It would only serve to encourage them as the argument could be then made that, since the water vapor is all but impossible to control that mankind's efforts to control the remaining gases need be increased to buy a few more generations a little more time.
I've already bumped into that arguement with people I've talked with. I poiunt outn that Mars and Pluto are both heating up and that solar output has increased and would be a far more logical explanation than CO2 on climate change. Their response, after point out the data supporting this is just what you said. If the sun is getting warmer (i.e. global warming is a NATURAL, phenomenon), they then insist that stopping greenhouse gas emissions is even more critical since we don't want to make a bad situation worse. No matter what you say, it's alsways Humanity's (i.e. Bush's Fault).