When Scott Walker announced his campaign for president in July, I wrote that he was the most dangerous candidate for the environment, because he embraced an anti-environmental platform as a central feature of his potential presidency. That was before the Wisconsin governor's campaign floundered, sputtered, and finally expired last month. 

Americans may have dodged the grim future promised by a Walker administration, but the GOP provides no shortage of other candidates to assume his “most dangerous” title. If the strongest contender for this dubious honor were the candidate with the craziest sound bytes, the leading contenders would be clear: Donald Trump, Dr. Ben Carson, or Carly Fiorina, the current leaders in national polls, or Senator Ted Cruz, who's polling in single digits.

The candidate who should cause environmentalists to shudder the most is a less-obvious choice: Senator Marco Rubio. Rubio argues—as seriously as he possibly can—that there is nothing in the world that could convince him it’s wise for America to reduce its reliance on fossil fuels. We’re “not going to destroy our economy the way our left-wing government now wants to do,” he said in last month's GOP debate. “Every proposal they put forward are going to be proposals that will make it harder to do business in America, that will make it harder to create jobs in America.”

During the same debate, Rubio made perhaps his only accurate comment on the subject since he began running for president: “America is not a planet." This was central to his argument: The United States can do nothing on its own to affect the climate, or the weather. And we shouldn't try. 

Rubio hasn't always taken quite such a hard line. Over the course of his career, he's taken more contradictory positions on global warming than any other candidate. In Florida’s state Senate, he supported cap-and-trade, which in those days still had conservative proponents. He once advocated for government incentives to spur businesss innovation in sunny Florida and make it the "Silicon Valley" of energy innovation. “Global warming, dependence on foreign sources of fuel, and capitalism have come together to create opportunities for us that were unimaginable just a few short years ago,” Rubio said in a 2007 speech unearthed by BuzzFeed. "Today Florida has the opportunity to pursue bold energy policies, not just because they are good for the environment, but because people can make money doing it."

That Rubio is unrecognizable today. He fully aligned himself with the skeptics in the build-up to his presidential bid, declaring on ABC’s This Week in 2014, “I do not believe that human activity is causing these dramatic changes to our climate the way these scientists are portraying it.” Last April, he downplayed the human role again, saying, “Humans are not responsible for climate change in the way some of these people out there are trying to make us believe.” Not surprisingly, he voted against a Senate amendment in January that said climate change is real and “significantly” caused by human activity. 

It's not Rubio's comments on science that make him stand out; it's also not his dismal voting record in the Senate. (According to the League of Conversation Voters, Rubio has voted against the environment 39 times as a senator, earning a 9 percent score from the group on his conservation record.) What makes Rubio uniquely dangerous is his assertion that the U.S. is powerless to shape the future of the planet. Without sounding nearly as extremist as Trump or Carson or Cruz, he argues the U.S. should discourage policies that promote a growing American clean energy industry, and that we should follow China’s cues in our energy policy. “We are not even the largest carbon producer anymore, China is,” he pointed out at the recent debate. Of course, America has reason enough to cut down on fossil fuels without China pitching in, for the public health benefits alone. And it isn’t true that we’re acting alone: The only reason the world’s biggest polluters, including China, have cut deals to reduce their carbon emissions in the next decade is because the current, pro-climate president has worked to get everyone else on board. 

Rubio’s second go-to argument against action on climate change is that these policies would “destroy the American economy"—directly contradicting his earlier view that clean energy could be a boon to business, and ignoring the fact that clean energy is increasingly seen as a smart investment and climate change costs the global economy trillions of dollars. It’s no surprise, then, that Rubio has promised that he would reverse not just President Barack Obama’s landmark plan to limit carbon pollution from power plants, but introduce a "sweeping overhaul of the regulatory system" as well. 

