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 (CNN) - Mitt Romney found himself taking hits from both a Republican rival and Democrats Friday over his stance on climate change, which both sides said had changed over the course of a few months. 
 
 On Thursday, the former Massachusetts governor told voters at a campaign event in Pittsburgh that he didn't know what was causing climate change. 
 
 
 
 "My view is that we don't know what's causing climate change on this planet," Romney said. "And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us." 
 
 Romney's opponents quickly noted that his stance seemed to have changed, using comments Romney had made in the past to support the notion he was flip-flopping on the issue. His campaign immediately pushed back calling the critics full of hot air. 
 
 Perry's campaign and the Democratic National Committee highlighted a statement Romney made when speaking to voters in New Hampshire in June, when he said "I believe based on what I read that the world is getting warmer. And number two, I believe that humans contribute to that." 
 
 In a statement, Perry's spokesman said the comments reflected a substantial change in position. 
 
 "Mitt Romney's positions change, often dramatically, depending on the audience or location," Ray Sullivan said. "Voters need to consider the fact that Romney, in one week, changed positions on manmade global warming, capping carbon emissions, and Ohio's efforts to curb union powers." 
 
 The DNC produced a web video using a similar criticism, punctuated with the question, "Which Mitt?" 
 
 Romney's campaign said that the governor's full quote provided more context. 
 
 "I don't speak for the scientific community, of course, but I believe the world's getting warmer," Romney said in June. "I can't prove that, but I believe based on what I read that the world is getting warmer. And number two, I believe that humans contribute to that. I don't know how much our contribution is to that, because I know that there have been periods of greater heat and warmth in the past but I believe we contribute to that. And so I think it's important for us to reduce our emissions of pollutants and greenhouse gases that may well be significant contributors to the climate change and the global warming that you're seeing." 
 
 Andrea Saul, Romney's spokesperson, said the criticism doesn't hold up. 
 
 "This is ridiculous," Saul said in a statement. "Governor Romney's view on climate change has not changed. He believes it's occurring, and that human activity contributes to it, but he doesn't know to what extent. He opposes cap and trade, and he refused to sign such a plan when he was governor. Maybe the bigger threat is all the hot air coming from career politicians who are desperate to hold on to power." 
 
 Between the comments in June and his comments Thursday, Romney has taken a skeptical approach to climate change. On Aug. 8, Romney told a group in Nashua, New Hampshire, "I'm not a scientist. I think the Earth is probably getting warmer, but I'm not sure how much we contribute to it." 
 
 A few weeks later, on Aug. 24, Romney's expressed similar skepticism at an event in Lebanon, New Hampshire. 
 
 "Do I think the world's getting hotter? Yeah, I don't know that but I think that it is," he said. "I don't know if it's mostly caused by humans." 
 
 He continued, "What I'm not willing to do is spend trillions of dollars on something I don't know the answer to." 

 Mitt Romney is still trying to plant himself in the global warming skeptic camp. 
 
 "My view is that we don’t know what’s causing climate change on this planet," the GOP presidential front-runner said Thursday during a fundraiser in Pittsburgh. "And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us." 
 
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 The liberal blog Think Progress released a video and a transcript of the exchange Friday. 
 
 Romney’s doubts about climate science — countering the widespread understanding that human activity and greenhouse gas emissions are a contributing factor to rises in global temperatures — have become a regular feature in his presidential campaign and represent a gradual rightward shift for someone who once considered putting a limit on power plant emissions. 
 
 In late August, as conservative lawmakers questioned his stance on the topic, Romney tried to align himself with those who dispute global warming science by questioning the role that humans play. 
 
 "Do I think the world's getting hotter? Yeah, I don't know that, but I think that it is," Romney said in New Hampshire, according to Reuters. "I don't know if it's mostly caused by humans." 
 
 Romney aides argued then that he wasn’t changing his position, noting that he’d made a similar case in his book "No Apology." 
 
 "I believe that climate change is occurring — the reduction in the size of global ice caps is hard to ignore,” he wrote. “I also believe that human activity is a contributing factor. I am uncertain how much of the warming, however, is attributable to man and how much is attributable to factors out of our control." 
 
 But in June, Romney ignited a conservative firestorm when he seemed to walk a bit too close to the line. 
 
 "I don't speak for the scientific community, of course, but I believe the world is getting warmer, and I believe that humans have contributed to that," he said. "I can't prove that, but I believe based on what I read that the world is getting warmer. 
 
