This is a science board, surely we could substitute a first-order scientific analysis for partisan bickering in order to explore this question. The current solar constant is 1.37 kW/m^2 at the top of earth's atmosphere (2006 RASC Observer's Handbook). Three significant digits probably isn't enough to solve this question, but I'm sure more precise figures are available somewhere, and they are probably available for decades in the past. This should be enough to determine if there is indeed a solar variability component to global warming, providing these fluxes have been measured with sufficient precision in the past.

This figure is probably an average which takes into account variability in the earth's elliptical orbit, but over the last century or two, the orbit, precession, and the obliquity of the ecliptic have changed little; nutation's effect is small but cyclical and predictable. Since these effects alter the geometry of the illumination, but not the total amount (the earth is not of homogeneous albedo), it should be at least possible to model the variations in the solar constant and determine whether they are due to observational inaccuracies, geometrical variations, or true change in the solar output.

Without hard figures, we are forced to guesswork and intuition to determine the significance and cause of global warming. According to the political debate currently raging, we are expected to accept that someone is lying, either the scientific community that accepts global warming as anthropogenic in nature, or the business community which does not. I simply don't know the answer to this, but if I had to make a decision, the potential severity of the problem, as well as my estimate of the persuasive value of conflict of of interest, leads me to believe that the burden of proof is with the denialists. I simply find it harder to believe that this a liberal/careerist plot by the scientific community than it is a cynical maneuver to protect profits. If I'm wrong, then I am advocating a policy which will probably lead to some economic dislocation. If they're wrong, it could potentially cause climatological change which would alter civilization on this planet forever. Therefore, I believe the burden of proof should be on the denialists.

As Lyndon Johnson once put it, "Don't piss in the soup, we've all got to eat." And those who consume the soup should not be required to conclusively disprove the assertions of those doing the pissing.