A few years ago I was at a conference in Holland, where an atmospheric scientist was discussing some data related to the levels of certain air pollutants at several sites in Europe. The purpose of his presentation was to compare methods of measuring pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), and particles. As part of the talk he showed some slides that had graphs of the concentrations of these and other pollutants over the past several years. I was a bit bored because this was a meeting of a multidisciplinary research group and my own discipline was far removed from the one being discussed. But although his voice had ceased to register in my mind, I did notice something odd about the slides he was showing.
On every slide showing pollutant levels over time, the graph went down. In other words, every year, for every chemical, at every site, and for every method of measurement, the amount of pollution was decreasing. I raised my hand and asked him if this was some sort of error or if it reflected reality. He looked at me with the weary patience that an expert in any field feels when asked a stupid question by a nonexpert. He explained that of course it was real, “and everyone knows that air pollution levels are constantly decreasing everywhere.” He then continued from where I had interrupted him.
I looked around the room. I was not the only nonexpert there. Most of