- Spurred largely by pending global regulations, the race is on to develop low- and zero-carbon fuels for ships and scale up their use.
- There are “bridge fuels” that could be used during a transition period or in a limited way for the long term, such as biofuels, and then there are options that are more sustainable at scale, such as green methanol and green ammonia.
- Experts continue to debate the pros and cons of green methanol and green ammonia, which are generally seen as the best options in the medium to long term.
- A net-zero framework for shipping that would drive the adoption of alternative fuels is coming up for a vote in mid-October at a meeting of the International Maritime Organization in London.
This is Part 3 of a short series on efforts to decarbonize the global shipping industry. Part 1 addressed international policy and politics, Part 2 efficiency measures. This part looks at alternative fuels.
In mid-October, more than 100 nations will gather at a London meeting to decide whether to enshrine a set of carbon reduction rules into international shipping law. If they do, high-emitting conventional shipping fuels will be heavily penalized within a few years. But the rules don’t specify how they’re to be replaced; industry members would get to decide which low- and zero-carbon fuels to use instead.
And so, spurred largely by these pending global regulations, the race is on to develop alternative fuels and scale up their use. Shipping accounts for about 3% of human-caused greenhouse gas emissions, and activity is expected to rise in coming decades. Efficiency measures to reduce fuel use of any kind, such as in hull design, will help but aren’t anywhere near a complete solution.
“You’re not going to get to zero” with just efficiency, Lee Kindberg, former head of environment and sustainability for the North America division of Danish shipping giant A.P. Møller-Mærsk, often known simply as Maersk, told Mongabay.
“Think about moving this huge mountain of metal with all these boxes of cargo on top of it,” she added. “It takes a lot of energy to do that, so it’s going to take low-carbon fuels.”
Experts may disagree on the best alternative fuels, but they generally agree on which ones should be in the discussion. There are “bridge fuels” that could be used during a transition period or in a limited way for the long term, such as biofuels, and then there are options that are more sustainable at scale, such as green methanol and green ammonia.
Bridge fuels?
Alternative fuels are currently in limited use. More than 90% of the fuel burned on ships is an oil-derived marine fuel such as heavy fuel oil, which is a “bottom-of-the-barrel” derivative of the fossil fuel refining process.
The next most common fuel in current use, accounting for a small percentage, is in fact another fossil fuel: liquefied natural gas (LNG). Fossil-based LNG can emit roughly 20% less greenhouse gas than conventional fuels, but methane leaks can effectively undo the gains. New forms of LNG, such as bio-LNG, offer more savings, but only if the feedstock is sustainable, a caveat that applies to all biofuels. LNG, like many alternative fuels, requires a different engine than conventional fuels, so resources are required to make the switch.
Tristan Smith, professor of energy and transport at University College London, told Mongabay that fossil LNG has “absolutely no contribution” to make to decarbonization efforts and is a “dead end,” and that other forms of LNG are also bad options. He said one problem is that once shipping companies have invested in something like LNG, they tend to lobby for a regulatory system that continues to allow it.
“Money has power,” he said. “Once you’ve sunk capital into a molecule, you’re going to defend it to the hilt.”
Some biofuels do have a role to play as bridge fuels, experts generally agree. However, as with bio-LNG, the climate benefits they offer depend on where the feedstock comes from. Biofuels are controversial, and not just in shipping, because demand for them can spur land use change and raise food prices. For example, when forests are cleared or peatlands drained in Southeast Asia to grow oil palm (Elaeis guineensis), some of which goes to make biofuel. The problem exists elsewhere, as well.
“If you want to be hyperbolic, there’s a risk to burn the Amazon,” Felix Klann, a maritime transport policy officer at Transport & Environment, a Brussels-based advocacy group, told Mongabay with regard to biofuel use.
Experts regard biofuels that come from waste-derived biomass, such as used cooking oil or agricultural residues, as more sustainable than “first-generation” biofuels that come from soy or corn. (They are especially skeptical of “renewable diesel,” which has been linked to deforestation in Brazil.) They say the main problem is scale: There is only so much waste-derived feedstock available.
Quantifying the full life cycle carbon emissions of biofuels, as with other fuels, is complicated. The text of the net-zero framework, which is the set of global regulations that could be adopted in mid-October by the International Maritime Organization (IMO), the U.N. agency that oversees shipping, doesn’t fully specify how such life cycle assessments will be made. If the framework is adopted, technical discussions on these and other contentious issues remain to be completed.
Better long-term options
In recent years, Maersk has launched 17 cargo vessels that can run on green methanol, which is produced from non-fossil sources. The vessels have “dual fuel” engines that can also run on conventional fuels, as needed. Meanwhile, green methanol production facilities are coming online in Denmark and China.
Green methanol is a squishy term that generally refers to either “e-methanol” made from renewable hydrogen and captured carbon dioxide or “bio-methanol,” a biofuel that can be produced from, for example, the anaerobic digestion of organic waste. (“E” because production of the hydrogen requires electricity.) Like other fuels, the degree of climate benefit offered by green methanol varies widely depending on how it’s produced. Some experts feel it’s generally a good option.
