Gwynne Dyer: World's dwindling larder

We are still living off the proceeds of the Green Revolution, but that hit diminishing returns 20 years ago. Now we live in a finely balanced situation where world food supply just about meets demand, with no reserve to cover further population growth. But the population will grow anyway, and the world's existing grain supply for human consumption is being eroded by three different factors: meat, heat and biofuels.

For the sixth time in the past seven years, the human race will grow less food than it eats this year. We closed the gap by eating into food stocks accumulated in better times, but there is no doubt that the situation is getting serious. The world's food stocks have shrunk by half since 1999, from a reserve big enough to feed the entire world for 116 days, to a predicted low of only 57 days by the end of this year.

The miracle that has fed us for a generation now was the Green Revolution: higher-yielding crops that enabled us to almost triple world food production between 1950 and 1990 while increasing the area of farmland by no more than 10 per cent. The global population more than doubled in that time, so we are now living on less than half the land per person than our grandparents needed. But that was a one-time miracle. Since the beginning of the 1990s, crop yields have essentially stopped rising.

One reason we are getting closer to the edge is the diversion of grain for meat production. As incomes rise, so does the consumption of meat, and feeding animals for meat is a very inefficient way of using grain.

It takes between 11 and 17 calories of food (almost all grain) to produce one calorie of beef, pork or chicken, and the world's production of meat has increased fivefold since 1950. We now get through five billion hoofed animals and 14 billion poultry a year, and it takes slightly over a third of all our grain to feed them.

Then there's the heat. The most visible cause of the fall in world grain production - from 2.68 billion tonnes in 2004 to 2.38 billion tonnes last year and a predicted 1.98 billion tonnes this year - is drought, but there are strong suspicions that these droughts are related to climate change.

Moreover, beyond a certain point, hotter temperatures directly reduce grain yields. Current estimates suggest that the yield of the main grain crops drops 10 per cent, on average, for every 1C that the mean temperature exceeds the optimum for that crop during the growing season. Which may be why the average corn yield in the US reached a record 8.4 tonnes a hectare in 1994, and has since fallen significantly.

Finally, biofuels. The idea is elegant: the carbon dioxide absorbed when the crops are grown exactly equals the carbon dioxide released when the fuel refined from those crops is burned, so the whole process is carbon-neutral. And it would be fine if the land used to grow this biomass was land that had no alternative use, but that is rarely the case.

In Southeast Asia, the main source of biofuels is oil palms, which are mostly grown on cleared rainforest. In the US, a "corn rush" has been unleashed by government subsidies for ethanol, and so many ethanol plants are planned or already in existence in Iowa that they could absorb the state's entire crop of corn (maize, mealies). The amount of ethanol needed to fill a big SUV just once uses enough grain to feed one person for a year.

There is a hidden buffer in the system, in that some of the grain now fed to animals could be diverted to feed people directly in an emergency. On the other hand, the downward trend in grain production will only accelerate if it is directly related to global warming.

It's only in the past couple of centuries that a growing number of countries have been able to stop worrying about whether there will be enough food at the end of the harvest to make it through to next year. The Golden Age may not last much longer.