90% of world trade occurs through the sea. Compared to other modes of transport, shipping is 80% cheaper and produces relatively less carbon emissions. But the sheer scale at which shipping operates makes it a contender for carbon emissions reductions as any optimizations can have massive effects.
In addition to ships, optimizations are possible across the marine ecosystem at ports, terminals, and offshore facilities. In this article, however, we are going to restrict our focus to reducing carbon emissions from ships as a measure towards climate neutrality.
Share of shipping in overall carbon emissions
Of the total carbon emissions across all sectors, the contribution of the transport sector was 20.2%. Of this, close to 2.8% was contributed by shipping. While the number does not seem large, we can put it into perspective by understanding that roughly about 100,000 ships are operating throughout the world.
In contrast, there are about 1.47 billion vehicles in the world as per 2023 estimates. That means for every vessel, there are about 15,000 cars. The kind of disparity makes it easier to understand why reducing the emissions from one ship can contribute significantly towards reduced greenhouse gas emissions. If we can improve on common shipping factors such as the quality of fuel, we can have a massive, long-term effect on climate change.
Sources of carbon emissions on ships
To understand how we can reduce emissions, we must first identify the different carbon emission sources on a ship. Let us take a look at some of these:
The main engine on ships is a massive machine that can consume hundreds of liters of fuel every hour. And since we use a highly carbon-intensive fuel onboard (Heavy Fuel Oil), it can add several tons of CO2 to the environment every single day.
A thick plume of dark smoke coming from the funnels of a large ferry while manoeuvring.
Auxiliary engines are the source of all electrical power onboard. They maintain a continuous flow of electricity to the ship’s equipment and systems besides the accommodation.
Auxiliary engines also use the same grade of carbon-intensive fuel as the main engines. Thus, by optimizing the auxiliary engine consumption, we can reduce the carbon emissions from ships.
Can they be neutralized?
While it seemed like a pipe dream a few years ago, significant investment into the renewable energy sector has produced some viable alternatives. To make a zero carbon emission ship, we have to eliminate the use of fuel oil from ships. We must use renewable energy to propel and power a ship. Many renewable energy concepts are being pursued to make them as reliable as IC engines. Let us take a look at some of these.
Types of Zero Carbon Emissions Ships
Hydrogen is the most abundant element on Earth. To make things better, one kg of hydrogen releases 4x the amount of energy in 1 kg of coal and almost 3x the amount of energy in a liter of gasoline. It’s also possible to extract hydrogen from seawater through an electrolysis process.
A hydrogen fuel cell ferry ship (concept).
This method produces electrical energy which can be used to propel vessels using electric motors as well as power all electrical equipment onboard. If pure hydrogen is used, the only byproduct is water as it has a clean energy source. There are no carbon dioxide emissions in this method which enables us to achieve net zero emissions from a ship. The FCS Alsterwasser, delivered in 2008, was the first passenger ship to use a hydrogen fuel cell to power itself.
Several shipping companies are pursuing the commercialization of hydrogen fuel cells to use them on ships. If successful, hydrogen power will be one of the most feasible replacements for IC engines.
For ships trading in areas with sunlight, solar energy can be used to propel and provide electrical power onboard. This has been in practice for many small-sized boats for several years now.
A large vessel does not have enough area for solar panels to generate propulsion power. Placing solar panels on the deck becomes becomes an option only when ships are not dependent on deck space such as car carriers.
MV Auriga leader with solar panels on the open deck. Credit: By Goodwillgames – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=54545313
However, the amount of power generated is sufficient to power some of the auxiliary machinery on such large vessels. In the case of MV Auriga, the installation of solar panels on the deck lead to a reduction of 13 tonnes in fuel usage and 40 tonnes in CO2 emissions. 
Thus, solar power by itself may not be enough to run a ship fully but it can be used in conjunction with other energy sources to advance towards carbon neutrality.
Wind power is also a formidable renewable energy source for propulsion emissions reductions. Studies are showing that the effective use of wind energy would reduce the lifetime emissions of a vessel by 30%. Thus, it is a very lucrative channel to cut down on fuel costs along with emissions. The fuel savings may go up to 50%.
The Scandlines hybrid ferry with its iconic Flettner rotor sail arriving Gedser port
Several different configurations and sail types are being enhanced and tested for efficiency. Some of the most popular ones are flexible sails, rigid sails, kite sails, and rotor sails. These designs are being developed by some of the biggest names in the industry such as NYK, STX, and Wallenius Wilhelmsen.
Research is also underway to use wave energy as a step towards making a ship carbon neutral. The setup would allow a ship to harness the power of waves and cut down its dependence on fossil fuels.
The Fraunhofer Center for Manufacturing Innovations has designed equipment that allows ships, whether moving or stationary, to produce electricity from the movement of waves. Presently, they intend to have small boats go out for about 20 hours, harness wave energy, store it onboard and supply it to the electrical grids at shore during peak demand hours.
But this system can also be commercialized for merchant vessels and reduce dependency on fossil fuel-based power generation systems.
What does the future look like?
Climate change is a harsh reality. If we want to limit global warming and rising sea levels, we have to work on our carbon footprint and greenhouse gases (GHG) emissions. While waning our dependency on burning fossil fuels for energy may have been impossible a few decades ago, new solutions are now emerging every year. These solutions will allow us to reach net zero sooner than originally assumed originally.
The future of shipping would involve a lot more use of renewable energy sources. It is likely to be a mix of different sources of energy as not all renewable energy sources perform reliably at all times as in the case of some ships today.
We already have electric and hybrid ships that combine renewable energy sources with IC engines to improve sustainability. The vessels use sources such as batteries, fuel cells, and renewable energy to run electric motors for propulsion. The vessels can switch between traditional and green propulsion as required.
It may likely take a few more decades to run on 100% renewable energy as it is a comparatively new and growing industry. Unique challenges appear at every junction as we move toward commercialization. But they are being solved one by one and the future of shipping is bright and zero carbon emission ships are definitely not inconceivable.