There are all kinds of bubbles. We had the financial bubble that burst in 2008. There is the Chinese property bubble, which may take the world economy down with it when it bursts. But nothing compares with the food bubble.
In 2008, the Organisation for Economic Co-operation and Development published a report predicting that the food price surge of that year would quickly revert to normal: 'Barring any underlying climate change or water constraints that could lead to permanent reductions in yield, normal higher output can be expected in the very short term.' And barring age, disease and accidents, we will all live forever.
Between April 2010 and April this year, the average world price of grain soared 71 per cent: a catastrophe for poor people who already spend more than half their incomes just to keep their families fed. And that is before 'climate change and water constraints' get really serious. They will.
But let's ignore the effects of climate change for now; let's just talk about what happens to the world food supply when the irrigation water runs out.
The first great food crisis was in the early 1970s, when consumption was outrunning production due to rapid population growth. Grain prices were even higher in real terms than they are now, and there was near-starvation in some areas. But the problem was solved by the 'Green Revolution', which increased yields of rice, wheat and corn.
The only drawback was that the revolution wasn't all that green. Higher-yielding strains of familiar crops played a part in the solution, but so did a vast increase in the use of fertiliser. And, above all, there was an enormous expansion of the world's irrigated area.
Only 20 per cent of the world's cropland is irrigated, even now, but that provides about 40 per cent of the world's food, so it is vital. A large part of the new irrigated land uses water pumped up from deep underground aquifers. Eventually, these will be pumped dry. Not all at once, but most will go dry at some point in the next 30 years.
The irrigated area in the US has probably passed its peak already. A World Bank study reported in 2005 that the grain supply for 175 million Indians is produced by overpumping water, and some 130 million Chinese similarly depend on a dwindling supply of underground water for their grain.
A great many countries will lose their ability to feed their own people once the irrigation bubble bursts - and will not be able to afford to import food at the vastly inflated prices that ensue.
Never mind what climate change will eventually do to the world food supply. The crisis is coming sooner than that, and it is quite unavoidable. We are living way beyond our means.
Gwynne Dyer is a London-based independent journalist