Today's cargo ships use massive diesel engines that emit large amounts of air pollutants, contributing to climate change and affecting human health. Maritime transport accounts for nearly 3 percent of global carbon dioxide emissions, and the industry's negative impacts on air quality cause about 100,000 premature deaths each year.

Decarbonizing shipping to reduce these harmful effects is the goal of the International Maritime Organization (IMO), a United Nations agency that regulates maritime transport. One potential solution is to switch the global fleet from fossil fuels to sustainable fuels like ammonia, which could be nearly carbon-free when considering its production and use.

However, a new study by an interdisciplinary team of researchers from MIT and other institutions warns that burning ammonia as a maritime fuel could further degrade air quality and lead to devastating public health effects unless enhanced emission regulations are adopted.

Burning ammonia generates nitrous oxide (N2O), a greenhouse gas about 300 times more potent than carbon dioxide. It also emits nitrogen in the form of nitrogen oxides (NO and NO2, known as NOx), and unburned ammonia can leak, ultimately forming fine particles in the atmosphere. These fine particles can be inhaled deeply into the lungs, causing health issues like heart attacks, strokes, and asthma.

The new study shows that under current laws, switching the global fleet to ammonia could cause up to about 600,000 additional premature deaths each year. However, with stronger regulations and cleaner engine technology, that switch could result in about 66,000 fewer premature deaths than currently caused by emissions from maritime transport, with a much smaller impact on global warming.

“Not all climate solutions are created equal. There's always some price to pay. We need to approach the problem holistically and consider all the costs and benefits of different climate solutions, not just their decarbonization potential,” says Anthony Wong, a postdoc at MIT's Center for Global Change Science and the study's lead author.

His co-authors include Noelle Selin, a professor at MIT in the Institute for Data, Systems, and Society and the Department of Earth, Atmospheric and Planetary Sciences (EAPS); Sebastian Eastham, former principal research scientist now a senior lecturer at Imperial College London; Christine Mounaïm-Rouselle, a professor at the University of Orléans in France; Yiqi Zhang, a researcher at the Hong Kong University of Science and Technology; and Florian Allroggen, a researcher at MIT's Department of Aeronautics and Astronautics. The research was published this week in Environmental Research Letters.

Greener, cleaner ammonia
Traditionally, ammonia is produced by extracting hydrogen from natural gas and then combining it with nitrogen at extremely high temperatures. This process is often associated with a significant carbon footprint. The maritime industry is relying on the development of "green ammonia," produced using renewable energy to create hydrogen via electrolysis and to generate heat.

“In theory, if you burn green ammonia in a ship engine, carbon emissions are almost zero,” says Wong.

But even the greenest ammonia produces nitrous oxide (N2O), nitrogen oxides (NOx) when burned, and some ammonia can leak unburned. This nitrous oxide would escape into the atmosphere, where the greenhouse gas would linger for more than 100 years. At the same time, nitrogen emitted as NOx and ammonia falls to Earth, damaging sensitive ecosystems. As these emissions are processed by bacteria, additional N2O is created.

NOx and ammonia also mix with gases in the air to form fine particles. A primary contributor to air pollution, fine particles kill an estimated 4 million people annually.

“To claim ammonia is a ‘clean’ fuel is a bit of an exaggeration. Just because it's carbon-free doesn't necessarily mean it's clean and good for public health,” says Wong.

Multiple models
The researchers wanted to present the full picture, capturing the environmental and health impacts of transitioning the global fleet to ammonia. To do this, they designed scenarios to measure how pollutant impacts change according to certain technological and policy assumptions.

On the technological side, they considered two types of ship engines. The first burns pure ammonia, which creates more unburned ammonia but emits less nitrogen oxides. The second engine technology involves mixing ammonia with hydrogen to improve combustion and optimize the catalytic converter's efficiency, which controls both nitrogen oxides and unburned ammonia pollution.

They also considered three policy scenarios: current regulations, which limit NOx emissions only in certain parts of the world; a scenario adding ammonia emission limits above North America and Western Europe; and a scenario adding global limits on ammonia and NOx emissions.

The researchers used a shipping route model to calculate how pollutant emissions change in each scenario, then fed the results into an air quality model. The air quality model calculates the impact of shipping emissions on particle and ozone pollution. Finally, they assessed the effects on global public health.

One of the biggest challenges was the lack of real-world data, as no ammonia-powered ships currently sail the seas. Instead, the researchers relied on experimental data on ammonia combustion from collaborators to build their model.

“We had to come up with some clever ways to make those data useful and informative for technological and regulatory situations,” says Wong.

Range of outcomes
In the end, they found that without new regulations and with ship engines burning pure ammonia, transitioning the entire fleet could cause 681,000 additional premature deaths each year.

“While a no-new-regulation scenario isn't very realistic, it serves as a good warning of how dangerous ammonia emissions could be. And unlike NOx, ammonia emissions from shipping are currently not regulated,” says Wong.

However, even without new regulations, using cleaner engine technology would reduce the number of premature deaths to about 80,000, which is about 20,000 fewer than currently attributed to emissions from maritime transport. With stronger global regulations and cleaner engine technology, the number of people dying from air pollution from shipping could be reduced by about 66,000.

“The results of this study highlight the importance of developing policies alongside new technologies,” says Selin. “There's potential for ammonia in shipping to be beneficial for both climate and air quality, but this requires regulations designed to cover the full range of potential impacts, including both climate and air quality.”

The impacts of ammonia on air quality would not be felt equally around the world, and fully addressing these issues would require coordinated strategies in very different contexts. Most premature deaths would occur in East Asia, as air quality regulations in this region are less stringent. Higher levels of existing air pollution cause more fine particles to form from ammonia emissions. Additionally, the volume of shipping traffic over East Asia is far greater than elsewhere on Earth, amplifying the negative effects.

In the future, the researchers hope to continue refining their analysis. They hope to use these findings as a starting point to encourage the maritime industry to share engine data that can be used for better air quality and climate impact assessments. They also hope to inform policymakers about the importance and urgency of updating emissions regulations for shipping.

This research was funded by MIT's Climate and Sustainability Consortium.

Source: Massachusetts Institute of Technology