Grey water embodied in global imports and exports has risen faster than the world’s overall grey water footprint, the team found. From 1995 to 2009, the EU and US effectively outsourced water pollution by importing goods requiring lots of grey water. Meanwhile, China in particular suffered from water scarcity by generating pollution in the production of goods for export.

The researchers believe that more attention should be paid to virtual trades in grey water. "It’s important to focus on the water scarcity caused not only by the decrease of quantity, but also by the degradation of quality," said Can Wang of Tsinghua University in Beijing. "International transfers of embodied pollution through bilateral trade, driven by globalization, could aggravate the uneven distribution of water resources on the planet."

Virtual trading occurs when one country imports a good that uses a resource in the exporter’s country. The virtual trade of water is a well-known example – if a country exports corn, it is also in effect exporting the water used to grow that corn.

Now Wang and his colleagues have explored the virtual trade not of water quantity but water quality – specifically nitrogen pollution. Using the same example, if a country discharges an amount of nitrogen fertilizer during the growth of corn for export, then it is a grey water exporter, because it needs to draw on its own fresh water resources to dilute that nitrogen pollution to its local water-quality standards.

"The tangible transfer of pollution never happens in reality," Wang explained. "However, pollution caused by the production of exported goods consumed by importing countries happens in the exporting countries. So the pollution that would have happened in the importing countries is shifted to the exporting countries."

Wang’s group developed a multi-region computer model to track international supply chains and their associated grey-water impacts for the period 1995 to 2009. Overall, they found, the grey water footprint rose by 2.2%, while traded grey water rose by 3.4%. China, with 28% of the world total, ranked first for grey water associated with exports; the EU was the biggest grey-water importer, with 23% of the world total, closely followed by the US.

The researchers believe policies could address the rise in virtually-traded grey water in a two-pronged attack. Big exporters could implement tighter regulations on domestic pollution discharges and restructure industries to lower the grey water generated, while big importers could label levels of associated grey water on the imported products, encouraging consumers to choose those that are more environmentally friendly. "Eventually, this will influence the international trade structure," said Wang.

Now the team, which published the study in Environmental Research Letters (ERL), wants to explore the pricing imposed on water consumption and pollution by different countries, in the hope that it will explain the drivers of grey-water trade at a fundamental level.

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