The two questions are for sorting and demographic purposes.
You might also be suprised to know that currently 42% are currently coming in at no on both questions. But that is preliminary, so I am not counting my chickens just yet.
I'm a bit surprised, but not that much, given the BINARY nature of your question concerning a complicated topic, you force people to take very polarized views.
I'd say that many people who answer NO NO are reading your questions as: Do you believe ANTHROPOGENIC Global Warming is happening?
Thus if you answer NO to the first question then you would of course answer NO to the second question.
My guess would be a lot of people who acknowledge that SOME GW is caused by humans would still CHOOSE to knowingly answer the question this way simply because GW is as such a POLITICAL issue.
Tell me, what percent are answering NO GW but YES man is responsible?
You really only have three logical answers to your questions:
YES there is GW and YES man is at least partially responsible
YES there is GW but NO man isn't even partially responsible
NO GW so NO man can't be responsible for what is not happening