priznam sa, ze sa uplne do hlbky o toto nezaujimam, ale v Rational Optimist som sa mimo ineho docital toto:

So I am going to make an outrageous proposal: that the world could reasonably set a goal of feeding itself to a higher and higher standard throughout the twenty-first century without bringing any new land under the plough, indeed with a gradual reduction in farmland area. Could it be done? 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.
Well, let me assume that despite Clark’s conservatism about photosynthesis, this is still wildly optimistic. Let me quadruple his number and assume that earth cannot feed an average human from less than 100 square metres. How close are we to that point? In 2004, the world grew about two billion tonnes of rice, wheat and maize on about half a billion hectares of land: an average yield of four tonnes to the hectare. Those three crops provided about two-thirds of the world’s food, both directly and via beef, chicken and pork – equivalent to feeding four billion people. So a hectare fed about eight people, or about 1,250 square metres each, down from about 4,000 square metres in the 1950s. That is a long way above 100 square metres. In addition, the world cultivated another billion hectares growing other cereals, soybeans, vegetables, cotton and the like (pasture land is not part of this calculation) – that is about 5,000 square metres each. Even if you increase the number of people to nine billion, there is still an enormous amount of room for improvement before we start hitting the limit of agricultural productivity. You could double or quadruple yields and still be nowhere near the maximum practical yields of land, let alone the photosynthetic limit. If we all turned vegetarian, the amount of land we would need would be still less, but if we turned organic, it would be more: we would need extra acres to grow the cows whose manure would fertilise our fields: more precisely, to replace all the industrial nitrogen fertiliser now applied would mean an extra seven billion cattle grazing an extra thirty billion acres of pasture. (You will often hear organic champions extol the virtues of both manure and vegetarianism: notice the contradiction.) But these calculations show that even without vegetarianism, there will be a growing surplus of farmland.
So let’s do it: let’s continue to cut down the area of farmland per person to the point where we can begin to turn the rest over to wilderness.

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