That leaves Martin O'Malley, the former mayor of Baltimore and two-term governor of Maryland, who's trailing all of the above in the polls. In the wake of the pope's headline-grabbing encyclical on the environment, which champions the moral need to fight global warming and calls for a swift transition from fossil fuels to clean energy, the presidential contender published an op-ed in USA Today outlining an extremely ambitious plan to do precisely that.

"I believe, within 35 years, our country can, and should, be 100% powered by clean energy, supported by millions of new jobs," O'Malley writes. "To reach this goal we must accelerate that transition starting now."

"As president, on day one, I would use my executive power to declare the transition to a clean energy future the number one priority of our Federal Government."

-Create a national Renewable Energy Standard that would mandate 100 percent clean energy by 2050
-Launch a seemingly New Deal-styled Clean Energy Job Corps that would retrofit buildings for efficiency and build green spaces
-Have the EPA enforce a "zero tolerance" methane leak policy (currently, natural gas production releases a staggering amount of the super-greenhouse gas into the atmosphere)
-Call on Congress to enact a carbon cap that would charge companies for their carbon pollution, and return the revenue to lower and middle class families
-Deny the Keystone XL and halt offshore oil drilling in Alaska

Have the EPA enforce a "zero tolerance" methane leak policy (currently, natural gas production releases a staggering amount of the super-greenhouse gas into the atmosphere)

One of the natural gas industry’s advertising claims is that natural gas burns cleaner than coal and is therefore better for the environment. While that is true, that is not the whole story.

According to the U.S. Environmental Protection Agency, when fugitive methane emissions exceed 3.2 percent, the climate benefits of natural gas are negated and climate change is accelerated. Recent scientific studies of natural gas operations have reported fugitive methane emissions of 9 percent.

Colorado, Ohio and Wyoming have all enacted stringent and enforceable regulations to curb such emissions. In Colorado, a “zero tolerance” policy requires natural gas companies to find and fix methane leaks, as well as install technology that captures 95 percent of emissions of both volatile organic compounds (VOCs), which cause air pollution, and methane.