How, in short, can we enjoy “sustainable development” on a very crowded planet?

The answer has two parts, and each portends a difficult journey over several decades. The first part requires a change of technologies — in farming, energy, industry, transport and building — so that each of us on average is putting less environmental stress on the planet. We will have to make a worldwide transition, for example, from today’s fossil-fuel era, dependent on coal, oil and gas, to an era powered by low-carbon energies such as the sun and wind. That will require an unprecedented degree of global cooperation.

The second key to sustainable development is the stabilization of the global population. This is already occurring in high-income and even some middle-income countries, as families choose to have one or two children on average. The reduction of fertility rates should be encouraged in the poorer countries as well. Rapid and wholly voluntary reductions of fertility have been and can be achieved in poor countries. Success at reducing high fertility rates depends on keeping girls in school, ensuring that children survive, and providing access to modern family planning and contraceptives.

Two centuries ago, the British thinker Thomas Robert Malthus famously warned that excessive population growth would cut short economic progress. That is a threat still with us today, but it is a warning, not an inevitable outcome.