>> explain the law. >> congress passed the clean air act in 1990. bush i signed it. many provisions of it have been upheld in the supreme court, including epa's authority to regulate greenhouse gases. so what the president is basically saying is, i went to congress, i tried to do it with you guys, you're not responsive. i'm going to take the authorities you gave me previously, you gave presidents previously, i'm going to take the clean air act and use it to regulate these dangerous pollutants. >> so if you take the overall emissions in the u.s. economy, how much of them are coming from power plants? >> a lot. the way people generally think about it, we have a third from transportation, a third from electricity generation, and then a third from buildings and industrial -- >> other stuff.