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Mitt Romney may be the GOP presidential candidate that is most often criticized for flip-flopping , but as Newt Gingrich rises to challenge him at the top of the polls, the former House speaker may also be giving him a run for his money on that label of inconsistency.

When it comes to global warming, Gingrich’s position seems to have changed faster than the climate.

“I don’t know if he’s just being opportunistic or of he’s had a real change of heart, but it is a bit disconcerting,” said Jim DiPeso, the policy director for Republicans for Environmental Protection.

In the more than 30 years since Gingrich was first elected to the House, he has said there is both sufficient evidence to prove the climate is changing and also that there is no conclusive proof. He supported a cap-and-trade program to limit carbon emissions and then later testified against it before a Congressional committee.

And while in the House he co-sponsored a bill that said climate change was “resulting from human activities,” but he later said he did not know if humans were to blame.

“There is no compelling evidence on either side to either rule it out or rule in it,” Gingrich’s spokesman R.C. Hammond said of the candidate’s position on global warming and the impact of man-made pollution. “But at the end of the day he’s somebody who does care about the environment.”

DiPeso said the Republican “orthodox” position on climate change is that “you can’t deal with this issue because it will kill the economy.”

“It’s politically dangerous for prominent Republicans to acknowledge climate change is real and that human activity plays a prominent role,” he said. ”It could be that Gingrich is just trying to play a political game and stick with the political orthodoxy to keep himself from being vulnerable to attacks.”

DiPeso added: “It’s not the first time he’s done that and it’s probably not going to be the last.”

Gingrich has recently gone on the air defending his past support for green initiatives, particularly a 2008 ad in which he appeared with then-House Speaker Nancy Pelosi, D-Calif., promoting federal action to combat global warming.

The ad opens with Gingrich and Pelosi sitting side-by-side on a couch in front of the Capitol Building.

“We don’t always see eye to eye, do we Newt?” Pelosi asks.

“No, but we do agree that our country must take action to address climate change,” Gingrich responds.

Pelosi then says, “We need cleaner forms of energy and we need them fast,” after which Gingrich calls on voters to “demand action from our leaders” in order to “spark the innovation we need.”

In an interviewthis week with Fox News’ Sean Hannity, Gingrich said the ad “is probably the dumbest thing I’ve done in recent years.”

“I was trying to communicate that conservatives care about the environment,” he said. “All I did was make conservatives mad, and liberals didn’t particularly like me. It was a mistake, and I wouldn’t go back and do it again.”

In the same interview Gingrich said he was “deeply opposed” to cap-and-trade, a regulatory system that puts a mandatory cap on greenhouse gas emissions and then allows emitters like utility and manufacturing companies to trade pollution permits.

In a 2007 on PBS’s “Frontline,” Gingrich said the cap-and-trade program was something he “would strongly support.”

“I think if you have mandatory carbon caps combined with a trading system, much like we did with sulfur, and if you have a tax-incentive program for investing in the solutions, that there’s a package there that’s very, very good,” he said according to the show’s transcript.

Two years later, Gingrich told the House Energy and Commerce Committee that a cap-and-trade bill was a “huge mistake” that amounted to a “massive energy tax.”

Hammond said the statements Gingrich made in 2007 were referencing a “very limited program” unlike the 2009 Waxman-Markey bill, which Gingrich testified against.

“He has been a strong advocate in stopping the Obama administration’s efforts to implement a cap-and-trade controlled carbon economy,” Hammond said.

But Gingrich is not the only GOP presidential hopeful that has changed his tune on cap-and-trade.

His Republican rival Jon Huntsman has also tried to distance himself from an ad he filmed while serving as the governor of Utah that urges congress to cut greenhouse gas emissions. Huntsman also signed onto an emission-cutting pledge in 2007 that promoted a regional cap-and-trade program.

But in an interview with Time magazine this past May, Huntsman said cap-and-trade “hasn’t worked.”

“Our economy’s in a different place than five years ago,” he said, adding that until the economy recovers, “this isn’t the moment” to push for cap-and-trade.

DiPeso argues that the candidates’ current positions on climate change are not as important as their consistency on the issue.

“Whether you’re talking about climate or any other issue, voters want to know – well, where do you really stand?” DiPeso said. “Where Gingrich and/or Romney would face some issue with the voters is matters of — are you being straight? Are you telling us what you really think? Or are you trimming your sail?”