Feeding the world’s population on an ongoing basis requires that farmers enrich their soils with fertilizers. The levels of atmospheric nitrogen, along with biological nitrogen fixation, are not enough to provide adequate supplies of this element to the growing crops. Farmers add nitrogen in the form of synthetic fertilizers and manure. However, the production and use of fertilizers has environmental consequences, not least of which is the emission of certain greenhouse gases.
If we are to reduce global emissions of greenhouse gases in order to limit the extent of global warming in future, we need to consider the role of fertilizer production and use. According to a new study by researchers from the University of Cambridge, fertilizer production and deployment accounts for five percent of humanity’s greenhouse gas emissions, including carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). We will also need to walk a delicate line between changing the way we use fertilizers and ensuring food security for people.
The study, published in Nature Food, is the first to quantify the contribution of fertilizers to global greenhouse gas emissions. The researchers calculate that one-third of the emissions arise during the industrial production of the fertilizers, while two thirds arise after the fertilizers have been spread on the fields. Their analysis found that manure and synthetic fertilizers emit the equivalent of 2.6 gigatons of carbon per year, which is more than the total from aviation and shipping combined.
“Incredibly, we don’t actually know how many chemicals we produce globally, where they end up, where and how they accumulate, how many emissions they produce, and how much waste they generate,” said co-author Dr. André Cabrera Serrenho from Cambridge’s Department of Engineering.
Serrenho and his co-author Yunhu Gao undertook the current study in order to accurately measure the total impacts of fertilizers, one of the two main products of the petrochemical industry. Of all the products made by the petrochemical industry, the vast majority – as much as 74 percent – are either plastics or fertilizers.
“In order to reduce emissions, it’s important for us to identify and prioritize any interventions we can make to make fertilizers less harmful to the environment,” said Serrenho. “But if we’re going to do that, we first need to have a clear picture of the whole lifecycle of these products. It sounds obvious, but we actually know very little about these things.”
The experts mapped the global flows of manure and synthetic fertilizers and their emissions for 2019, along all stages of the lifecycle, by reconciling the production and consumption of nitrogen fertilizers and regional emission factors across nine world regions. After completing their analysis, the researchers found that unlike many other products, the majority of emissions for fertilizers occur not during production, but during their use.
“It was surprising that this was the major source of emissions,” said Serrenho. “But only after quantifying all emissions, at every point of the lifecycle, can we then start looking at different mitigation methods to reduce emissions without a loss of productivity.”
The researchers say that carbon emissions from fertilizers urgently need to be reduced but this must be balanced against the need to produce food for the world’s populations. Earlier research has estimated that 48 percent of the global population is currently fed with crops grown using synthetic fertilizers; and the world’s population is set to increase by 20 percent before 2050. Fertilizers are thus expected to be more in demand than ever.
The Cambridge researchers outline a combination of scalable technological and policy solutions to reduce fertilizer emissions while maintaining food security. They estimate that, if such solutions could be implemented at scale, the emissions from manure and synthetic fertilizers could be reduced by as much as 80 percent, to one-fifth of current levels, without a loss of productivity.
For example, the synthesis of ammonia, from which all synthetic fertilizers are produced, involves the use of natural gas, coal and oil as raw materials for the industrial process. The extraction and combustion of these substances produces greenhouse gases, as do the chemical reactions along the production pathway.
The researchers say the most effective mitigation at the production stage would be for the industry to decarbonize heating and hydrogen production. They suggest that electrolysis could be used to supply hydrogen for ammonia synthesis, thus avoiding greenhouse gases from steam methane reforming. Additionally, fertilizers could be mixed with chemicals called nitrification inhibitors, which prevent bacteria from forming nitrous oxide. However, these chemicals are likely to make fertilizers more expensive.
“If we’re going to make fertilizers more expensive, then there needs to be some sort of financial incentive to farmers and to fertilizer companies,” said Serrenho. “Farming is an incredibly tough business as it is, and farmers aren’t currently rewarded for producing lower emissions.”
The experts listed and quantified the maximum theoretical impact of different mitigation methods – most of these are already known, but their maximum potential effect had not been quantified. Since two-thirds of emissions arise after the fertilizers have been spread on the fields, increasing the efficiency of fertilizer use is the single, most effective strategy to reduce emissions.
“We’re incredibly inefficient in our use of fertilizers,” said Serrenho. “We’re using far more than we need, which is economically inefficient and that’s down to farming practices. If we used fertilizer more efficiently, we would need substantially less.”
The researchers also looked at the mix of fertilizers used around the world, which varies by region. They say that replacing some of the fertilizers with the highest emissions, such as urea, with ammonium nitrate, worldwide, could further reduce emissions by between 20 and 30 percent. However, this would only be beneficial after decarbonizing the fertilizer industry.
“There are no perfect solutions,” said Serrenho. “We need to rethink how we produce food, and what sorts of economic incentives work best. Perhaps that means paying farmers to produce fewer emissions, perhaps that means paying more for food. We need to find the right mix of financial, technological and policy solutions to reduce emissions while keeping the world fed.”
Serrenho and Gao estimate that by implementing all the mitigations they analyzed, emissions from the fertilizer sector could be reduced by as much as 80 percent by 2050.
“Our work gives us a good idea of what’s technically possible, what’s big, and where interventions would be meaningful – it’s important that we aim interventions at what matters the most, in order to make fast and meaningful progress in reducing emissions,” said Serrenho.
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