By David Biello, Scientific American

The Keystone XL Pipeline would move enough tar sands oil to result in another 181 million metric tons of greenhouse gases entering the atmosphere yearly. A new report prepared by environmental group Oil Change International (OCI) analyzes what the climate change impacts of the proposed pipeline might be.

Consultants hired by the U.S. State Department determined that completing the Keystone XL Pipeline that would transport tar sands from Canada to Texas would have no impact on greenhouse gas emissions, largely because they assumed that the tar sands oil would flow regardless. But the new report challenges that assertion, noting that the tar sands are stranded in Alberta and face few good pipeline prospects, either to Canada’s west coast or via reversing the flow of existing pipelines to North America’s east coast. “Other options like rail or truck are not feasible for the transportation of large quantities,” said Elizabeth Shope, anti-tar sands advocate with environmental group the Natural Resources Defense Council, in a conference call with reporters, noting that such alternative transportation more than triples the cost of moving tar sands oil. “It’s increasingly clear that without Keystone XL, the tar sands will not be able to expand at such a reckless pace.”

If Keystone XL is built, and an additional 830,000 barrels of tar sands oil flows south each day, the climate change impacts will be “unacceptable,” said former NASA climatologist James Hansen on the conference call. “Yet, governments are not only allowing the development of any fossil fuel that can be found, but particularly unconventional oil like tar sands and shale oil.” Based on an estimate of 598 kilograms of greenhouse gases per barrel of oil, Keystone’s more than 300 million barrels a year would result in more pollution than that emitted by 37.7 million passenger cars.

Of course, Keystone XL might not be used at full capacity at all times and industry estimates of the greenhouse gases associated with producing and burning tar sands oil can be as low as 482 kilograms per barrel, depending on whether the tar sands were mined or not. “We’ll continue to drive [that number] down,” says Greg Stringham, vice president for the Canadian Association of Petroleum Producers (CAPP). “If the oil is going to be consumed anyway, then it has to come from some source, and we think we should be the preferred source.”

The U.S. Environmental Protection Agency estimated that Keystone XL tar sands oil would result in additional greenhouse gas emissions of 27 million metric tons annually compared with conventional oil. Regardless, the tar sands represent a significant chunk of potential carbon emissions, and those from tar sands have increased in recent years-up 16 percent since 2009, according to CAPP. Keystone XL itself would exacerbate that-the U.S. State Department notes that the greenhouse gas emissions from just the pipeline’s pumps would be 4.4 million metric tons per year, roughly the same as one average U.S. coal-fired power plant.

Present economic trends may help keep tar sands carbon underground, however. The recent gusher in shale oil from North Dakota and elsewhere may reduce the demand for tar sands oil here in the U.S., at least in the short term. But such shale oil may not represent a significant improvement in the long run for the climate. As Steve Kretzmann, executive director of OCI, noted in response to Scientific American, the flares of methane from such oil wells are visible from space. “Methane is a potent greenhouse gas as well,” he added. “Frankly, I don’t think we even have a very good estimate of how bad that [shale oil] is.”

Ultimately, the campaigners hope that the Obama administration will reject the Keystone XL Pipeline proposal, noting that its social cost-in health care, property loss, agricultural impacts and more from climate change-would amount to some $4.5 billion annually. “We need to put a price on carbon emissions, which makes fossil fuels pay their true costs to society,” Hansen added. And although Alberta currently boasts a carbon price of $15 per metric ton, it is not enough-at less than a dollar per barrel-to keep greenhouse gas pollution from the tar sands in check.