What would net-zero shipping look like?

The International Maritime Organization has set a net-zero goal "by or around 2050". What is needed to reach this?

A large cargo ship fitted with huge rigid sails has set sail on its maiden voyage from China to Brazil in an attempt to prove the technology can help the shipping industry slash its carbon footprint. Wind propulsion is considered to be one of the most promising energy sources available for the rapid decarbonisation of shipping.

Countries recently agreed to curb shipping emissions to net zero "by or around 2050" at a UN summit. At the annual meeting of the International Maritime Organization (IMO), member countries pledged to cut 20% of their shipping emissions by 2030 and 70% by 2040, compared to 2008 levels. They agreed to cut 100% of emissions by or around 2050. While ambitious, it doesn't go far enough for some – small island nations and richer countries had called for a 50% reduction by 2030 and 96% by 2040.

Kitack Lim, Secretary-General of the IMO, described the deal as a "monumental development [that] opens a new chapter towards maritime decarbonisation". But campaigners warn that the deal is flawed and will fail to bring the shipping industry in line with the Paris Agreement goal of limiting global temperature rise to 1.5C by the end of this century.

Shipping is a highly polluting industry, responsible for nearly 3% of global emissions and generating around one billion tonnes of greenhouse gases each year – roughly the same amount as Germany's carbon footprint. If it were a country, the shipping industry would be the sixth largest polluter in the world.

Reducing maritime emissions rapidly in the next three decades will require new regulations, infrastructure and fuels. But what might green shipping of the future look like?

The shipping industry can reduce its reliance on fossil fuels by turning to an ancient technology: sails. UK shipping firm Cargill aims to cut its fuel consumption by fitting cargo ships with wind power. The maiden voyage of the firm's ship, the Pyxis Ocean, from China to Brazil in August 2023 is the first real-world test of a rigid sail design known as WindWings, 123ft (37.5m) tall foldable wings made from the same material as wind turbines. The vessel was retrofitted with the wings.

And they are not the only ones taking this approach. Swedish company Oceanbird has also built a prototype ship using four rigid sails. Wind power not only propels the ship forward but also aids its manoeuvrability and agility on the water. One of the biggest challenges is encouraging governments and investors to adopt wind propulsion and retrofit ships, while wind propulsion is still early-stage. (Read more: Will shipping return to its ancient roots?)

Deploying clean fuels such as hydrogen is critical if the shipping industry is to reach net zero by 2050. Green hydrogen - generated by using renewable energy, such as wind or solar power, to extract hydrogen from water molecules - is emissions-free. But there are some major challenges when deploying hydrogen: the fuel must be stored at cryogenic temperatures of -253C (-423F) and crew must be trained how to handle it as the fuel is highly flammable. (Read more about the fuel that could transform shipping).

Maersk, the world's second-largest container shipping company, is betting big on green methanol to help it decarbonise. The company has ordered a total of 25 methanol-powered ships to date. Green methanol is a low-carbon fuel which can be produced from sustainable biomass or by using renewable electricity to split water into oxygen and hydrogen, which is combined with carbon dioxide. Unlike hydrogen, green methanol does not have to be stored under pressure or extreme cold, and many ports already have infrastructure in place to store the fuel. But the process is complex:  CO2 must be captured out of the atmosphere, technology which is still emerging, expensive and as yet unproven. 

Batteries charged using renewable electricity are another way to curb shipping emissions. But there are limits to the distances they can power. Currently, renewable batteries are an option only for smaller ships making short journeys, such as ferries and river boats, not for large cargo ships crossing oceans. Instead, ship owners are looking to power cargo ships with a combination of wind power and solar panels. Japanese renewable energy systems company Eco Marine Power, for example, has developed "EnergySails": rigid sails fitted with solar panels, which allow ships to use both solar and wind energy at the same time.

A rapid uptake of green fuels on vessels will require abundant new infrastructure at ports to produce and store them, and to allow ships to refuel. Ports must invest in hydrogen-generating electrolysers, renewable energy capacity, such as wind and solar power, as well as battery and hydrogen storage facilities.  Most ships will also need to be retrofitted to enable them to run on green fuels, use wind propulsion and digital software to improve their efficiency and optimise routes.  

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