According to Romney,
"My view is that the EPA in getting into carbon and regulating carbon has gone beyond the original intent of that legislation, and I would not take it there,"Romney last month ignited conservative complaints when he told a New Hampshire audience that he believed in the widespread scientific consensus that humans have contributed to a warming planet.
So it appears that Romney believes that humans have contributed to climate change, but he believes that it does not pose a threat to human health.
As governor in Massachusetts, Romney took the opening steps toward joining a cap-and-trade compact for power plants known as the Regional Greenhouse Gas Initiative. But he pulled out of the program just before it formed in late 2005. (Politico, 7/18/2011)