Climate policy and analysis often focus on energy production and consumption, but seldom consider how energy transportation infrastructure shapes energy systems. US President Obama has recently brought these issues to the fore, stating that he would only approve the Keystone XL pipeline, connecting Canadian oil sands with US refineries and ports, if it ‘does not significantly exacerbate the problem of carbon pollution’. Here, we apply a simple model to understand the implications of the pipeline for greenhouse gas emissions as a function of any resulting increase in oil sands production.