The US Supreme court has handed down a decision on carbon emissions that is decisive in a number of respects, but leaves us without an answer to the ultimate question at hand: should the EPA regulate automotive carbon emissions? The decision in Massachusetts v. EPA says that it could if it wanted to and compels the EPA to both make a decision and explain the logic behind it.
The case arose because Massachusetts asked the EPA to issue regulations that limited the carbon released by automobiles, arguing that it was a pollutant that posed dangers to the state as defined by the Clean Air Act. The EPA declined, triggering the suit, which was later joined by a number of other states and private organizations. The case reached the Supreme Court following a mixed decision by a D.C. Circuit Court in which each of the three judges that heard the appeal submitted their own opinion.
The case hinged on three questions: do the states involved have standing to bring the suit, does CO2 constitute a pollutant according to the Clean Air Act, and should the EPA regulate it accordingly. The EPA argued no on all three counts, focusing on the legal precedent associated with the Clean Air Act for the first two questions. They also argued that adopting regulations for CO2 would interfere with other governmental agencies and administration policy. At no point did the EPA suggest, however, that there was any problem with the science suggesting that CO2 posed a danger. This seems to have come back to haunt them.
On the first matter, a five judge majority ruled that states have standing on this issue due to their loss of land from rising oceans and the ecological and financial damage expected to be caused by changes in precipitation. A minority of justices—Alito, Roberts, Scalia, and Thomas—dissented on this point, and accepted the EPA's argument that the CO2 emitted by US automobiles only contributed to a fraction of the expected damage due to climate change, so regulating it would not prevent the states from suffering damage. This reasoning was rejected by the majority because it suggested that any regulatory effort must completely eliminate the risk, or not be worth attempting. In the majority view, the "EPA’s steadfast refusal to regulate greenhouse gas emissions presents a risk of harm to Massachusetts that is both 'actual' and 'imminent.'"