OK, so none of the other Republican contenders—possibly excepting former Florida Governor Jeb Bush—is any better on the environment. The entire field opposes Obama’s climate regulations, and warns with varying degrees of alarm of imminent economic disaster from the cost of transitioning to clean energy. Trump, in his emphatic way, says “I’m not a believer” in climate change. Carson posits that “temperatures are going up or temperatures are going down,” and when “that stops happening, that's when we're in big trouble." Fiorina's line, echoing Rubio, is that a “single nation acting alone can make no difference at all." Cruz claims "there's been zero recorded warming."

So what distinguishes Rubio?

Partly it's his viability. Rubio stands a better shot than the more vociferous climate skeptics at capturing the GOP nomination, assuming the outsiders fizzle as expected. (MSNBC, New York Times, Washington Post, among other outlets have already practically crowned him as the nominee). Rubio has a better chance, therefore, of turning his climate rhetoric into policy. And among the GOP contenders, he could prove uniquely adept at convincing Americans that he's right: Rubio's true threat is that he sounds less crazy when he says there is no point in bringing down greenhouse gas emissions, which would help him skate past the issue in a general election without sounding alarms among the majority of Americans who support strong govenment action on climate. 

Unlike Bush, Rubio has left little room to moderate his positions in the general election campaign—or as president. Admittedly, Bush's views are similar to Rubio's in many ways; he also opposes Obama's climate regulations because he doesn't want to "destroy the American economy." But Bush has a slightly more nuanced, or maybe muddled, argument: He is more forthright that humans contribute to climate change and that it's a threat; thinks the “proper role of government’ may be to help states adapt and prepare for the future; and calls for lawmakers to fund "the next breakthrough in disruptive technologies," as he explained at an October town hall in New Hampshire. Bush also a marginally better environmental record to lean on than Rubio, having advocated for a time against offshore drilling and promoted Everglades restoration as governor. (Rubio has voted for Everglades restoration in the state Senate and U.S. Senate, while also advocating for business interests to guide Everglades conservation. "Oftentimes, agencies that are regulating industries see the industries as the enemy, not as a partner in a joint endeavor," Rubio said in 2011.)

Rubio is crafty at disguising his hardcore views on climate change. Lately he's learned to tone down his outright denial of climate science—to focus only on his skepticism that climate action will do any good. He also manages to sound reasonable by appearing more pro-science on other topics, like evolution and vaccines. After a gaffe in 2012 where he said he did not know the age of the Earth, Rubio explicitly said he accepts the science. More recently, he endorsed the medical consensus on vaccines during a measles outbreak in February. Rubio's new tone came through when Pope Francis visited the U.S. last month. While some Catholic Republicans attacked Pope Francis' climate advocacy, Rubio struck a less adversarial tone. "I have no problem with what the Pope did," he said. "He is a moral authority and as a moral authority is reminding us of our obligation to be good caretakers of the planet." But while Rubio may appeal to moderate voters by talking eloquently about the need to protect the common good, his policy proposals suggest industry interests should always trump conservation. 

The details of Rubio's platform could inflict more damage than your average Republican to the entire environmental regulatory apparatus. In addition to supporting trademark Republican policies like lifting the oil and gas export ban and the Keystone XL pipeline, Rubio's big idea for a regulatory overhaul is to cap how much a federal agency can cost the economy, regardless of the benefits a regulation may provide. Rubio's cap would force the EPA to pick and choose arbitrarily between pollutants. According to Rubio's plan, the EPA "would be prohibited from issuing the new regulation unless it withdrew other regulations" and thus stayed under his cap. This ignores the real bottom line: the billions of dollars saved, and the many citizens' lives and health improved, by reductions in pollution. 

With proposals like this one, Rubio sounds serious about action and reform, without actually being serious. Trump, Carson, Cruz, and Fiorina are easy to dismiss as completely detached from reality on climate issues, while Bush is doing his best to pander to conservatives and reassure them he's no stealth green candidate. Rubio is different because he isn’t the craziest-sounding candidate of the bunch. He’s just artfully disguising how extreme his positions really are.