By Adam Vaughan
The amount of nitrogen pollution emitted just by global livestock farming is more than the planet can cope with, prompting scientists to say we need to eat less meat and dairy produce.
Fertilisers made for agriculture are high in nitrogen, but their use can contribute to air and water pollution, climate change and ozone depletion. Livestock waste is also a source of nitrogen pollution.
Aimable Uwizeye at the UN Food and Agriculture Organization and his colleagues found that the livestock sector accounts for about a third of all humanity’s nitrogen emissions, which are also released by burning fossil fuels and other activities.
The emissions from livestock farming amount to about 65 teragrams (Tg) of nitrogen a year. That means meat and dairy production alone breaches the lower limit of the 62 to 82 Tg a year considered to be the “planetary boundary” for nitrogen emissions, or the safe global level beyond which humanity’s future prosperity is endangered. Nitrous oxide, for example, is exacerbating global warming.
“The livestock sector contributes substantially to nitrogen emissions,” says Uwizeye. His team says that while there are technical fixes in agriculture, they may not be enough on their own to keep within planetary boundaries for nitrogen pollution, and some parts of the world will need to eat and produce less meat and dairy.
The group has called for a global initiative to tackle the problem. Rich countries in Europe and North America, as well as middle-income ones including Brazil, are among those that should consume less, says Uwizeye.
The overwhelming bulk of the emissions, 68 per cent, comes from crops grown to feed animals, followed by nitrogen released by the build-up and management of manure.
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Asia stands out as a hotspot for nitrogen emissions from livestock, at two-thirds of the global total. Uwizeye says that this is in large part down to China, where there is a growing consumer appetite for meat and dairy produce. Corn grown in the US but fed to pigs in China sees the nitrogen emissions related to this crop allocated to China in the analysis.
The other factor in China’s impact is the country’s shift to ever bigger farms, reducing their ability to recycle manure and leading to stinking mountains of the stuff building up. Much is discarded illegally, says Uwizeye.
The study, which looked at 275 countries, also reveals that rearing broiler chickens makes the most efficient use of the nitrogen needed to make feedstuff, while cattle are the worst. Pigs are in the middle.
Uwizeye says that while there is some uncertainty in the numbers for individual countries because of gaps and differing quality in the 2010 data underpinning the research, the headline global findings still stand.
Mark Sutton at the UK Centre for Ecology & Hydrology in Edinburgh says the study shows the “massive impact of livestock on global nitrogen pollution”. The findings could help inform actions stemming from a UN resolution last year on tackling nitrogen pollution’s environmental impact. One shortcoming of the research is how it treats dinitrogen (N2), which was excluded from the analysis of nitrogen emissions, Sutton says.
“This is a sober reminder to policymakers that while they’re already not dealing adequately with carbon dioxide from food, alongside it are equally pressing ecosystem demands such as nitrogen,” says Tim Lang at City, University of London.
Journal reference: Nature Food, DOI: 10.1038/s43016-020-0113-y
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