EcoWatch: Presidential decisions often turn out to be far less significant than imagined, but every now and then what a president decides actually determines how the world turns. Such is the case with the Keystone XL pipeline, which, if built, is slated to bring some of the dirtiest, carbon-rich oil on the planet from Alberta, Canada, to refineries on the U.S. Gulf Coast. In the near future, President Obama is expected to give its construction a definitive thumbs up or thumbs down, and the decision he makes...