Cripes...
I wish the Left had this kind of steel-trap evidence requirement for things like crippling the American economy because of human-caused global warming.
I believe the operative phrase from many environmentalists* has been:
"We don't have the solid evidence, but we need to go ahead (with Kyoto, etc.) as if it's true, since it's way too critical to take a chance!"
Hell, a lot of them still think socialism works, in the face of conclusive experimental evidence against it. Guess it just depends on where you want to put your faith, I suppose.
Not really relevant to the argument, just an observation or two.
*Do a Google search on the phrases "take that chance" and "global warming."