"I guess it doesn't occur to them that the increased solar activity could be the source of the warming, instead of the increase in CO2 due to human activity."
It also doesn't occur to them that, based upon other, scientific research, New York City (for example) was under a two-mile thick sheet of ice 12,000 years ago.
Now, if that isn't proof enough that global warming is a natural phenomenon independant of human activity, then nothing is. We're not dealing with just science, we're dealing with a science-as-religion, on this issue. Logic and "science" have nothing to do with it; it's all about controlling behavior.