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."