 "No. 2, I believe that humans contribute to that," Romney continued. "I don't know how much our contribution is to that, because I know there's been periods of greater heat and warmth than in the past, but I believe we contribute to that. And so I think it's important for us to reduce our emissions of pollutants and greenhouse gases that may well be significant contributors to the climate change and global warming that you're seeing." 
 
 As Massachusetts governor, Romney in 2005 considered signing onto the Regional Greenhouse Gas Initiative, a multistate cap-and-trade compact for power plant emissions. But he backed away from the program, citing a lack of economic safeguards. 
 
 This article first appeared on POLITICO Pro at 11:25 a.m. on October 28, 2011. 

 (Photo by Darren McCollester/Getty Images) 
 
 The longer he runs for president, the more doubts Republican front-runner Mitt Romney seems to have about the science behind global climate change. 
 
 Speaking at a closed-door fundraiser Thursday in Pittsburgh, Romney's position on the causes of global warming continued the rightward shift that has been underway for several months. "My view is that we don't know what's causing climate change on this planet. And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us," he told donors at the Consol Energy Center. 
 
 As recently as his 2010 book, No Apology, Romney wrote, "I believe that climate change is occurring. ... I also believe that human activity is a contributing factor. I am uncertain how much of the warming, however, is attributable to man and how much is attributable to factors out of our control." 
 
 In June of this year, Reuters reported that Romney said that the United States should "reduce our emissions of pollutants and greenhouse gases that may be significant contributors." Then in August, Texas Gov. Rick Perry, who openly questions the science of climate change factors, entered the GOP primary race, and Romney appeared to dial down his own certainty of the science, saying, "I think the earth is getting warmer. ... I think humans contribute to that. I don't know by how much. It could be a little. It could be a lot." 
 
 While Romney's expressed skepticism about climate change plays well with conservatives, Democrats are bound to use it against him with the moderate and independent voters who hold sway in a general election. His remarks at the Pittsburgh fundraiser were captured on video, which ended up in the hands of the liberal Center for American Progress. The center distributed the video to news outlets today. 
 
 Romney spokesman Ryan Williams said Friday, "Governor Romney has been consistent in his statements on global warming." 
 
 At the fundraiser, Romney was asked by a member of the audience to explain his position on man-made global warming and asked further whether he would reject a cap-and-trade bill to limit carbon emissions linked to climate change. 
 
 Romney replied, "Man-made global warming and cap and trade and so forth - I actually had in Massachusetts a consortium of states that came together with a cap and trade program. It was called the Regional Greenhouse Gas Initiative. And all the governors, Governor Pataki and so forth, signed it. I refused to sign. I do not believe in a cap and trade program." 
 
 By the way, they do not call it America warming, they call it global warming. So the idea of America spending massive amounts, trillions of dollars to somehow stop global warming is not a great idea. It loses jobs for Americans and ultimately it won't be successful, because industries that are energy intensive will just get up and go somewhere else. So it doesn't make any sense at all. My view is that we don't know what's causing climate change on this planet. And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us. My view with regards to energy policy is pretty straightforward. I want us to become energy secure and independent of the oil cartels. And that means let's aggressively develop our oil, our gas, our coal, our nuclear power." 
 
 Sarah Boxer contributed to this story.
– Mitt Romney's views on global warming are back in the news. This week, he reaffirmed his skepticism about humans contributing to climate change. CBS News detects a "rightward shift" as the campaign goes on, and both Democrats and Rick Perry agree, notes CNN. Judge for yourself, as Politico rounds up some of his statements on the subject: Thursday: "My view is that we don’t know what’s causing climate change on this planet. And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us." August: "Do I think the world's getting hotter? Yeah, I don't know that, but I think that it is. I don't know if it's mostly caused by humans." June: "I don't speak for the scientific community, of course, but I believe the world is getting warmer, and I believe that humans have contributed to that. ... I don't know how much our contribution is to that, because I know there's been periods of greater heat and warmth than in the past, but I believe we contribute to that. And so I think it's important for us to reduce our emissions of pollutants and greenhouse gases that may well be significant contributors to the climate change and global warming that you're seeing." His 2010 book No Apology: "I believe that climate change is occurring—the reduction in the size of global ice caps is hard to ignore. I also believe that human activity is a contributing factor. I am uncertain how much of the warming, however, is attributable to man and how much is attributable to factors out of our control."