National polls show that climate change is low on the list of voters’ priorities. For good reason: In the U.S., and for much of the world, the most dangerous environmental pollutants have been cleaned up. U.S. emissions of particulates, metals and varied gases—all of these: ozone, lead, carbon monoxide, oxides of nitrogen and sulfur—fell almost 70% between 1970 and 2014.

Further reductions will come from improved technologies such as catalytic removal of oxides of nitrogen and more-efficient sulfur scrubbers. This is a boon...