050 a mere 0.0015 degrees cooler! … Yes, carbon dioxide is a cause of global warming, but it’s so minor it’s not worth doing much about.

I would highly recommend you read the article.  I’ve read a number of articles on this topic over the last few years, but few have broken the issue down as clearly and concisely as Mr. Evans has.