If we don't allow the Keystone XL pipeline to be built through to the gulf we have a much better chance of stepping back from the cliff of going over one trillion metric tons of carbon in the atmosphere. Once we pass this threshold, we can't step back. Without the Keystone XL pipeline, we might still be able to give our grandchildren a world they can live on.
http://www.scientificamerican.com/article.cfm?id=tar-sands-and-keystone-...