The optimum population of Earth – enough to guarantee the minimal physical ingredients of a decent life to everyone – was 1.5 to 2 billion people rather than the 7 billion who are alive today or the 9 billion expected in 2050, said Ehrlich in an interview with the Guardian.

“How many you support depends on lifestyles. We came up with 1.5 to 2 billion because you can have big active cities and wilderness. If you want a battery chicken world where everyone has minimum space and food and everyone is kept just about alive you might be able to support in the long term about 4 or 5 billion people. But you already have 7 billion. So we have to humanely and as rapidly as possible move to population shrinkage.”

“The question is: can you go over the top without a disaster, like a worldwide plague or a nuclear war between India and Pakistan? If we go on at the pace we are there’s going to be various forms of disaster. Some maybe slow motion disasters like people getting more and more hungry, or catastrophic disasters because the more people you have the greater the chance of some weird virus transferring from animal to human populations, there could be a vast die-off.”