CNN: When Barack Obama took office in 2009, it was clear that if he could do four big things, his presidency would probably be judged a success. The first, of course, was to pull the country out of its desperate economic crisis. The second was to extricate it from the Iraq War. The third was to finally achieve comprehensive health care reform. And the fourth was to make meaningful progress on arresting global warming. This last challenge was in some ways easiest to ignore: While you were shoveling snow...