Nitrogen can be a threat to the environment and human health, so it's prudent to ask who bears the brunt of nitrogen pollution, and who's responsible for it. According to a new study, developed nations bear most of the responsibility, but the effects fall hardest on the world's poor—including, it turns out, some of the most vulnerable people here in the United States.
Though nitrogen makes up more than three-quarters of the air we breath, it's a major contributor to acid rain and a freakishly potent greenhouse gas in more chemically reactive forms, such as nitrous oxide and nitrates. According to one recent estimate, nitrogen pollution costs the European Union around €70 billion per year, roughly twice the economic value E.U. farms get from using nitrogen-based fertilizers.
Four countries—China, India, Brazil, and the U.S.—are responsible for nearly half of global reactive nitrogen emissions.
Yet like carbon dioxide, assigning responsibility for nitrogen pollution to particular countries isn't entirely straightforward. There's a certain amount of carbon dioxide that enters the atmosphere from cars, trucks, and manufacturing in the U.S. Then there's the carbon dioxide that enters the air in other parts of the world because of the U.S. Consider a T-shirt bought in the U.S but made in China. The factory that made the shirt and released carbon dioxide in the process may be in China, but it's U.S. consumers' demand for T-shirts that led to it.
The same goes for reactive nitrogen (Nr)—economic demand in one country increases nitrogen pollution in another—but it remains unclear which countries export, so to speak, the most Nr and what products contribute the most to the problem.
Enter Azusa Oita, Manfred Lenzen, and their colleagues, who combined data on Nr emissions and domestic and international trade with a model of the global nitrogen cycle to estimate 188 countries' nitrogen footprints. In broad strokes, their conclusions are about what you'd expect. Four countries—China, India, Brazil, and the U.S.—are responsible for nearly half of global Nr emissions, and the U.S. alone accounts for 10 percent of the global total.
More surprisingly, "[r]oughly a quarter of the global nitrogen footprint is from commodities that were traded across country borders," the researchers write—typically agricultural products and textiles produced in developing countries and sent almost exclusively to developed economies such as the U.S. "We conclude that substantial local nitrogen pollution is driven by demand from consumers in other countries."
Yet disadvantaged Americans suffer too. One in six residents of Tulare County, California, a major beef center and home to many low-income Latinos, has tap water with high levels of nitrates from fertilizer and manure, forcing them to choose either poor health or the expense of bottled water. Japan, a major importer of the region's beef, contributes 22 billion grams of Nr to Tulare County's problems, the researchers write.
Solutions, the authors argue, will require better coordination at the international level as well as looking at nitrogen emissions across a product's supply chain, from the farms that grow cotton to consumers who drive economic demand.
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