Republican candidates for the 2012 presidential nomination overwhelmingly agree in rejecting evidence that Earth is warming and that humans are substantially responsible. But just three years ago, both major party presidential candidates were pledging to cut greenhouse emissions. What’s changed?

Americans voting in November 2008 chose between two presidential candidates who had each pledged to take significant action on climate change.

“We stand warned by serious and credible scientists across the world,” Republican nominee John McCain said on the campaign trail in Oregon. “Time is short and the dangers are great.”

McCain proposed cutting greenhouse emissions by 60 percent by 2050. Then-candidate Barack Obama said he supported cuts of 80 percent.

Fast-forward three years, and scientists remain convinced that Earth is warming and that people are substantially responsible.

But the political landscape has shifted dramatically. All the Republican presidential candidates now either oppose greenhouse regulations or outright deny that climate change is occurring.

Take Minnesota Congresswoman Michele Bachmann, who has described global warming as “voodoo, nonsense, hokum, a hoax” and carbon dioxide as “a natural byproduct of nature.”

Business executive Herman Cain has said the Regional Greenhouse Gas Initiative should instead be called the “Regional Greenhouse Gas Rip-off.” Texas Congressman Ron Paul has said global warming is a hoax; former Pennsylvania Senator Rick Santorum has called it “a beautifully concocted scheme.”

There’s also Texas Governor Rick Perry, who has claimed that “there are a substantial number of scientists who have manipulated data so that they will have dollars rolling into their projects.”

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In 2008, former House Speaker and now candidate Newt Gingrich appeared in an advertisement with then-Speaker Nancy Pelosi on behalf of Al Gore’s Alliance for Climate Protection. “Our country must take action to address climate change,” he said in the ad.

But in the heat of the presidential campaign, Gingrich has said he regrets participating in the ad and that his appearance had been “misconstrued.”

In the GOP field, former Ambassador to China and Utah Governor Jon Huntsman stands virtually alone in his clear support for the evidence on climate change, which he expressed via Twitter.

That tweet prompted a swipe from GOP veteran Pat Buchanan, speaking on MSNBC: “If he’s running for the Republican nomination, he is crazy,” Buchanan said.

But even Huntsman doesn’t support legislation to address emissions. He told Time in May that “this isn’t the moment” to take action on climate change.

Mitt Romney’s Cooling Climate Views

The changed political prospects for climate change are most obvious in former Massachusetts Governor Mitt Romney’s shifting position on the issue. When Romney was governor, his administration capped emissions from coal-fired power plants. The limits, Romney said in a December 2005 press release, would provide “real and immediate progress in the battle to improve our environment.”

The Romney administration also helped negotiate a regional cap-and-trade initiative, although the governor ultimately backed out of the deal.

As recently as June 3, 2011, Romney offered support for the scientific consensus on climate change.

“I believe based on what I read that the world is getting warmer. And number two, I believe that humans contribute to that,” he said in Manchester, New Hampshire. “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.”

Then conservative radio host Rush Limbaugh seized on Romney’s comments. “Bye bye, nomination. Another one down,” Limbaugh said.

By August, Romney seemed to soften his stance.

“Do I think the world’s getting hotter? Yeah, I don’t know that, but I think that it is,” Romney said in Lebanon, New Hampshire. “I don’t know if it’s mostly caused by humans.” He added: “What I’m not willing to do is spend trillions of dollars on something I don’t know the answer to.”

In early September, Romney released his jobs plan, in which he pledged to move to amend the Clean Air Act so that carbon dioxide could not be regulated as a pollutant.

That didn’t stop Rick Perry from using Romney’s climate-change record as attack fodder.

“Massachusetts was one of the first states to implement its own cap-and-trade program, which included limits on carbon emissions from power plants,” Perry said in a Georgia speech that likened Romney to President Obama. The Perry campaign released a video describing Romney and Obama as “carbon copies.”

The Romney campaign replied with its own climate-tinged swipe: “Rick Perry supported Al Gore for president,” spokeswoman Andrea Saul told Politico. “Instead of distorting Mitt Romney’s record, Mr. Perry should explain why he lined up behind Al Gore’s radical environmental agenda.”

Then, speaking in Pennsylvania in late October, Romney reversed his position on climate change.

“My view is that we don’t know what’s causing climate change on this planet,” he said.

Four Reasons The Debate Has Changed

What’s changed since 2008? Drexel University’s Robert Brulle, professor of sociology and environmental science, says four factors help explain climate change’s political problems, even within the Obama camp. The administration is “not really talking much about climate change at all, either,” he said. “You have the Democrats sort of just getting quieter and quieter, and the Republicans getting louder and louder in denial.”

First, in the summer of 2008 — when the last presidential campaign was in high gear — unemployment was low. The opposite is true today. “Climate change becomes a peripheral issue when unemployment is so high,” Brulle said in a recent interview.

The second factor: the rise of the Tea Party.

In 2008, the Tea Party did not exist. By 2010, it was an organized political force.

According to a poll conducted in April and May of this year, led by Yale Forum publisher Anthony Leiserowitz, 53 percent of Republicans who do not identify with the Tea Party say that global warming is happening. In contrast, among Tea Party members — who make up 12 percent of the American public — only 34 percent say that global warming is occurring.

“All the Republican candidates have to go through the gauntlet of the Republican primary, which is where the Tea Party is going to have the most influence,” Brulle said. The result is that candidates are taking positions to appeal to their political base in the upcoming primaries.

Dwindling news coverage is also a factor in this year’s climate politics. Media coverage and public concern about climate change increased dramatically in 2006 and 2007, during the period when Gore’s “An Inconvenient Truth” was released, the Intergovernmental Panel on Climate Change issued its Fourth Assessment Report, and Gore and the IPCC won the Nobel Peace Prize.

But by 2010, coverage had returned to 2004 levels, Brulle said: “It’s just not a major item of discussion.”

Finally, the movement to oppose action on climate change has grown much stronger since 2008, Brulle said. In the wake of hacked e-mail controversies, for example, climate “skeptics” feel free to claim publicly, though falsely, that climate scientists faked their data.

During the same time period, the energy industry invested millions of dollars in lobbying, according to an analysis by the liberal Center for American Progress Action Fund.

“Five hundred million dollars has been spent since the last race to discredit climate change,” said Shawn Lawrence Otto, author of “Fool Me Twice: Fighting the assault on science in America.” The GOP candidates’ views, Otto said, represent “the triumph of ideology over knowledge.”

Speaking at the Clinton Global Initiative annual meeting in September, former President Bill Clinton also criticized the GOP candidates for denying climate change. “We look like a joke,” he said. “You can’t win the nomination of one of the major parties in our country if you admit that the scientists are right.”

‘Storify’ as a Reporting Tool

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Sara Peach is the editor-in-chief of Yale Climate Connections. She is an environmental journalist whose work has appeared in National Geographic, Scientific American, Environmental Health News, Grist,...