The conservative wing of the Roberts Court today ruled that the EPA cannot regulate greenhouse gases in certain contexts — but acknowledged that the agency can continue to do so in others. Despite the negative part of the ruling, the EPA retains its general authority to regulate greenhouse gases, a result that the EPA calls “a good day for all supporters of clean air and public health.” Environmental organizations such as the Natural Resources Defense Council and Earthjustice issued generally positive statements in response to the ruling.

In 2007, the Supreme Court ruled that greenhouse gases count as a pollutant under the Clean Air Act. While that case involved the regulation of cars and trucks, today’s case involved two sections of the Clean Air act relating to “stationary sources,” like power plants.

In the now-predictable 5-4 lineup, the Justices sided with polluters in ruling that the Clean Air Act prohibits certain permitting requirements for facilities that emit large amounts of greenhouse gases. Under the Clean Air Act, facilities emitting pollutants above a certain level (250 tons per year) are subject to EPA permitting requirements. Congress set that level with traditional pollutants in mind, but since greenhouse gases are emitted in far greater volumes than other pollutants, millions of industrial, commercial, and even residential sources exceed the statutory threshold. To avoid tremendous costs to both industry and state permitting authorities, the EPA chose to initially subject only the largest sources of emissions to mandatory greenhouse gas permitting – those emitting far, far more than 250 tons per year.

All the Justices agreed that when Congress passed the Clean Air Act, it wanted permitting requirements only for the largest polluters. So how do you reconcile the Act’s requirements for a permitting program for places emitting “any air pollutant” over 250 tons per year, with the fact that including greenhouse gases over that amount leads to results that Congress didn’t want?

As Justice Breyer stated in his dissent, all the Justices agreed that the statute has to be interpreted as having an implicit exception, or else it doesn’t make sense. But they disagreed sharply over what that implicit exception is. The conservative majority used this as an opportunity to poke a hole in the definition of “air pollutant” in the specific sections of the Act at issue in this case, so they basically apply to “any air pollutant except greenhouse gases.” This would allow the largest contributors to greenhouse gas pollution escape reasonable regulations under the sections of the Clean Air Act at issue in this case.

In contrast, the dissent would attach the implied exception to the types of sites that are subject to the mandatory permitting requirements. That would let the EPA adopt regulations affecting only the main sources of the problem, exactly as Congress intended. And it would prevent an interpretation of the Clean Air Act where greenhouse gases count as pollutants in one section but not another. The dissent’s approach lets the EPA respond flexibly to new information and advances in science since the Clean Air Act was adopted, just as Congress intended.

Yet, as noted above, this was not the only part of today’s opinion. Importantly, the Court also upheld EPA regulatory authority in another context, with only Justices Thomas and Alito dissenting. If a facility is already subject to EPA regulations due to more traditional air pollutants, then the EPA can require it to use the best available technology to control its greenhouse gas emissions, as well. During oral arguments, the Obama Administration said that this covers 83% of the greenhouse gas emissions that come from stationary sources in the United States.

Justices Alito and Thomas wrote that the greenhouse gases aren’t covered by any part of the Clean Air Act, period. Fortunately, their position did not carry the day. And the day ends with EPA’s authority mostly intact.