“I am betting on methanol, for the intermediate term, at least,” said Kindberg, who spoke as an independent expert, not on behalf of her former employer, Maersk. She said the future will involve multiple fuels.
“I don’t see really one fuel winning out,” she added.
For his part, Smith of UCL said methanol was not a good long-term option because of the amount of carbon required. The most sustainable way to source carbon is to suck it out of the atmosphere: direct air capture, as it’s called. Producing methanol this way — the only green methanol worth its name, in Smith’s view — virtually eliminates life cycle emissions if the production is powered by renewable energy. But scale is a challenge because direct air capture is an expensive, energy-intensive process, as carbon dioxide is very diffuse in the atmosphere.
Smith criticized Maersk for its investment in green methanol, saying it drew attention away from green ammonia, which his team’s research indicates is a better solution.
“They’ve distracted everyone with a molecule which is not a long-run solution and delayed the onset of ammonia,” Smith said of Maersk.
In response, a Maersk spokesperson whom the company did not name told Mongabay in an emailed statement that “we envision a future with multiple technological pathways.”
“No single fuel is likely to meet the diverse needs of global shipping, given differences in vessel types, routes, infrastructure availability, and regulatory frameworks,” the spokesperson said. “A portfolio approach, combining different low GHG [greenhouse gas] emission options, offers the flexibility to optimize emissions reductions, manage costs and mitigate the risk of lock-in to any one fuel.”
Maersk has indicated publicly that ammonia may be the best long-term option. Green ammonia contains no carbon, but rather nitrogen, which is easier and cheaper to source at scale. (E-methanol and green ammonia both require the production of hydrogen, a major energy and cost driver; bio-methanol can get its hydrogen from biomass feedstock, reducing that particular energy requirement.)
Price listings as of September 2025, adjusted for energy density, show that green ammonia is already slightly less expensive than green methanol; however, other associated costs such as storage could make ammonia more expensive to own in total. China and India have ramped up production of green ammonia using renewable energies, which could drive prices lower. Both green ammonia and green methanol are more than three times the cost of conventional fuels.
Jasper Verschuur, an assistant professor in engineering systems and climate security at Delft University of Technology in the Netherlands, said green methanol is the most viable solution for the next 10-15 years, but green ammonia has the potential to “become the most viable longer-term solution.” He said his thinking was “fully in line” with a new report issued by the DNV, a Norway-based ship classification society, outlining ammonia’s pros and cons. He told Mongabay that even if green ammonia becomes widely produced, the shipping industry will have to compete with other industries for its use and may not be able to afford it without government support. He also said building up the infrastructure and supply chain for green ammonia would be hard.
There are relatively few ammonia-fueled vessels on the water today. However, dozens have been ordered and dozens more, including container ships and other types of vessels, are being made “ammonia ready” so they can be easily retrofitted to use the fuel later, according to the Ammonia Energy Association, a trade group.
This is a positive development, according to Agustin Valera Medina, co-director of the Net Zero Innovation Institute at Cardiff University in the U.K. He considers green ammonia to be the most promising alternative fuel.
“The reason is that you don’t have any CO2 in the molecule, and therefore, if you just burn it properly, the only thing that you get out is nitrogen and water,” he told Mongabay.
Still, it’s not without risks. Without proper design and controls, burning ammonia can emit nitrogen oxides and nitrous oxide, a greenhouse gas, threatening local air quality, the ozone and the climate. And all experts who spoke to Mongabay agreed that the use of ammonia presents safety concerns, especially when vessels are at port. Ammonia, even more so than methanol, is highly toxic and corrosive and can cause severe respiratory damage if inhaled.
“You could kill people,” Kindberg said in explaining the stakes.
Ammonia has long been used in refrigeration and as a fertilizer feedstock, so there are well-established protocols for carrying it as cargo, and recent work has been done to prepare guidelines for its use as a marine fuel, including by the IMO.
Transition to any alternative fuels will involve design, retrofitting and storage costs. Port authorities will have to decide which fuels to stock as the fuel mix diversifies.
The fuel transition could come alongside other technological developments. Fuel cells could replace combustion engines for many fuels, especially those that contain hydrogen, while electrification and battery power could become more widely used for certain ship functions, especially at ports.
The rollout of alternative fuels does not depend entirely on the outcome of the London IMO meeting: There are national and regional regulations also driving change, most notably at the European Union level. But the transition would be weakened if the deal doesn’t go through, Smith of UCL said.
“[W]e would still see incentives creating energy transition, but not in as … coordinated and equitable [a] way as if [the] IMO drove the transition. And quite possibly as a result, not such a fast ramp-up [or] switch of energy,” Smith said in a text message.
Banner image: The Laura Mærsk arrives at port in Copenhagen in September 2023 for a name-giving ceremony. Laura Mærsk was the first vessel in Maersk’s “dual fuel” methanol fleet and the world’s first container ship to be methanol-enabled. Vessels with dual fuel engines can generally run on one alternative fuel such as green methanol as well as, when necessary, a more conventional option such as heavy fuel oil. Image courtesy of Maersk.
World’s first industry-wide climate mandate could be launched with shipping vote
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