PNAS Study: Population Growth Will be Constrained by the Limits of Trading Virtual Water (Food)

A new study has been released by the Proceedings of the National Academy of Sciences (PNAS), which calls into question the unsustainable global food export system based upon unsustainable export volumes of virtual water.

The first sentence sums it up:

Population growth is in general constrained by food production, which in turn depends on the access to water resources. … Most of the water we use is to produce the food we eat. With the world’s population that has doubled every 40 years, there is a growing concern that water limitations will soon impede humanity to meet its food requirements.

The study refers to a global water unbalance and challenges the long-term sustainability of the food trade system as a whole, based upon current food export rates. It projects that growing demographic requirements of water-rich nations will result in less exports to water-poor regions.

Water-rich regions are likely to soon reduce the amount of virtual water they export, thus leaving import-dependent regions without enough water to sustain their populations.

The concern is that virtual water trade is allowing for some populations to exceed the limits imposed by their local water resources, and that by sustaining demographic growth above the regional carrying capacity, virtual water trade has mitigated the effects of drought and famine in many regions of the world.



Using the estimate that the carrying capacity of one-third of all nations today depends upon food availability, and thus water availability, the study estimates maximum sustainable populations. Because both water-rich and trade-dependent populations are growing to rely on the same pool of resources, in the long run the virtual water exports are unsustainable.

One of the study’s scenarios takes into account plausible crop expansions, increases in agricultural production efficiency, and changes in diet and consumption rates.

These are huge variables, each one of them, making this a difficult assessment.

The study concludes that water will eventually limit population growth. This analysis estimates that the decline in the trade-dependent population is expected to start around 2030, but possibly not until 2040 to 2060 given the right amount of international cooperation. Interestingly, they compared their conclusion as coming out similarly to Heinz von Foerster’s Doomsday prediction for human population growth to cease expansion on November 13, 2026.

This is a hugely important subject, but one that is riddled with “what if” questions.

In my opinion, the greatest current threats to global food security are politics and policy of individual nations, economic conditions of individual nations, biofuels, and yes, water, energy, climate, and land constraints. On the subject of water, the energy-water nexus will compete for agricultural water, in addition to the growth of human populations. Policies can also create large water demands. Corn ethanol policy results in huge amounts of aquifer water use, for example. Another great challenge in the coming decades might be production costs, since industrial farming method costs seem to be increasing at an unsustainable pace. Were it not for today’s biofuels policies and generous government agricultural subsidies, today’s high input costs might not be recovered. Energy costs and availability are a key factor behind today’s food exports, or surpluses, and may dictate the farming methods of tomorrow, which in turn will determine food output amounts. Energy is required for irrigation and is embedded throughout today’s food system and food trade system. Furthermore, there is opportunity for more water conservation and innovation in today’s food production systems. As water becomes more expensive, that innovation will occur.

Agriculture requires many and varied inputs making modern food security a very complex issue. According the the World Bank, in 2007, only 15 percent of farm output in terms of embedded water is traded internationally and though I suspect that number is headed upwards, it would be difficult to predict a close-term limits to growth of human populations based upon water trade embedded in food, and much of today’s virtual water trade most likely involves livestock. So, today’s agricultural production, trade, and export volumes are not limited by water availability alone.

Today’s food security specialists are encouraging the goal of regional food security. This is the best security, and takes into account water availability and production methods. For it to happen, small shareholder farmers need markets which often can’t compete with “virtual water” food exports and they also need good, supportive governments.

Preserving the water that we have by using aquifers in a wise and limited way, and keeping our water uncontaminated, may be the biggest and most worthwhile water challenge of all. But unfortunately, like soil conservation, water is a natural resource too easily exploited.

Thankfully, there is a lot of resilience available in the food system, and today we continue to have a large amount of overproduction and waste, so I’m not certain that I agree with the doomsday dates of this study. If climate change results in less water from snow melt and more weather-related crop failures, however, agricultural growing regions will necessarily shift, which could change food producing regions significantly, even within the U.S.

