On the night of his re-election, President Barack Obama described grand ambitions for his second term, including a desire to bequeath to future generations a nation not only free of debt and unencumbered by inequality, but also one "that isn't threatened by the destructive power of a warming planet." The laws of both physics and politics suggest he'll have his work cut out for him, and his second-term success will surely be measured on far more concrete terms. The president, after all, faces several lingering and highly divisive decisions, including whether and how to clean up the nation's aging fleet of coal-fired power plants, which pump vast amounts of carbon dioxide and other greenhouse gases into the atmosphere. He also must decide whether or not to approve the controversial Keystone XL pipeline project, which would transport heavy, carbon-intensive oil from the scarred landscape of Alberta, Canada, to ports on the American Gulf Coast.