Wednesday, March 21, 2012

Humanity's Growing Impact on the World's Freshwater by Sandra Postel

As the human population has climbed past seven billion, and the consumption per person of everything from burgers to blue jeans has risen inexorably, the finiteness of Earth’s freshwater is becoming ever more apparent.

It takes water to make everything, and the explosion of demand for all manner of products is draining rivers, shrinking lakes, and depleting aquifers.

Consider this: on average it takes 2,700 liters (713 gallons) to make a cotton shirt and 9,800 liters (2600 gallons) to make a pair of blue jeans.  The cotton crops growing in farmers’ fields consume most of that water; a smaller share is used in the factories that churn out the clothes.

On any given day we’re likely wearing more than 15,000 liters (~4,000 gallons) worth of water.  And if we slip on a pair of leather loafers, well, add another 8,000 liters (~2,100 gallons).  It takes a lot of water to grow the grain to feed the cow whose skin is turned into shoes.

Such figures might not matter if there was abundant water whenever and wherever we needed it – or if water had a substitute.  But water is limited, and there’s no substitute for it.  We need water to quench our thirst, to grow our food, to cool electric power plants, and to make cars, computers and all those cotton shirts.

And that’s why the size of humanity’s water footprint – and of yours and mine – matters.

In a study published this week in the Proceedings of the National Academy of Sciences, researchers Arjen Hoekstra and Mesfin Mekonnen of the University of Twente in the Netherlands, have made the most detailed estimate to date of the scale and patterns of humanity’s water consumption.

This is a tricky and complicated task.  Using a high level of spatial resolution, the researchers tabulated all the water from both rainfall and irrigation that’s consumed in making goods and services for the global population. To complete the picture, they added in the volume of water needed to assimilate the pollution generated along the way.  They calculated the annual average global footprint for 1996-2005, the most recent ten-year period for which the necessary data were available.

The result is a large number – 9,087 billion cubic meters (2,400 trillion gallons) per year.   That’s a volume equivalent to the annual flow of five hundred Colorado Rivers.

Agriculture accounts for a whopping 92 percent of that global water footprint.  Not only are crops naturally thirsty, we’re feeding more than a third of the global grain harvest to livestock to satisfy our desires for meat and other animal products. Added up, the average beef burger takes 2,400 liters (634 gallons) of water to make.

In fact diets heavy in meat largely explain why the average water footprint for the United States is twice the global average.  U. S. consumers eat 4.5 times more meat than the global average.

One of the most interesting findings of Hoekstra and Mekonnen is that one-fifth of humanity’s water footprint travels across national borders in the form of “virtual water” – the water embedded in products that are traded between countries. For Egypt, Israel, Iran, Jordan, Saudi Arabia and other water-scarce nations, the ability to externalize their water consumption by importing wheat and other thirsty grains allows them to save their scarce water for industrial production and other higher-value uses.

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