The pictures from Beijing tell their own story: pollution there is catastrophic. Bad news for residents, and awkward for me too. Just over a decade ago, I wrote a book, The Undercover Economist, which among many other things cheerfully asserted that particulate air pollution in urban China was sharply falling as the country grew richer. It’s a claim I believed at the time (based on well-regarded research in the 2002 Journal of Economic Perspectives) but with each new report of smog over China, I felt a nagging sense that I had led readers astray. I figured it was time to do some more research and to set the record straight.

There is a broader question here. When countries become richer, do they pollute their environment more or less? For a while it seemed obvious that pollution and riches went hand in hand: industrialised nations spewed out more of everything.

But then the leading countries began to crack down on pollution. London no longer suffers from smog. The European Union reduced sulphur dioxide emissions by more than 80 per cent between 1990 and 2011. At the same time, the United States has reduced atmospheric lead by 98 per cent.