The 21st century promises to be quite exciting. It's certainly been a lot of fun so far

Another unsustainable arrangement recently came to light when Marc Bierkens of Utrecht University (the Netherlands) released his results about groundwater depletion in agriculural breadbaskets all over the world—

In an upcoming study, Bierkens and his colleagues find that not only is global groundwater extraction outstripping its natural recharge rate, this disparity has been increasing. Groundwater represents about 30 percent of the available fresh water on the planet, with surface water accounting for only one percent. The rest of the potable, agriculture friendly supply is locked up in glaciers or the polar ice caps. This means that any reduction in the availability of groundwater supplies could have profound effects for a growing human population. Using a database of global groundwater availability and estimates of groundwater usage and recharge rates, Bierkens and his team have been able to model both where, and how quickly, groundwater stores are being depleted. They find that the rate at which global groundwater stocks are being used has more than doubled between 1960 and 2000, increasing the amount lost from 126 to 283 cubic kilometers (30 to 68 cubic miles) of water per year. Because the total amount of groundwater in the world is unknown, it’s hard to say how fast the global supply would vanish at this rate. But, if water was siphoned as rapidly from the Great Lakes, they would go bone-dry in around 80 years. “The rate of depletion increased almost linearly from the 1960s to the early 1990s,” says Bierkens, of Utrecht’s Physical Geography Department. “But then you see a sharp increase which is related to the increase of upcoming economies and population numbers; mainly in India and China.” [My note: I will disregard the inconsistent statements "groundwater represents ... about 30% of the potable water on the planet" and "the total amount of groundwater in the world is unknown."



Global map of groundwater depletion, where 1000 is equal to one cubic kilometer of depletion per year.

While the largest increases in the depletion rates have been in China and India, you can see from the map that there is some impressive depletion going on in America's Midwest and California's Central Valley.

Then there is our system of "virtual water" trading in which places with water (where you can grow food) support human populations living in arid areas—

It is not just that there are more thirsty and hungry people catalyzing groundwater depletion, using the water for drinking water, irrigation and industry. It’s also, says Bierkens, that as a population grows, “people start to live in locations where there is less precipitation. If you want to feed those people, you need to find water somewhere else.” In another study, Paolo D’Odorico of the University of Virginia in Charlottesville and his colleagues model a world which is increasingly dependent on a globalized water supply. D’Odorico says that we are moving towards a system of trading what is called ‘virtual water’. He is referring to a network where food is produced in a region that has water for irrigation, and then sold to feed inhabitants of other regions. The study by him and colleagues at the Politecnico di Torino, in Turin, Italy, finds that as we become ever more efficient at producing food in areas with a plentiful water supply and shipping to regions lacking sufficient water to feed a hungry population, we reduce our ability to cope with shocks to the network, like droughts or crop failure... When taken together, Bierkens’ and D’Odorico’s works erect warning signs that we may be living on another kind of virtual water. “You’re living on loaned money, in this case loaned water,” says Bierkens. “It’s a water debt you build up because these aquifers are not being recharged, but it allows you to raise your standard of living. "I don’t want to insult you, but it sounds a bit like how some people in the U.S. and Europe live when it comes to money.”

No offence taken, Marc. Although these acquifers are renewable in the sense that they are recharged through precipitation, it takes a long time for groundwater systems to fill up again to their natural levels (i.e. the level we would measure without human interference.) Bierkens views this as a "water debt" in the sense that we are "borrowing" water that might eventually be replaced. However, we are "borrowing" water at a rate well in excess of the recharge rate, so it might be more accurate to say that we are living off the principal of the water loan, meaning the system must crash at some point unless we change our irrigation practices.

Another interesting result concerns where all this groundwater ends up—

If the water being pumped is not returning to the aquifers, where is it going? More than 95 percent ends up in the ocean, the authors say, where the quantity of groundwater reaching the sea is making an important contribution to sea level rise. The team concluded that groundwater has contributed 25 percent of the sea level rise observed since 2000.

Wow! I'm a little skeptical of this result—I'd have to read the paper to see how Bierkens arrived at this conclusion—because that's an immense amount of water. Sea level is now rising at a rate of a few millimeters per year, but 25% of that, given the size of the world's oceans, is still a lot of water being transferred from underground aquifers to the sea.

What's the real meaning of this groundwater depletion fiasco situation?

Unless the depletion rate is less than or equal to the recharge rate, aquifers will be effectively drained at some point, which might mean that they are bone dry or that it will have become prohibitively expensive to get at the remaining water (i.e. it is no longer economic to pump the water).

As we continue our current irrigation practices, the "virtual water" trading system allows us to maintain larger populations than we can actually support in the longer run. In this sense, thriving human populations living in arid regions are an illusion. In various regions at various times, this system must coming to a grinding halt unless we change behavior.

The situation is actually worse than Bierkens and his colleagues say with respect to groundwater depletion. Typical climate change scenarios indicate that many of the regions where withdrawals are highest are the same regions that can expect more drought in future decades. As these agricultural regions dry out, the recharge rate for the aquifers will no doubt decline, which makes the water we are "borrowing" all the more precious.

Right now, I can't tell you when Phoenix, Arizona will become a ghost town. But it will be.

A situation that can't continue won't continue. Groundwater depletion is a case in point. As with other planetary habitability disasters, humans will learn this lesson the hard way. I've given up on trying to influence people in the hopes that they will change their behavior. Nonetheless, I report this stuff so that at least some people are fully aware of the problem. And for the rest, at least they can't say nobody warned them.