What if they are wrong?

Because the theory of anthropogenic global warming (AGW) depends on a feedback mechanism between increase in CO2 and an increase in atmospheric water – a mechanism about which there is considerable, scientifically justified doubt – it is possible that CO2 has effectively no influence on global climate.

There is now considerable data collected, and being collected, that gives a fairly accurate view of the global temperature, insofar as such a thing can be defined. And the temperature record shows reasonably clearly that a heating took place from around the beginning of the industrial age in the early 20th century until around 1940, followed by 30-40 years of cooling, followed again by 20 years of warming until ~1998. In an interesting admissionthe (British) Met Office and the University of East Anglia Climatic Research Unit have now admitted that the global temperature has ceased rising for the last 15 years. Don Surber has done a nice little riff on this report here.

The main point is that there are other potential mechanism to account for the warming trend of 1980-1998 than CO2. Notably, ocean climates and interactions between solar wind and cosmic rays relating to earth's cloud formation are scientifically established mechanisms for the change.

Here I ask this. Suppose it turns out that CO2 has essentially nothing to do with the earth's climate. How will the history of this colossal mistake be written?

They will say that a mechanism called the "greenhouse effect," was postulated long ago (~1824 by Joseph Fourier) and gained adherents in the late 20th century. They will say that the theory was seemingly invalidated by the decrease in global temperatures from 1940-1975, but that the adherents patched this up by explaining the cooling with pollution, specifically sulfur, from industry

They will say that the theory was challenged by the noted vast gap between the amount of CO2 produced by civilization and the substantially smaller increase in CO2 in the atmosphere, but that the theory was patched up by examining the increased CO2 uptake by the hydrosphere and the biosphere.

They will say the theory was seemingly invalidated by the evidence that the atmosphere was already nearly opaque in the wavelengths that are absorbed by CO2 and so the additional CO2 could have, on its own, little effect, but that the theory was patched up by positing a feedback mechanism between the small temperature increases directly due to CO2 and the production of water vapor which is the main greenhouse gas.

They will note that the theory of anthropogenic global warming (AGW) proceeded much like any scientific theory (cf. Thomas Kuhn, The Structure of Scientific Revolutions) in that it was modified and patched up and adjusted to fit empirical challenges until it finally collapsed altogether under the weight of incontrovertible evidence. But, the scientific historians will have a new phenomenon to consider, and that is the social and political context of this particular scientific theory.

Kuhn describes very well the build-up of evidence that ultimately leads to the over-turning of accepted orthodoxywithin the scientific community, of some particular theory. But AGW is intrinsically wrapped up with political ideology and, increasingly, with economics and government (cf. "Solyndra"). The only apt comparison I can think of is Lysenkoism, the anti-genetics theory of Trofim Lysenko that was bought wholesale by Stalin and ultimately hobbled the entire Soviet biological establishment for generations (to say nothing of its role in leading to the starvation of people who followed its tenets in regard to things like agriculture).

Scientific revolutions are difficult and traumatic enough without the added inertia of government sponsorship. To put it more bluntly, scientists have difficulty enough admitting that they have egg on their faces. Throw in the Solyndras of the world and the United Nations and the entire anti-capitalist Global Left and the backing out of this theory will be nothing short of a fiasco.

If someone were, for instance, to come up with indisputable evidence tomorrow that CO2 has essentially no impact on earth's climate, could the world accept it? With the development of frakking and the concomitant extension of carbon based energy resources hundreds of years into the future, what would they do with all the windmills?

Well, the truth of this issue should be apparent within about 15 years…at which point we may be allowed to buy incandescent light bulbs again.