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The production and use of nitrates also worsens climate change, both directly and indirectly. The Haber-Bosch process is energy intensive and gobbles up 5 percent of the world’s entire annual natural gas production—emitting millions of tonnes of additional CO2 as a by-product. One of the consequent forms of reactive nitrogen, nitrous oxide (N2O), is an extremely powerful greenhouse gas—fully three hundred times as potent in trapping the sun’s heat as carbon dioxide—and hangs around in the atmosphere for on average more than a century. As with CO2, methane, and other greenhouse gases, concentrations of nitrous oxide have been rising, from 270 ppb (parts per billion) in preindustrial times to 319 ppb in 2005. Agriculture is one of the biggest contributors to nitrous oxide emissions, and it barely matters whether farms use artificial fertilizers or cattle manure.21 Nitrogen is nitrogen, pure and simple.

With all these impacts in mind, the planetary boundaries expert group proposes that the flow of human-fixed nitrogen should be reduced to slightly more than a third of its current value—from 100 million tonnes to about 35 million tonnes per year (phosphorus is included with a separately defined boundary). In case this seems a little rash, and given the many complexities and the heavy dependence of humanity on nitrogen use, the experts include the proviso that “much more research and synthesis of information is required to determine a more informed boundary” in due course.

This nitrogen boundary affects many others, as the previous discussion illustrates—consider the impacts on biodiversity, the ozone layer, climate change, and fresh water, just for starters. Consider also that by mid-century there will likely be another 3 billion mouths to feed, not to mention the higher production needs implied by changing diets and the worldwide increase in biofuels. At a first approximation, the nitrogen boundary could be even more difficult to meet than the carbon one: After all, we are only technically dependent on carbon, but we are biologically dependent on nitrogen. There are no possible substitutes for the nitrogen we need to build muscle proteins, secrete enzymes, and construct cellular DNA. This may not be a boundary that we can ever meet in practice with the number of people alive on the planet today. But there is a lot we can do to try.

MEETING THE NITROGEN BOUNDARY

There are three ways that humanity can move away from our profligate use of nitrogen and toward a safer level as defined by the planetary boundaries expert group. We can synthesize less ammonia and produce less reactive nitrogen overall. We can create the conditions for microbial denitrification on a wider scale, to get reactive nitrogen turned back into harmless N2 faster and across larger areas. And we can also use what is produced more efficiently, reducing runoff and ensuring that more nitrogen is incorporated into the crop rather than wasted. There are many opportunities for all of these, some of which confirm conventional Green and development wisdom—but some of which environmentalists will find very hard to swallow indeed. I am not doctrinaire about any of these options, but I would ask readers to consider what follows with an open mind.

One of the easiest win-wins would be eliminating the nitrogen that is produced unnecessarily in various processes, brings no benefits at all to agriculture, and is also dangerous to human health. A significant proportion of humanity’s additional reactive nitrogen is created as a by-product of burning fossil fuels, such as the harmful nitrogen oxides (NOx) that come out of your car’s exhaust pipe. Technologies already exist that could slash the nitrogen produced from this source by two thirds, reducing the smog pollution that is highly damaging to human lungs and hearts.22 For power stations burning coal, “selective catalytic reduction” systems pump ammonia through the boiler exhaust gases and turn the nitrous oxides back into harmless N2 or water. These should be made mandatory everywhere. Gas-powered