As President Obama moves to constrain U.S. carbon emissions, casting doubt over prospects for the Keystone XL pipeline, things could be getting hairy for tar sands. An article in the July 2013 issue of Scientific American detailed how turning tar sands into oil and burning it as fuel produces enormous amounts of carbon dioxide, and how -- if built -- the Keystone XL pipeline would become "a spigot that speeds tar sands production, pushing the planet toward its emissions limit." Now that President Obama has said he would only approve Keystone if it did not "significantly exacerbate" carbon emissions, further tar sands development may face obstacles.