Maybe the Earth can handle 9 billion people

See Room for all by Matt Ridley. Excerpts:

"... it is actually likely that the ecological impact of nine billion in 2050 will be lighter, not heavier: there will be less pollution and more space left over for nature than there is today.

Consider three startling facts. The world population quadrupled in the 20th century, but the calories available per person went up, not down. The world population doubled in the second half of the century, but the total forest area on the planet went up slightly, not down. The world population increased by a billion in the last 13 years, but the number living in absolute poverty (less than a dollar a day, adjusted for inflation) fell by around a third.

Clearly it is possible at least for a while to escape the fate forecast by Robert Malthus, the pessimistic mathematical cleric, in 1798. We've been proving Malthus wrong for more than 200 years. And now the population explosion is fading. Fertility rates are falling all over the world: in Bangladesh down from 6.8 children per woman in 1955 to 2.7 today; China - 5.6 to 1.7; Iran - 7 to 1.7; Nigeria - 6.5 to 5.2; Brazil 6.1 to 1.8; Yemen - 8.3 to 5.1.

The rate of growth of world population has halved since the 1960s; the absolute number added to the population each year has been falling for more than 20 years. According to the United Nations, population will probably cease growing altogether by 2070. This miraculous collapse of fertility has not been caused by Malthusian misery, or coercion (except in China), but by the very opposite: enrichment, urbanization, female emancipation, education and above all the defeat of child mortality - which means that women start to plan families rather than continue breeding.

Increasing prosperity means eating more food, though. Can we really feed today's let alone tomorrow's billions? In 60 years we have trebled the total harvest of the three biggest crops, wheat, rice and corn. Yet the acreage devoted to growing these crops has barely changed. This is because fertilizer, irrigation, pesticides and new varieties have greatly increased yields.

They continue to do so. Growth regulators boost the yield of wheat. Genetic modification boosts the yield of cotton (while increasing the biodiversity in fields). New enzymes promise to cut the phosphate output and increase weight gain of pigs. These technologies save rain forest, by sparing land from the plow. If we went back to organic farming, the world would have to cultivate more than twice as much land as we do."

"What could possibly prevent this golden vision? Running out of fossil fuels? Not a chance: the discovery of how to extract shale gas has just given the world a quarter of a millennium's worth of cheap fossil fuel. Running out of water? No: far more frugal uses of water are already in play where price and technology combine. Climate change? Hardly. Rising carbon dioxide is already measurably boosting yields of crops and the slow and small warming we have had so far - roughly half a degree in 50 years - has probably boosted rainfall slightly. Even the UN's own models predict that a big warming by 2050 from here is unlikely."