Switching from burning fossil fuels to burning ammonia is a promising way for the shipping industry to cut its carbon dioxide emissions. But if it isn’t careful, this swap could also boost harmful nitrogen pollution.
“That’s a major alarm bell that’s ringing,” says at the University of Maryland Center for Environmental Science.
The tens of thousands of ships on the oceans are responsible for about 3 per cent of global CO2 emissions, burning more than 300 million tonnes of fossil fuels each year. A climate-friendly alternative is “green ammonia”, a zero-carbon fuel made from hydrogen and nitrogen using clean electricity – one that could be produced at larger volumes than other low-emission fuels.
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There aren’t yet any ammonia-powered ships at sea, but several are under construction. Under an aggressive decarbonisation scenario, the International Energy Agency projects ammonia could by 2050.
There is a potential hitch though. While use of green ammonia would mean less CO2 released, it could boost nitrogen pollution. In the atmosphere, reactive forms of nitrogen can cause and ozone depletion; in the water, they can spur the growth of harmful algal blooms. Nitrous oxide can also form, a potent greenhouse gas that might offset the climate benefits of switching to the zero-carbon fuel.
Zhang and her colleagues estimated that making enough ammonia to supply 44 per cent of shipping demand would require increasing industrial production of nitrogen compounds by more than 200 million tonnes per year. That increase is greater than the total amount of nitrogen pollution currently resulting annually from human activities, mainly agriculture.
Much of the additional nitrogen in the green ammonia would be returned to the atmosphere in an inert form when it is burned as fuel. But if only a tiny fraction of the ammonia leaked out or failed to combust, the researchers found it could still boost nitrogen pollution by several per cent.
At the moment, such pollution estimates are uncertain because there are no existing full-scale ammonia engines to test, says at the Polytechnic University of Turin in Italy. “The biggest question mark is about burning ammonia. How clean is ammonia combustion?”
To reduce pollution, ships could install ammonia scrubbers that remove unburned fuel or engines designed to minimise nitrogen pollution – but such measures aren’t requirements, notes at the Massachusetts Institute of Technology.
“If we don’t have any specific ammonia legislation, using ammonia as shipping fuel would be only slightly cleaner than fossil fuels, from an air pollution standpoint,” he says.
Nature Reviews Earth & Environment
Article amended on 16 January 2025
We clarified Xin Zhang’s affiliation
