Paul C. Knappenberger, researcher and assistant director of the Center for the Study of Scienceat the Cato Institute, a libertarian think tank, said opponents' attempts to "frame the issue in terms of climate change is sort of a hollow claim," claiming they exaggerate the global warming potential of tar sands development. On Jan. 24, writing in the Wall Street Journal, he said "the expansion of tar-sands development will happen with or without the approval of Keystone XL."

Still, Knappenberger said pipeline protestors are "wise to try to put this in a climate change framework," given the rise in concern over global warming in the wake of Sandy.

Previous environmental impact assessments of the Keystone XL carried out by the State Department did not consider the entire suite of greenhouse gases associated with operation of the project.

Environmentalists are hopeful that will change this year, because newly appointed Secretary of State John Kerry, a proponent of climate action, will lead the review. In testimony during his nomination hearing before a Senate committee, Kerry listed global warming as one of the "life threatening issues" that defines U.S. foreign policy.

The U.S. Environmental Protection Agency could also assert considerable influence over the review, InsideClimate News reported earlier this week.