There is no reason to believe the IPCC given its history of presenting a brief for one side of a scientific argument while ignoring persuasive counter arguments, confirmation bias, and outright political activism. Here is a criticism citing some of the IPCC's shortcomings.

http://ross.mckitrick.googlepages.com/EPA-ANPRsubmission.pdf

The new Administration in Washington seems to have bought into the AGW hypothesis, and is planning on implementing a ‘carbon cap and trade’ scheme that is ripe for corruption, and will be harmful to the economy of the US (and ultimately the world).

This article by James A Peden, reproduced below, has a good description of atmospheric science. I would have included more discussion of the role of water vapor in the AGW hypothesis. As I understand them, James Hansen and the IPCC agree that CO2 itself does not have the potential to increase the Earth’s temperature much. Rather they think that a slight increase in air temperature, by the Clausius-Clapeyron equation, will increase the amount of water vapor in the air, thus further increasing temperature due to the greenhouse gas effect of water vapor. (I would wager that Al Gore would be unable to solve the Clausius-Clapeyron equation to show that the amount of water vapor the air will hold increases almost exponentially with temperature.) The further increase in temperature causes ice caps to melt, reducing the amount of solar flux reflected, thus further increasing temperature. Elaborate General Circulation Models (GCMs) are used to make projections of future temperature. Some important phenomena such as cloud cover, which obviously has a large effect on temperature, are not well modeled, so are parametrized. Basically, they use what those of us who make large numerical models call ‘fudge factors’ to characterize these parameters. The ‘fudge factors’ make the reliability of the models highly questionable. And, satellite data now being obtained by Spencer and others casts severe doubt on the accuracy of the GCMs.



It is amazing to me that a group of people with a socialist agenda were able to set up the IPCC and convince politicians around the world, who are mostly scientifically ignorant, that CO2 is significantly changing the climate of the world. Vaclav Klaus appears to be an exception, but I think that is more because he recognizes tyrants based on his personal experience than his scientific knowledge. Lord Monckton appears to have good scientific knowledge, but other politicians don’t pay any attention to him. I don’t understand how the US can adopt the policies that Obama espouses without a serious scientific debate on this subject that, as far as I can tell, has never happened anywhere in the world. The IPCC clearly does not follow the scientific principle, and cannot be relied on. Their position was founded on the Mann ‘hockeystick’ which has been shown to be bogus, but they have never acknowledged the simple fact that the base of the AGW hypothesis has collapsed.

Here is Peden's editorial:

http://www.middlebury.net/op-ed/global-warming-01.html