As has been covered extensively during the campaign, Mitt Romney believed humans caused climate change before he didn't believe it and before it became a punch-line in his speeches. In response to a question on the climate a science questionnaire from Nature this week, also filled out by Obama, Romney seems to be trying to have it both ways:
I am not a scientist myself, but my best assessment of the data is that the world is getting warmer, that human activity contributes to that warming, and that policymakers should therefore consider the risk of negative consequences. However, there remains a lack of scientific consensus on the issue — on the extent of the warming, the extent of the human contribution, and the severity of the risk — and I believe we must support continued debate and investigation within the scientific community. Ultimately, the science is an input to the public policy decision; it does not dictate a particular policy response.
So he does believe that humans are causing climate change and that lawmakers should consider the subject, but doesn't believe the science is settled. And even if it were, the science shouldn't dictate a "particular policy response." This is what happens when statements are tailored to avoid any assertions that could later be contradicted by either real-world events or the speaker's own actions.