Even given that the globe is warming, the evidence that man is the cause is pretty shaky. But then, liberals tend to be consumed with guilt, anyway...all it needs is a focus.
If there is an evidence that is weak is the evidence on the theory of solar output causing the increasing temperatures in recent decades. And this has been proven by directly measuring solar radiation, which we couldn't do in the past. The U.S Global Climate Change Research Program says it. A group commissioned by Bush to investigate global warming says it. Most scientists say it.
This may have been a valid statement in the early 1990s, but the evidence now is much more established.