A lot of reporters were distracted by the big number in yesterday's announcement of proposed reductions in carbon dioxide emissions: 30 percent by 2030. Indeed that was the lead sentence in almost every news story about the new rules.
But the 30 percent figure is not how the Environmental Protection Agency will measure success of the new regulations. The figure is arbitrary, chosen to give some nationwide context to what the state-by-state goals would mean.
The goals the EPA actually set vary quite a lot from state to state. And, indeed, how the agency arrived at those figures is a good deal more complicated than just picking a nice, round number.