The Rational Optimist: How Prosperity Evolves

by Matt Ridley

In the early 1960s the economist Colin Clark calculated that human beings could in theory sustain themselves on just twenty-seven square metres of land each. His reasoning went like this: an average person needs about 2,500 calories of food per day, equivalent to about 685 grams of grain. Double it for growing a bit of fuel, fibre and some animal protein: 1,370 grams. The maximum rate of photosynthesis on well-watered, rich soils is about 350 grams per square metre per day, but you can knock that down to about fifty for the best that farming is in practice able to achieve over a wide area. So it takes twenty-seven square metres to grow the 1,370 grams a person needs. On this basis and using the yields of the day, Clark calculated in the 1960s that the world could feed thirty-five billion mouths.