The fuels these proposed projects would transport are among the filthiest on earth, some scientists warn. Environmentalists argue that burning coal and Canadian tar sands oil could saturate the atmosphere with a critical amount of carbon dioxide — so much that the climate would heat up even faster than it is already warming. Many are calling the northwestern United States, with Montana at its heart, a carbon choke point — that is, a place where opponents might stop dirty fossil fuels before they can be burned.

Montana is "potentially a real cork in the carbon bottle," said author and activist Bill McKibben. "And it's a perfect illustration of the emerging, sprawling fossil-fuel resistance. It's necessarily centered in local concerns."