Geoff Bennett:Today, the Biden administration rolled out its most aggressive effort yet to combat climate change with tougher emissions limits for cars and trucks. Michael Regan, Environmental Protection Agency Administrator: William Brangham: Michael Regan: William Brangham:Transportation is the largest source of America's greenhouse gas emissions, accounting for around 27 percent of the U. John Bozzella: William Brangham:Because these EPA rules are an expansion of existing statutes, analysts believe they will likely be challenged in the courts