Manure and fertilizer generates more emissions annually than the shipping and aviation industries together, according to the first fully comprehensive estimate of its footprint. But if we adopt a wide-ranging set of mitigation measures, these emissions could be scaled back by more than 80%, the new Nature Food study says.
To date, most estimates on fertilizer impacts have focused either on the production or the application stage. Until now no one has mapped the impact of this agricultural commodity from its manufacturing all the way through to what happens after it goes onto the field—which is what these researchers set out to do.
To connect the dots they mapped the flows of synthetic nitrogen fertilizers and manure across nine world regions in 2019, then meticulously tallied up the greenhouse gas emissions emitted at each lifecycle stage.
Seeing the whole picture like this showed that overall, manure and synthetic nitrogen fertilizers contribute 2.6 gigatons of carbon to the atmosphere annually. Of that, synthetic fertilizers account for about half—1.31 gigatons of carbon dioxide equivalent, or 5% of emissions globally, which is more than is emitted by the entire plastics industry.
There’s a striking split in the source of those synthetic fertilizer emissions. One-third is released at the production stage as ammonia is synthesized. Gas, oil and coal are used to power this process—accounting for 0.8% of global greenhouse gas emissions and chowing down 2% of global energy. But surprisingly, the majority two-thirds of emissions occur after the nutrients have been applied to the field, where the excess not absorbed by plants interacts with soil microbes and is released back into the atmosphere as nitrous oxide, a potent greenhouse gas.
Having this full view on the industry helps pinpoint where interventions could help to bring down this enormous emissions load. First, we need to increase fertilizer efficiency on field — which is the single most effective way to bring down emissions, the researchers calculated—and we need to decarbonize ammonia production for synthetic fertilizers.
Over-applications of fertilizers on farms currently leaves large amounts unused—and so there are clear opportunities here to reduce impacts. Increasing efficiency from 42 to 67% could cut synthetic fertilizer demand by almost 50% by 2050. This would involve a suite of measures: growing crops that have been bred to use fertilizers more efficiently, improving irrigation on fields and applying fertilizers more precisely and at the right growing times for crops.
On the manufacturing side, powering the production of ammonia through a mix of electricity and renewable energy sources could bring down overall fertilizer emissions by between 21% and 27%, by 2050.
But combining these measures is where the true benefits become apparent: this could reduce greenhouse gas emissions of synthetic fertilizer by 78%. What’s more, switching more emissions-heavy fertilizer varieties like urea to lower-impact ammonium nitrate could reduce it further, to 84%.
Most excitingly, all these interventions are tech-ready, the researchers say: we could switch to them relatively fast. And, all of this could be achieved without affecting food production. But now that we know the potential, actually implementing these measures will require a careful process of weighing up costs and benefits, and leveraging policy to support the necessary transitions. It will also mean tailoring measures to suit specific regions: there is no one size fits all, and no perfect solution to this hefty challenge.
“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,” the researchers say. “We need to find the right mix of financial, technological and policy solutions to reduce emissions while keeping the world fed.”
Serrenho et. al. “Greenhouse gas emissions from nitrogen fertilizers could be reduced by up to one-fifth of current levels by 2050 with combined interventions.” Nature Food. 2023.