The logic goes like this: The Keystone pipeline will carry 830,000 barrels of oil per day from the tar sands of Alberta down to the Gulf Coast. Expanding the market for Canadian crude will allow the tar-sands industry to produce even more oil and keep growing. Surely that additional production would "significantly exacerbate the problem of carbon pollution," right?
But here's the flip side: Back in March, the State Department issued a draft environmental impact statement finding that Keystone XL wouldn't lead to significantly more carbon pollution than would otherwise be the case. The State Department's argument was that, if the pipeline gets blocked, oil-sands producers will just find other routes to ship their product, such as by rail. So the extra emissions will happen regardless. (Other groups, including the EPA, have disputed this analysis.)
So Obama might have left himself some room to approve the pipeline. We'll have to see how the State Department ultimately comes down on this emissions question. My colleague Juliet Eilperin reports that "the administration will examine whether vetoing the project—which would mean the oil would likely be shipped by rail—would translate into higher emissions than building it."
That said, this is the first time that Obama has put climate change at the debate over Keystone XL, which is a real shift in how he's talked about the subject.