PBS: HARI SREENIVASAN: President Obama is proposing the toughest regulations in U.S. history to combat climate change. In a video previewing tomorrows announcement, the president says climate change is not an opinion, but a fact and a threat. He calls power plants the biggest domestic source of polluting emissions that contribute to global warming. The most significant rule would require existing power plants to cut emissions from 2005 levels by 32 percent by 2030. Other rules would limit construction...