Rising agricultural nitrogen emissions around the world are steering us toward global temperature increases that will rise 3 °C above pre-industrial levels—putting the Paris Agreement goals well out of reach. And yet, this doomsday scenario can be avoided, if we turn to mitigation measures that have already shown striking success in bringing down regional greenhouse gas emissions, according to a new study published in Nature.
Nitrogen is a crucial nutrient for plant growth, hence why it’s used to enrich synthetic fertilizers that are applied liberally to farmland across the planet. It also occurs in livestock manure, and even in aquaculture it may be poured into fish pens to boost phytoplankton growth. But in these settings—especially when it’s excessively applied—nitrogen leads to microbial reactions that generate the greenhouse gas known as nitrous oxide (N2O).
At 300 times the potency of carbon dioxide, and persisting for 100 years in the atmosphere, this gas has a profound shaping effect on climate change.
The new study, which brings together 48 institutions, takes an inventory of nitrogen emissions—both natural sources, and anthropogenic—and reveals that anthropogenic N2O emissions have leapt by 30% over the past four decades. What’s more, the researchers discovered that the vast majority of that increase (87%) was driven by agriculture, as cropland expanded across the planet, and livestock populations ballooned. (The remaining 13% came from industry and other sources.)
The expanding use of synthetic fertilizers across large swathes of the planet is the biggest culprit, featuring prominently in the United States, India, Europe, and China. The growth of the livestock industry, and with it, the application of manure-based fertilizer to farmland, was the major driver of N2O emissions in South America and Africa. For example, in Brazil alone, the excessive application of manure-based fertilizers has caused a 120% increase in agricultural N2O emissions there, over the last 40 years.
The researchers note that an expanding appetite for meat and dairy across the planet is also increasing N2O emissions through the ever-mounting livestock byproduct of manure. It’s also occurring as more land is converted into pasture, either to feed cattle and dairy cows directly, or to grow crops for their feed – both of which require fertilizing.
The lion’s share of N2O emissions currently arise from countries like Brazil, China, India—places with intensive use of synthetic fertilizers, and also where large shares of land are devoted to growing animal feed and farming livestock, for consumption across the planet.
Looking at longer timescales, the researchers found that compared to pre-industrial levels in 1750, N2O emissions have leapt by 20%. Notably, the majority of this increase has only occurred in the last 50 years because of human activities—again pointing the finger at agriculture’s outsized role. And the combined effect of all this spells bad news for our climate: the researchers show that the increase in N2O emissions has occurred at a faster pace than any of the climate scenarios projected by the Intergovernmental Panel of Climate Change.
At the pace that N2O emissions are growing, it will take us far above the Paris Climate target of limiting global warming to less than 2 °C above pre-industrial levels. This fact underscores “the urgency to mitigate N2O emissions,” the researchers write.
But how do we do that, in a world that needs more food to feed a growing population? The key to solving this crisis lies in the fact that fertilizer is dramatically over-applied, worldwide. So we at least have the option to reel it back. That could mean using precision agriculture to apply exact amounts fertilizer to crops, ensuring that they use it all up instead of leaving large quantities behind in the soil to be broken down by microbes and released as N2O. Conservation measures like no-till on farms can also help soil retain more nutrients, so that less fertilizer is required.
Already, there’s evidence that these approaches can lead to relatively swift and large scale emissions reductions. In Europe in the 1990s, a policy to optimize fertilizer use and reduce its over-application ended up driving down N2O emissions from soils by 21%, over a 20 year period.
While farmers inevitably bear the brunt of the responsibility for bringing down fertilizer use, consumers also have a considerable role to play, by reducing the consumption of meat and dairy products, and limiting food waste, the researchers say. These steps can both bring down the amount of fertilizer needlessly applied to the soil, and lost to the atmosphere.
“There is a conflict between the way we are feeding people and stabilizing the climate,” the researchers say. All these steps will be crucial for fixing that balance, and bringing agriculture in line with a planet that can sustain us for longer.
Source: Tian et. al. “A comprehensive quantification of global nitrous oxide sources and sinks.” Nature. 2020.