"The same policies that protect the climate also promote energy efficiency, smart business practices, and improve the environment in which our citizens live and work," Romney wrote in a 2004 letter.
But the governor also opted his state out of a regional cap-and-trade program, the Regional Greenhouse Gas Initiative.
Even if it's not a new position, Romney's acknowledgment of man-made climate change is likely to stoke skepticism among conservatives who view him as too moderate. The view that humans are contributing to climate change is a highly controversial position within the GOP, with most conservatives fiercely disputing the notion that Earth is warming at all.
A National Journal analysis of GOP Senate nominees in 2010, for instance, found only one candidate in 21, now-Illinois Sen. Mark Kirk, said he thought man was contributing to climate change.
He's not the only presidential candidate with a problematic climate change history, either. Tim Pawlenty supported cap-and-trade legislation in the past - a position he has now apologized for - while Newt Gingrich cut a TV ad in 2008 with then-Speaker of the House Nancy Pelosi urging action against climate change.