It's fairly easy to back it up. Take all the peer-reviewed scientific papers on global warming and devide them up along the human activity/other causes line, and count them. Alternatively, you count the authors of said papers. IIRC, the last time this was done the number turned out to be 97% on the human activity side.
I think the Russians (and probably others too) are following this with great interest. They're amazed how much chaos they'll be able to create with 100 euros/dollars/pounds.....
