by Julia Rosen Monday, March 9, 2015
Crumbling aqueducts crisscross the lands that once belonged to Rome, relics of a water system that sustained an empire. But now, research suggests that much of the water that flowed into Roman cities did not come gurgling down a conduit — it came in copious quantities of grain, imported to feed a growing population.
The grain purchased each year by Rome alone required more than 100 million tons of water to produce — water Rome effectively gulped down. This hidden water trade nourished Roman cities, fortifying their food supply against the fickle Mediterranean climate, according to a new study published in the journal Hydrology and Earth System Sciences. However, trade may have also pushed the empire closer to collapse.
Brian Dermody of Utrecht University in the Netherlands and his colleagues wanted to understand the exchange of so-called virtual water — water used to produce goods or services in one place that ultimately get consumed somewhere else — and how it affects the resilience of civilizations. The researchers focused on the Roman Empire because its cities grappled with some of the same challenges many face today: growing urbanization, water scarcity and unpredictable climate. “It was quite a challenge for [the Romans] to maintain a stable supply of food to the cities, but one they achieved remarkably well,” Dermody says.
His team developed a unique multidisciplinary approach to explore how the Romans did it. First, the researchers used a global hydrological model to estimate grain yields in the agricultural areas that were controlled by the Roman Empire around the year A.D. 200. They ran the model for 50 years to try to capture the Mediterranean climate’s substantial variability.
They also enlisted the help of classics scholars from Stanford University who study ancient Rome. Using Stanford’s database of Roman trade networks — including the locations of ancient roads, shipping routes and transportation costs — the team estimated how much grain and thus, how much virtual water, moved throughout the region.
As expected, the researchers found that Rome and other major cities imported the greatest amounts of virtual water. Most of it came from Egypt, southern Spain, France and northern Italy. However, the researchers also found some discrepancies between their results and historical records — for instance, Dermody says that while rain-fed lands in southern Europe could have supplied plenty of grain, the Romans appeared to prefer importing from irrigated regions like the Nile, which produced predictable yields every year.
Overall, Dermody says, maintaining a diverse network of trading partners — and possessing total military control of the Mediterranean, which allowed the Romans to ship goods without fear of raids — likely improved the empire’s resilience to weather variations. That’s because different areas of the region “would not be affected by the same climate variations at the same time,” says Carole Dalin, a post-doctoral researcher in hydroeconomy and climate at the Grantham Research Institute at the London School of Economics, who was not involved in the work. However, Dalin notes that the researchers used modern data to drive their hydrological model, which may not capture the true range of historical climate variability.
Unfortunately, the Romans' effective use of trade may also have allowed the populations of cities like Rome to grow beyond their ability to sustain themselves. “In an isolated society, you have to remain below the minimum variability in the food supply to avoid famine,” Dermody says. “Trade removes much of this variability in supply, allowing populations to grow unchecked.” However, even trade has limits — at some point, there are no new suppliers available. Dermody says the voracious demands of Rome’s 1 million residents may have exceeded the ability of the Mediterranean region to support them, perhaps contributing to the empire’s demise.
The double-edged nature of virtual water trade may contain lessons for modern civilization, says Megan Konar, a hydrologist at the University of Illinois at Urbana-Champaign, who was not involved in the study. “There’s currently a debate about whether trade makes cities or countries more or less resilient to climate variations,” she says. “This paper suggests that there might be time-varying implications,” meaning the immediate benefits may give way to long-term liabilities.
The study’s authors also propose that trade may introduce new vulnerabilities into the food-supply chain. Not only are cities vulnerable to food shortages in the regions from which they import, “but they are also quite vulnerable to breakdowns in the trade network,” Dermody says. These concerns are particularly relevant to modern megacities, which depend heavily on trade, Dermody says. “Virtual water is a nice way to highlight how interconnected we all are.”
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