Food for thought on greenhouse gas emissions

By Rachel Berkowitz

In a world dependent on nitrogen for food production—it has been estimated that more than half Earth’s human population is fed as a result of synthetic nitrogen fertilizer use—agriculture is responsible for more than 80% of anthropogenic nitrous oxide emissions.

Synthetic nitrogen fertilizers spread onto croplands, along with the use and storage of livestock manure, provide a feeding ground for microbes that break down the nitrogen and release it as potent greenhouse gas N₂O.

The third highest greenhouse gas contributor to climate change after carbon dioxide and methane, N₂O is the most potent of the three gases because it’s a better absorber of IR radiation. We humans generate 6 million metric tons of nitrogen as N₂O emissions, and 10 billion metric tons of carbon as CO₂, per year.

But it’s going to be tricky to reduce N₂O emissions, as food production processes have been accelerating to feed Earth’s growing human population, which is currently at 7 billion. The nitrogen-use efficiency of crops is far from perfect, and efficiency is further reduced when crops are used as animal feed.

In its fifth assessment report, the Intergovernmental Panel on Climate Change (IPCC) presented several scenarios, referred to as representative concentration pathways, that represent possible ways to reduce major greenhouse gas emissions. Many combinations of cultural and technological scenarios could be consistent with each pathway. The most aggressive involves stabilizing N₂O concentrations by 2050 via a combination of reducing humans’ meat consumption, improving agricultural efficiency, and reducing emissions from industry.

Reducing the demand for fertilizer

Eric Davidson , president and a senior scientist at the Woods Hole Research Center in Massachusetts, believes improved management of both fertilizer and manure sources and decreased meat consumption will reduce fertilizer demand as well as the amount of manure produced, and thus reduce emissions.

In a study published in April in the Institute of Physics’s Environmental Research Letters, Davidson evaluated the scale of improvements needed to match the IPCC pathways for N2O stabilization. His model calculates the annual atmospheric increase of N2O as the difference between anthropogenic biological sources and sinks that have changed the natural balance since the Industrial Revolution.

The United Nations’ Food and Agriculture Organization estimates an increase in global population to 8.9 billion by 2040 and a per capita increase in daily calorie intake to 3130 nutritional calories. The FAO also projects that from 2002 to 2030 meat consumption per capita per year will increase from 78 kg to 89 kg in the developed world and from 28 kg to 37 kg in the developing world.

From those data, Davidson calculated that to stabilize atmospheric N₂O concentrations at 345 ppb, people would need to reduce both their meat consumption and industry emissions by 50%.

That will involve some major cultural, industrial, and dietary modifications, for which there is no “silver bullet,” according to the paper.

The study, however, lends new value to Albert Einstein’s observation that “Nothing will benefit human health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet.”