Let me introduce you to the idea of virtual water: the water supplies that make possible the world trade in commodities, especially food.
The term was the invention of water scientist
Tony Allan from King's College London and, I think, it goes a long way to explaining why the world is currently in the grips of a food crisis.
Most crops take extraordinary amounts of water to grow: a thousand tonnes for a tonne of wheat, for instance. In fact, two-thirds of all the
water abstracted from the world's rivers and underground reserves goes for crop irrigation. Unsurprisingly, dry countries, like most of the Middle East, don't have enough water to feed their growing populations.
So they import water. They import virtual water. This trade is huge, the equivalent of 20 times the flow of the world's longest river, the Nile. Without it, hundreds of millions would starve. But the trade is in trouble.
The world's biggest supplier of virtual water is, or was until a couple of years ago, Australia. It exported 70 cubic kilometres of virtual water, in the form of fruit and crops, a year. That's 70 billion tonnes, if you can imagine that better.
Then came drought, which has more than halved that figure. Australia's wheat exports are down 60%, its rice exports down 90%. The US, the second biggest virtual water exporter, has been diverting much of its water to growing corn for
biofuels and hence has been reducing its own exports.
Meanwhile global demand for virtual water is soaring, especially from China, where water is the main constraint on food production.
China has effectively run out of water in its traditional breadbasket region in the north of the country, where the Yellow River now rarely reaches the sea in any volume. China can't feed itself any more.
And other countries will follow. India has been one of the great success stories of the revolution in agricultural production in the past 40 years. Once a byword for famine, it feeds itself today. But at great cost.
Its farmers are estimated to pump, from below ground, 250 cubic kilometres of water a year, of which only 150 cubic kilometres is replaced by the monsoon rains. Water tables are plunging as a result.
This over-pumping cannot go on. Farmland is already falling out of production. Water is very heavy stuff, too expensive to move great distances. So there is no global trade in water itself, in the way there is in oil. But virtual water is traded. And as
water shortages emerge round the world –
the US, for example, has its own problems – the virtual water trade is turning those local crises into a global crisis.
In Europe, we often consider ourselves immune from water shortages in other parts of the world. But we rely on others' water more than we know.
Spain is currently importing water ("real" water, in transport ships) from France.
Britain imports 40 cubic kilometres of virtual water a year, mostly in the form of food. Right now, we are feeling other peoples' water shortages in
higher food prices. One day, if countries decide to hang onto their water, the supplies themselves may dry up.
Fred Pearce, senior environment correspondent Labels: freds-footprint, water
I think I was first struck by the potential impact of climate change when a US climate scientist I met at a conference told me he had bought a house in Canada "because it makes good financial sense: that's where the water will be".
It's World Water Day today, an international day that was initiated by the United Nations Conference on Environment and Development, in 1991. Each year has a different theme, and this year's is "Coping with water scarcity".
