Guardian | March 21, 2009

Spring, the season of fertility, began yesterday, yet it is warnings of scarcity that are notably abundant. John Beddington, the government's chief scientific adviser, warned this week of a "perfect storm", with food, water and energy all dangerously depleted by 2030, thanks to population growth and rising prosperity. Next week the Optimum Population Trust will hold a conference at the Royal Statistical Society, arguing that the planet has room for 5 billion people at the most, and that the United Kingdom should be home to no more than about 18 million.

Such figures are unhelpful: they describe an alternative planet with an entirely notional history. Thomas Malthus, who warned that population growth would outstrip food supply, has been dismissed because food production has more or less kept up with population growth. That is one reason why we are all here, and why some are clinically obese.

But the Malthusian question has stimulated argument about the Earth's carrying capacity, which depends as much on human optimism as on ingenuity. "If the world's population had the productivity of the Swiss, the consumption habits of the Chinese, the egalitarian instincts of the Swedes, and the social discipline of the Japanese, then the planet could support many times its current population without privation for anyone," wrote Lester C Thurow in the very different world of 1986. 

Yet human numbers continue to swell, at more than 9,000 an hour, 80 million a year, a rate that threatens a doubling in less than 50 years. Land for cultivation is dwindling. Wind and rain erode fertile soils. Water supplies are increasingly precarious. Once-fertile regions are threatened with sterility. The yield from the oceans has begun to fall. To make matters potentially worse, human numbers threaten the survival of other species of plant and animal. Humans depend not just on what they can extract from the soil, but what they can grow in it, and this yield is driven by an intricate ecological network of organisms. Even at the most conservative estimate, other species are being extinguished at 100 to 1,000 times the background rate observable in the fossil record.

Malthus's arguments were part of the inspiration for Charles Darwin's theory of evolution, and they have validity in the natural world. On the savannah, in the rainforests, and across the tundra, animal populations explode when times are good, and crash when food reserves are exhausted. Is homo sapiens an exception? Perhaps. Humans can consider each other's needs, and cooperate; there is also plenty of evidence that they choose not to. The Optimum Population Trust does not have the answers, but the questions remain, quite literally, vital.