Not only that, but each successive nominee has started out less convinced of the realities of human-driven climate change than the last. In 2008, Republicans nominated someone who ran an ad featuring a smoke stack and promising smart solutions to climate change. In 2012, climate change wasn't mentioned in the presidential debates. Now, the nation has a president who refuses to clarify if he still thinks climate change is a “hoax” put on by the Chinese and who may not accept a new report from his own scientists that says climate change is happening now.

Here are key climate change moments in the conservative world since Al Gore's “An Inconvenient Truth” came out a decade ago, as told by Republican presidential nominees — and the research.

2008: “I believe climate change is real”

Sen. John McCain (R-Ariz.) could be considered the most pro-climate-change-action Republican to ever win the nomination. When he launched his campaign for president, McCain was a leader in the Republican Party on climate change.

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He ran an ad that actually tried to out-climate-change the Democrats. It featured smokestacks, congested highways and a not-so-subtle setting sun, with news clips declaring: “McCain climate views clash with GOP,” scrolling across.

“I believe climate change is real,” he said on his campaign website. “I think it's devastating. I think we have to act and I agree with most experts that we may at some point reach a tipping point where we cannot save our climate.”

But as the campaign went on, McCain slowly and subtly backed away from his act-or-else position. Ultimately, he picked an open climate change skeptic, Sarah Palin, as his running mate.

After he lost the election and was back in the Senate, McCain's evolution as a climate change skeptic was complete. He started calling cap-and-trade — something he had supported since at least 2003 — a “cap and tax.”

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Key climate change moments: Al Gore and the U.N.'s Intergovernmental Panel on Climate Change win the Nobel Peace Prize in 2007 for “their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change.”

The 2007 report declares “warming of the climate system is unequivocal.”

2012: “We don't know what's causing climate change”

If McCain's position after the presidential race was confusing, GOP nominee Mitt Romney's position was hard to track during the presidential race. He fudged or switched his position on the degree to which humans contribute to climate change several times, and he never offered any specific policy proposals.

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Let's start from before he got the nomination. He wrote in his 2010 book, “No Apology,” that he believes humans are playing a role in climate change, but he wasn't sure to what degree.

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“I believe based on what I read that the world is getting warmer,” he told the New Hampshire Union-Leader in 2011.

Like McCain, as the campaign went on, Romney's skepticism toward climate change grew: “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 said at one point.

Finally, Romney used Barack Obama's support for climate change action as an attack against the president: “President Obama promised to begin to slow the rise of the oceans and heal the planet. My promise … is to help you and your family,” he said in his nominating speech.

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After the election Romney appeared to switch his positions — this time, back to his original assertion that climate change is a problem.

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“I'm one of those Republicans who thinks we are getting warmer and that we contribute to that,” he told the Associated Press in 2015.

Key climate change moments: A prominent climate-change skeptic scholar, Richard Muller, writes in the New York Times that, after research (funded by the Koch Brothers, climate change skeptics), he has decided climate change is real, and humans are the main cause.

And a Brookings Institution study finds that public opinion about whether climate change is real is rebounding, after dropping from a high of 78 percent in 2008 to a low of 52 percent in 2010. In the spring of 2012, 65 percent of Americans believe there is solid evidence that human activity is warming the planet.

2016: “I am not a great believer in man-made climate change”

When Donald Trump won the nomination for president, he was on record denouncing climate change as a hoax (before he ran for president, but he refuses to this day to clarify or elaborate).

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He fit right into the GOP primary. Sen. Ted Cruz (R-Tex.) denied the planet is warming, and Sen. Marco Rubio (R-Fla.) said he doesn't think humans are causing “dramatic changes to our climate the way these scientists are portraying it.”

And, as The Washington Post's Philip Bump documents extensively, Trump took just about every position possible on climate change when he got into the race. But the overriding theme was skepticism.

“I am not a great believer in man-made climate change. I’m not a great believer,” he told The Post as he was on his way to the nomination.

The New York Times reports that Trump's advisers saw an applause line — and a political opening — with blue-collar coal and mine workers in the Rust Belt by questioning climate change.

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This time, the Republican presidential candidate won. And this time, the politician didn't veer from his position. In fact, you could argue that since Trump has become president, he's increased his skepticism by pulling out of the Paris climate change accord that all but two countries are a part of and putting in place climate change skeptics into Cabinet positions, like Rick Perry at the Energy Department and Scott Pruitt at Environmental Protection Agency.

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Key climate change moments: Reporters get hold of a government climate change report in August that says it is “extremely likely” that half of the rise of temperatures over the past 40 years are thanks to humans. In other words: man-made climate change is very real, and it's happening now: “There are no alternative explanations, and no natural cycles are found in the observational record that can explain the observed changes in climate.”

Also, a 2015 Gallup poll found that only the most conservative Republicans think climate change won't affect them in their lifetime.