I strongly beg to differ. Economics, like political science, is an attempt to apply the scientific method to a field where pure experimentation is impossible, but it's not searching for a physical basis for natural phenomena in the way that scientific fields do, like chemistry, physics, biology, and so forth. How do you possibly propose, for instance, to control for experimental error among the inherently irrational decision-making processes of millions of individual humans? Nowhere more than in political science and economics have I seen such gross oversimplifications of complex processes, such as treating international relations in a black box theory, or in economics the periodic waves of zealots for import substitution industrialization, or trade liberalization, or other policies which are institutions of belief and dogma and not of experimental basis.

You deeply misunderstand the scientific justification for climatology if you think we need a separate control Earth for the field to have an experimental basis. Climatology is the extension of established, testable scientific principles, to a larger scale, predicting how interactions that are well established in laboratory conditions operate in the aggregate will influence the climate as a whole. The infrared activity of carbon dioxide--- the degree to which it traps heat where diatomic atmospheric gases do not--- was established in the laboratory in the 1850s by John Tyndall. The degree to which surface air temperatures, air pressure, and solar radiation produce evaporation from bodies of water, is similarly based on laboratory experimental evidence conducted by Howard Penman in the first half of the 20th century. Changes in the albedo of sections of the Earth's surface, or the effects of permafrost methane release, are similarly quantifiable and attributable to established physical principles that have been studied at the laboratory scale.

The AOGCMs synthesize, albeit imperfectly, these established physical principles into an understanding of how, in the aggregate, they drive changes in Earth's climate, as tested against millions of years of evidence, as preserved in ice cores, foraminifer sedimentation, dendrochronological data, and so forth. This is comparable not to the methods of economics or to political science, which have no physical basis, but instead to the conclusions of scientists who similarly extrapolate laboratory-established scientific principles of gravity or spectroscopy, to conclude the composition of distant stars or the orbits of their exoplanets. We don't need a separate control Earth to make well-substantiated predictions about the effects of increased carbon dioxide on global temperatures any more than we actually need to send a probe to the star Altair in order to know its temperature or surface gravity. Our understanding has a fundamental physical basis, just as it does with our understanding of climate science. You can make no such claims for economics, and I think if you did a bit more research into the basis for climatology, you would wholeheartedly agree.