Four countries generate almost half the world’s nitrogen emissions, with developing countries tending to suffer local pollution caused by foreign demand.

An international collaboration led by the University of Sydney traced the flow of emissions from 188 countries and found the United States, China, India and Brazil are responsible for 46% of the world's nitrogen emissions.

As a measure of a country’s contribution to global emissions, its nitrogen footprint was defined as the quantity of reactive nitrogen emitted during the production, consumption and transportation of commodities consumed within that country, and whether those commodities are produced domestically or internationally.

The study's model grouped the nitrogen footprint into top-ranking bilateral trade relationships and found developed nations are largely responsible for emissions abroad for their own consumption. The study revealed that developing countries tend to embody large amounts of nitrogen emissions from their exports of food, textiles and clothing.

Net importers and net exporters of nitrogen. Image credit: University of Sydney “High-income nations are responsible for more than 10 times the emissions of the poorest nations,” says PhD candidate Arunima Malik, who carried out the study with fellow University of Sydney colleagues Professor Manfred Lenzen and Dr. Arne Geschke, as well as researchers from Yokohama National University and Kyushu University, in Japan. “This reflects greater consumption of animal products, highly processed foods and energy-intensive goods and services.”

The vast bulk of emissions came from industries such as agriculture, transport and energy generation. Emissions from consumers' end use were mostly from sewage.

The study also found:

• High-income exceptions to the pattern included Australia, New Zealand and Argentina, which export significant nitrogen embodied in livestock products.
• Developing countries such as China, India, Pakistan and Thailand embody large amounts of nitrogen emissions into their exports of textiles and clothing.
• Per-capita nitrogen emission ranged from more than 100 kg annually for wealthier entities such as Hong Kong and Luxembourg to less than 7 kg for developing nations such as Papua New Guinea, Côte d’Ivoire and Liberia.
• Of the 189 teragrams (Tg) of nitrogen emitted worldwide in 2010, 161 Tg was emitted by industries and agriculture, while 28 Tg was emitted by consumers.