If the first rule of holes is to stop digging, the first rule of climate change is to stop putting carbon dioxide in the air. President Barack Obama seemed to take that approach in his historic June 25 climate speech when he addressed the proposed Keystone XL pipeline, meant to carry tar sands crude from the vast strip mines of Alberta, Canada, to refineries on the U.S. Gulf Coast. "Our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution," he said.

It seems like a simple calculation. If built, Keystone would carry 830,000 barrels of carbon-intensive tar sands oil a day. Burning it, Oil Change International estimates, would add 181 million metric tons of CO2 equivalent to the atmosphere each year, as much as is emitted by 37.7 million cars or 51 coal-fired power plants.

Case closed, right? Not so fast. The State Department's draft environmental assessment of Keystone declared that the pipeline's climate impact would be "minor," reasoning that if the pipeline weren't built, the oil would get out some other way. The Canadian government insists the same: The "oil will get to market," Canadian ambassador to the United States Gary Doer told an industry journal.

The main alternative to Keystone is the proposed Northern Gateway pipeline to Kitimat on the coast of British Columbia. In May, however, the B.C. government formally opposed the plan, largely because of fears of oil spills; many of the First Nations people whose land it would cross oppose it for the same reason.

That leaves the method currently being used to get tar sands oil out of Alberta: rail. To make its case that Keystone won't really change anything, the State Department maintained that rail shipments could increase by 175,000 barrels per day each year and that the increased transportation cost (at least twice that of a pipeline) wouldn't change the project's economic viability.

Goldman Sachs, an organization with no environmental bones to pick, begs to differ. Without pipelines, a company report says, "the potential for Canadian heavy crude oil supply to remain trapped in the province of Alberta is a growing [financial] risk." When asked by Reuters if rail-only transportation would slow tar sands production, even Canadian natural resources minister Joe Oliver admitted, "Yes, I would say it would."

The question comes down to whether turning on the spigot for the world's dirtiest oil will "significantly exacerbate" carbon pollution or not. President Obama's decision is expected within the next few months.