Emissions from manure and synthetic fertilisers could be reduced by as much as 80 per cent, to one-fifth of current levels, according to a new study.
Researchers have quantified that two thirds of emissions from fertilisers take place after they are spread on fields, with one third of emissions coming from production processes, the study said.
The researchers, from University of Cambridge, UK, have calculated the carbon footprint for the full life cycle of fertilisers, the study said.
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Fertilisers are responsible for approximately five percent of total greenhouse gas emissions, the first time this has been accurately quantified, and found that carbon emissions could be reduced to one-fifth of current levels by 2050, the study said.
Although nitrogen-based fertilisers are already known to be a major source of greenhouse gas emissions, this is the first time that their overall contribution, from production to deployment, has been fully quantified, the study said.
Their analysis found that manure and synthetic fertilisers emit the equivalent of 2.6 gigatonnes of carbon per year - more than global aviation and shipping combined, the study said.
Carbon emissions from fertilisers urgently need to be reduced; however, this must be balanced against the need for global food security, the study said.
According to the study, earlier research has estimated that 48 per cent of the global population are fed with crops grown with synthetic fertilisers, and the world's population is expected to grow by 20 per cent until 2050.
The Cambridge researchers said that a combination of scalable technological and policy solutions are needed to reduce fertiliser emissions while maintaining food security.
However, they estimate that if such solutions could be implemented at scale, the emissions from manure and synthetic fertilisers could be reduced by as much as 80 per cent, to one-fifth of current levels, without a loss of productivity.
The results are reported in the journal Nature Food.
"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 Andre Cabrera Serrenho from Cambridge's Department of Engineering.
Serrenho and his co-author Yunhu Gao undertook a project to accurately measure the total impacts of fertilisers, one of the two main products of the petrochemical industry, the study said.
Of all the products made by the petrochemical industry, the vast majority, as much as 74 per cent, are either plastics or fertilisers, the study said.
"In order to reduce emissions, it's important for us to identify and prioritise any interventions we can make to make fertilisers 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," said Serrenho.
The researchers mapped the global flows of manure and synthetic fertilisers and their emissions for 2019, along all stages of the lifecycle, by reconciling the production and consumption of nitrogen fertilisers and regional emission factors across nine world regions, the study said.
After completing their analysis, the researchers found that unlike many other products, the majority of emissions for fertilisers 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," said Serrenho.
The researchers 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.
Emissions from the production of synthetic fertilisers are mostly from ammonia synthesis, partly due to chemical reactions used in the production process.
The most effective mitigation at the production stage would be for the industry to decarbonise heating and hydrogen production.
Fertilisers could also be mixed with chemicals called nitrification inhibitors, which prevent bacteria from forming nitrous oxide.
However, these chemicals are likely to make fertilisers more expensive.
"If we're going to make fertilisers more expensive, then there needs to be some sort of financial incentive to farmers and to fertiliser companies," said Serrenho.
"Farming is an incredibly tough business as it is, and farmers aren't currently rewarded for producing lower emissions," said Serrenho.
The single most effective way to reduce fertiliser-associated emissions, however, would be to reduce the amount of fertilisers that we use.
"We're incredibly inefficient in our use of fertilisers," said Serrenho.
"We're using far more than we need, which is economically inefficient and that's down to farming practices.
"If we used fertiliser more efficiently, we would need substantially less fertiliser, which would reduce emissions without affecting crop productivity," said Serrenho.