We attempt to put the issue of sustainable population on the agenda. The number of people whose CO2 can be absorbed and not cause other species to become extinct may be only two or three billion. The present level of seven billion has global warming as an unavoidable consequence. The more immediate threat comes from migration. Evidence is presented that anti-immigration feelings, the Arab Spring, war in Iraq and Syria and disasters involving boat people are consequences of more than two births per woman. Women's choices are considered. Perhaps prospective parents could consider this question: my baby will require the equivalent of a tank of petrol every five days for its whole life: where will the oil come from? Some possibilities are discussed for these countries: Afghanistan, Britain, China, Nigeria, Syria and Zimbabwe.