We know that reducing emissions from cars and factories is essential for cleaning our air, but new research reveals a chemical quirk in the atmosphere whereby slashing one pollutant can inadvertently increase the amount of another – in this case a harmful form of nitrogen deposited into the ocean.
The study used a sophisticated global atmospheric model to project how nitrogen deposition to the world’s oceans will change by 2050, finding that the future depends heavily on which pollutants we target.
Nitrogen from human activities – primarily ammonia (NH₃) from agriculture and nitrogen oxides (NOx) from combustion – is carried through the air and eventually falls onto the ocean surface. This acts as a fertiliser, potentially boosting marine plant growth but also disrupting delicate ecosystem balances.
Under a ‘sustainable development’ scenario with stringent climate policies, the amount of nitrogen deposited into the ocean is projected to drop by 24% by 2050, driven by the cuts we are making to NOx emissions. However, the model revealed a counter-effect: while total nitrogen fell, the share coming from agricultural ammonia increased significantly. In coastal China, for example, the proportion of nitrogen from ammonia jumped from 37% to 64%.
The study revealed a key chemical reaction. In the air, ammonia from farms and nitric acid (created from vehicle and industrial pollution) normally stick together to form small particles but, when NOx pollution is cut, there’s less nitric acid around. That means more ammonia is left floating freely as a gas and in this form, it falls much faster and more directly onto nearby coastal waters, a process called dry deposition.
‘If you reduce NOx emissions alone, you can actually enhance the dry deposition of ammonia to the oceans,’ the authors say.
Conversely, in a high-emission scenario with weak environmental policies, total nitrogen deposition to the ocean increases slightly by 6%, driven by rising ammonia from agriculture.
Changes in this nutrient supply have real-world consequences. In 2015, the nitrogen raining into the ocean helped grow around 290 million tonnes of marine plants annually. If we follow a low-emission path, that fertilising effect could shrink by 23%, preventing as much carbon from being absorbed as would be saved by scrapping 50 million cars. If emissions stay high, it could rise by 5%.
On the other hand, more nitrogen also stimulates the ocean to produce more nitrous oxide (N₂O), a potent greenhouse gas. The research found that changes in N₂O emissions could offset roughly 60% of the climate benefit gained from any increased carbon storage.
Calling for integrated strategies that address both agricultural and combustion sources simultaneously, the researchers conclude: ‘Jointly controlling NH₃ and NOx emissions would be more effective than managing individual nitrogen species.’
The full research can be read here.
Photo: Ant Rozetsky
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