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Without tackling nitrous oxide, the third most important greenhouse gas and the most significant ozone-layer depleting substance, there is no plausible route to limit warming under 1.5 degree Celsius (°C), warned experts speaking at a COP29 event.
Instead, the greenhouse is likely to contribute additional warming of around 0.2°C by 2100. But implementing certain solutions could lead to a 0.1°C reduction in warming beginning late this century, the United Nations Global Nitrous Oxide Assessment report highlighted.
“The report shows a very clear science-driven pathway to reduce N20 with benefits for climate, ozone, biodiversity, health and agriculture,” Kaveh Zahedi, director of FAO’s Office of Climate Change, Biodiversity and Environment, said at a press conference at COP29.
Agriculture is currently the source of 75 per cent of those emissions, while industrial sources contribute roughly 5 per cent of anthropogenic nitrous oxide emissions. The remaining 20 per cent come from fossil fuel combustion, wastewater treatment, aquaculture, biomass burning, and other sources.
Compared to the preindustrial era, N2O emission due to human activities have seen a 20 per cent increase. The gas has a lifetime of 120 years and is 270 times more powerful than carbon dioxide per tonne of emission at warming the Earth. This means, its warming effect will accumulate and will be long-lasting. It has already contributed to 0.1°C of net global warming to date since the industrial revolution.
Further, nitrous oxide is the most significant ozone-layer depleting substance being emitted to the atmosphere and is not part of the Montreal Protocol, a global agreement to protect the stratospheric ozone layer by phasing out the production and consumption of ozone-depleting substances.
The report highlighted that currently, available abatement measures could reduce N2O emissions by more than 40 per cent below current levels. Transformations in food production and societal systems could lead to even deeper reductions, it added.
Some of these measures include controlled-release fertilisers or formulations that inhibit nitrogen losses, improved manure management and behavioural changes, such as lowering the consumption of animal protein in some populations, the report explained.
The report also recommended that industries can eliminate N2O emissions by adopting existing and relatively low-cost abatement measures that could cost $1,600–6,000 per tonne of nitrous oxide. “It is a low-hanging fruit for near-term abatement and even though it currently represents approximately 5 per cent of anthropogenic emissions, this could increase in the future,” the report warned.
“This assessment sounds the alarm on a relatively forgotten super pollutant that contributes greatly to climate change and air pollution,” Martina Otto, head of secretariat of the UNEP-convened Climate and Clean Air Coalition, said in a statement. “By using the abatement tools highlighted in the Assessment that are already available to us, we can yield multiple benefits across climate, clean air and health,” she added.
As ozone in the stratosphere protects life on Earth from harmful ultraviolet (UV) radiation, abating nitrous oxide could avoid a 0.2–0.8 per cent increase in cataract cases and a 2-10 per cent increase in skin cancers by 2080-90, depending on latitude, according to the report.
Further, using ammonia as a fuel for marine shipping and biofuels derived from fertilised crops could produce significant nitrous oxide emissions, partially or completely offsetting their intended climate benefits.
Ammonia does not emit carbon dioxide when burnt. The experts also added that more studies need to be done to understand the trade-offs in carbon dioxide, methane and nitrous oxide abatement in all sectors.