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By Root 786 0

cars in most Western countries already use smaller catalytic converters to reduce harmful pollutants, including NOx. Diesel cars, which do not usually come fitted with catalytic converters, produce many times more nitrogen pollution but a fifth less CO2, facing drivers with a difficult choice about which is “greenest.” Ultimately, as with power stations, the only long-term solution will be to eschew fossil fuels altogether and switch to electric vehicles.

On the international stage there are already some relatively successful efforts to control nitrogen pollution: The 1999 Gothenburg Protocol, part of the 1979 Convention on Long-range Transboundary Air Pollution, focuses on cutting both sulfur and nitrogen emissions. The Convention was the result of concerns about acid rain, to which both chemical elements are contributors. Although hardly noticed by anyone (or perhaps because), the measures have been quite a success: Between 1990 and 2004, nitrogen oxide emissions fell by a third, while ammonia emissions have dropped by a fifth.23 Overall, 25 million tonnes of reactive nitrogen are still produced by fossil-fuel burning, however.24 All of this can be eliminated as a by-product of meeting the climate change boundary—a happy synergy that we should exploit to the full.

Another promising strategy is to create the conditions for microbial denitrification on the largest scale possible. Streams and rivers are vital nitrogen sinks: Wetlands and flood meadows create the low-oxygen conditions in their sediments that denitrifying bacteria need, while also helping support freshwater biodiversity.25 This is a win-win for at least four planetary boundaries: those on freshwater, biodiversity, land use, and nitrogen. Towns and cities could pay to protect wetlands downstream from their wastewater outlets, while major agricultural areas—with a small levy payable by farmers on tonnages of fertilizer used—could do likewise.

Human encroachment onto fragile wetland areas at river outlets should be strictly controlled, for big deltas provide our last chance to stop nitrates washing out of rivers and causing oceanic dead zones. At the mouth of the Mississippi large areas of wetland are being lost annually because the artificially channeled river no longer delivers sediment downstream, and nitrates leached out of the heavily fertilized corn belt wash straight into the Gulf of Mexico rather than being trapped in reed beds. Reversing this process would be economically as well as environmentally beneficial because of the benefit to fisheries of reducing the size of the dead zone. None of this requires the great expense of high technology: We just need to provide the space for plants and microbes to work their magic.

Removing nitrates from wastewater is one of the major functions of sewage treatment plants, which are designed to encourage denitrifying bacteria in settling sewage sludge. However, 3.2 billion city-dwellers around the world are still not connected to modern sewage systems. Instead, in many urban slums in Africa, Asia, and Latin America, children play in fetid swamps while open sewers run past the front doors of their shacks. Millions of young lives are lost to preventable diseases as a result. Helping meet the nitrogen planetary boundary is probably the least compelling reason for tackling this injustice, but nevertheless one estimate suggests that delivering modern sewage facilities to everyone in poor countries could reduce reactive nitrogen pollution by 5 million tonnes a year.26 World Health Organization estimates suggest a cost of about $11 billion a year to provide both clean water and adequate sanitation to half the world’s population by 2015, as demanded by the UN’s Millennium Development Goals.27 Because of the lives saved, the reduction of health-care costs for preventable diseases, and the waste of productive time involved in fetching and carrying water, this expenditure would be strongly cost-positive. Each dollar invested would yield a return of between $5 and $46, depending on the region. There can’t be many better deals