The planet is getting warmer, human activity is a main factor, and the consequences will be catastrophic. It is here that the claims cease being mostly scientific in nature and begin to become political and economic questions. Unsurprisingly, it is here that the emotional tenor of the debate starts to become shrill, with visions of maritime nations lost, New York City under water, and the like. In truth, the IPPC predicts a warming, over the next century, of 1–3.5 degrees Celsius. Much of that warming, IPPC concludes, will take place at the poles; it will not be evenly distributed around the globe, and the organization writes that “on regional scales, confidence in future climate projections remains low. . . . The degree to which regional climate variability will change also remains uncertain.”
In economic terms, which Jim Manzi has considered extensively, the damage is equivalent to 1–2 percent of global GDP — a century from now. Yale economist Robert Mendelsohn concludes that the damage is more like 0.08–0.24 percent of global GDP — again, 100 years from now, when global GDP is expected to be many times larger than it is now. Real damage, to be sure, but something less than Armageddon.
http://www.nationalreview.com/article/228756/what-think-about-global-warming-kevin-d-williamson
This is an old piece, but still sums up the situation. They can't prove, but they have a strongly held belief that, human action is a cause of warming. The warming of late is so small it is within the rather large margin of error in measurements. There are several issues with the accuracy of the data - adjustments and assumptions made by the government keepers of the data sets often seeming like these are intentionally hidden.
The killer of any real AGW action though is - why bother? How do we know what would happen if things warmed? Who would it hurt, who would it help? Never mind the matter who what would or could be done to mitigate human contributions to warming, who would enforce it, and why on earth would non-wealthy nations support these efforts?
Calm down, be ready if warming happens, be rich enough to adapt, focus on economic growth around the world to have options to help humankind in the event AGW real, causes big problems, and needs a world leader to intervention.