The geopolitics of water-rich nations cooperating with water-poor nations, however, is a very interesting one indeed. The agricultural “land-grabs” of today are in part, at least, an effort to gain water resources. Disputes over water, food security, and other natural resources have made for human migrations, multiple conflicts, and re-drawing of borders in the past. No doubt they also will in the future.


Source: PNAS “Water-controlled wealth of nations” by Samir Suweis, Andrea Rinaldo, Amos Maritan, and Paola D’Odorico Jan. 2013. []

8 thoughts on “PNAS Study: Population Growth Will be Constrained by the Limits of Trading Virtual Water (Food)

  1. Mike Cahill

    THE DODGY concept of what is called the ‘water footprint’ of food appears to have slipped below the media radar since Australia’s drought of the century broke late last year. In 2006, New Scientist magazine had told readers: ‘It takes 20,000 litres of water to grow 1 kilo of coffee, 11,000 litres for a quarter pounder and 5000 litres for 1 kilo of cheese… no wonder the earth is running dry…’
    Really? Truth is ‘it takes’ and it gives back in one big global and seemingly eternal cycle.
    A couple of years ago Canadian geneticist and environmental activist Dr David Suzuki was interviewed on ABC Radio National. Here’s part of what he said: “Like … I think one of the craziest things in Australia is that you are one of the exporters of rice. When you export rice, you are basically taking a very limited resource called water and you are shipping that water to other countries. Now to me that’s crazy…”
    We all have a right to expect more of Doctor Suzuki. He’s a scientist. The world is literally his stage thanks to his profile in nature based science documentaries and in the news media. Wherever he goes, well-intentioned journalists flock to sit at his feet. But, David Suzuki is a born showman and, like all of us, he’s more than capable of talking utter nonsense. A little porkie-pie to save the planet perhaps? Let’s revisit for a moment the wonderful hydrological science of the water cycle about which many of us would have been taught at school. The water cycle is critical to all life on earth. The water cycle doesn’t recognise trade or national boundaries. Environmental water, which is subject to its own immutable natural laws, can’t be ‘exported’. Environmental water, as a liquid or a gas, passes between land, rivers and waterways to oceans and from land and oceans to the atmosphere. It does this by life-giving processes called evaporation, condensation, precipitation, infiltration, run-off and also via brilliant underground flows and storage chambers. The water cycle is exactly that … a cycle in which all life is sustained and nothing, but nothing, is lost… Australia’s SunRice is one of the largest rice food producers, processors and marketers in the world and it says it typically feeds up to 40 million people per day in more than 60 countries with high quality Australian produce.

    1. John Troughton

      Hi Mike
      Great video. Briefly, we want to create an event on World Food Day in October this year at Wollondilly, Sydney, based on food and water and to be educational. Wollondilly is both agricultural and the mountains around supply Sydney water. Could I make contact? Ring 0419970594. John

  2. John Troughton

    Thanks for your analysis. This is good quantitative data that is necessary for the debate but as you note the food system has resilience. The waste factor, overeating, feeding pets, feeding animals, wasted land are five factors of significance. The humans right to food is prevented, firstly by economic exclusion and financial manipulation (primarily by the large banks) then political incompetence, malpractice and manipulation. The failure of the institutions is central to the problem; UN, FAO, IMF, World Bank.

    1. K. McDonald Post author

      Though some institutions that are on your list are well meaning, top-down approaches don’t always work and its HOW you implement the small shareholder help and earn their trust without any infrastructure and without energy and machinery and without a political system that is solid and uplifting that is the biggest problem of all.

      1. John Troughton

        Great comment and I agree.
        Bottom up is to be encouraged and supported but lacks the resources available to the top. Need is to provide a scale of support to small scale activities that allows them to operate with the benefits of scale. At least the US Governement is now acknowledging support for small scale farming I.e. local farm enterprises although minuscule help compared to the scale of support it helps industrial farming and large operators, and distorts the arable land use for food problem.


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