ON ITS TAR SANDS in northern Alberta, Suncor Energy scrapes vast open-pit mines and drills down deep into the ground to extract the viscous bitumen that has turned it into one of the largest energy companies in North America. The process is so energy-intensive that it has also made the firm into Canada’s largest carbon emitter: it belches roughly 28 million tonnes into the atmosphere every year, equivalent to the entire emissions of Tunisia.