Should slowing population growth become part of the international push to tackle climate change? That question is raised by US environmentalists writing in the journal Science . They argue that the Intergovernmental Panel on Climate Change ought to view it as a potential “policy lever” in a warming world.

To their credit, the authors advocate only voluntary fertility-reducing measures such as education and access to family planning services. Unfortunately, history shows that such good intentions are not enough to prevent abuses, that voluntary programmes frequently turn into the opposite.

In November …