In a draft assessment of the proposed Keystone XL pipeline, consultants for the U.S. State Department judged that building it would have no significant impact on greenhouse gas emissions. Why? Because the analysts assumed the tar sands oil would find a way out with or without the new pipeline.

The U.S. Environmental Protection Agency does not agree. Keystone XL's ability to carry an additional 830,000 barrels of tar sands oil per day is vital to expanded production of the tarry crude in Alberta. The EPA contends that the analysis by State got the economics all wrong. In particular the consultants were too optimistic about the ease with which the oil could be moved by railroad—an alternative already in use. But such tar sands oil transportation alternatives can more than triple the cost of moving crude. State's report also neglected to consider the potential for congestion on the railroads with an uptick in oil transport, EPA contends. Of course, from a greenhouse gas perspective, transport by pipeline results in fewer emissions than transport by rail, truck or barge.

The bottom line, from a climate perspective: "oil sands crude is significantly more [greenhouse gas] intensive than other crudes, and therefore has potentially large impacts," wrote EPA's Cynthia Giles about the State Department's attempts to assess the full implications of Keystone. "Lifecycle emissions from oil sands crude could be 81 percent greater than the average crude refined in the U.S.," a difference that can grow "depending on the assumptions made."

The EPA also cited its experience from cleaning up after the spill of tar sands oil from a pipeline near the Kalamazoo River in Michigan. This pipeline, smaller than Keystone XL, managed to spill some 20,000 barrels in 2010, much of which ended up at the bottom of the river. Despite three years of clean up effort, the river will have to be dredged because the oil sands crude "will not appreciably biodegrade," Giles wrote. In other words, the kind of microbes that chewed up the oil from BP's blown out Macondo well in the Gulf of Mexico could find no purchase on diluted bitumen from Alberta. Such heavy oil results in the tarballs ubiquitous along the Gulf Coast and, apparently, a layer of tar at the bottom of the Kalamazoo River. All of that experience suggests that would-be pipeline operator TransCanada should be required to prepare for such submerged oil in the event of a leak from Keystone XL as well as having equipment in place to deal with a spill before it happens, the EPA suggests.

That's a particular concern because, despite a re-routing around ecologically sensitive regions in Nebraska, the Keystone XL pipeline would still cross over the nation's largest freshwater aquifer: the Ogallala.

All of that leads the agency to object to the State Department's analysis on the grounds of "insufficient information" and "significant" environmental objections. What impact, if any, that has on the approval or disapproval of the pipeline by the Obama administration remains to be seen but the impact of Keystone XL on climate change is clear.