WHAT is your carbon legacy – not the emissions you are personally liable for, but those of your descendants? Ask Paul Murtaugh, a statistician at Oregon State University in Corvallis.

If you have a child, he says, you and you partner are each responsible for half its emissions. If that child has kids, one-quarter of their emissions are down to you, and so on. How it adds up depends on population trends and emission changes in the future.

Murtaugh used UN population projections, which say that after 2050, birth rates in all countries will be 1.85 children per woman, on …