The climate of the Earth has changed considerably throughout history. It seems intuitive that something besides the activity of man was driving the wide variations in temperature, which varied widely from no ice at the poles to most of the surface being above 40 degrees latitude being covered with ice. It seems likely that the sun must somehow be involved in these large temperature changes. The UN IPCC projections based on the hypothesis that atmospheric concentration of CO2 causes a significant increase in temperature have now been shown by recent data to not be accurate. Here is a discussion of another theory that does successfully explain the past as well as current temperature trends.

http://www.scoop.co.nz/stories/SC0811/S00028.htm