President Obama's announcement on Tuesday that he will use his executive authority to regulate carbon-dioxide emissions from existing power plants, and to finalize rules for plants that have yet to be built, is reason to celebrate — but also an occasion for mourning. The announcement reflects Congress’s refusal to pass serious laws to curb greenhouse-gas emissions. Ultimately, if America is to fully contribute to an effective global response to the perils of a warming planet, Congress must stop dithering.