Planes Vs Ships: Who's The Bigger Polluter?

Transport is a major contributor to global warming, with aviation and shipping being the fastest-growing sources of greenhouse gas emissions. While both planes and ships emit large amounts of CO2, the relative difference in carbon footprint between the two modes of transport is significant. In this paragraph, we will explore the environmental impact of each and consider which is more polluting.

Cruise ships emit more CO2 per passenger kilometre than planes

Cruise ships emit more carbon dioxide per passenger kilometre than planes. According to the International Council on Clean Transportation (ICCT), the most efficient cruise ships emit an average of 250 grams of CO2 per passenger kilometre, while the industry average for planes ranges from approximately 10 to 130 grams of CO2 per passenger kilometre, with longer flights tending to have lower carbon intensity. This means that cruise ships have a higher carbon footprint per passenger than planes, even when the plane trips are shorter.

The high emissions from cruise ships are due in part to the use of scrubbers, which reroute pollution from the air into the water. Scrubber discharge water is contaminated with harmful substances such as polycyclic aromatic hydrocarbons and heavy metals, which have been linked to adverse health effects in marine life. Cruise ships use scrubbers to comply with fuel sulphur regulations while continuing to use cheaper high-sulphur fuels. However, scrubbers result in higher direct and life-cycle CO2 emissions, as well as higher emissions of climate-warming black carbon.

The European Union is taking steps to address the carbon pollution generated by ships. Starting in 2024, the EU will include maritime transport in its Emissions Trading Scheme, charging ships for their carbon pollution. This move is part of the EU's broader efforts to reduce emissions from both the aviation and maritime sectors, with a target of cutting the EU's emissions by 55% by 2030 and reaching net-zero emissions by 2050.

While flying is also a significant contributor to climate change, the relative difference in carbon footprint between flying and cruising may not outweigh the benefits of taking a cruise for some individuals. Cruising allows travellers to visit multiple destinations without the need for additional transportation once they reach their destination. Additionally, technological advancements are being made to support more sustainable cruising, such as the development of the world's first emissions-free cruise ship by 2030, which will utilise wind and battery power.

However, until cruise ships achieve carbon neutrality, they remain an extremely carbon-intensive mode of travel. To reduce their environmental impact, individuals can opt for more sustainable modes of transportation, such as trains, or choose holiday destinations that can be reached by less climate-intensive means.

Cruise ships pollute European ports with sulphur oxides and methane

Cruise ships are a major contributor to the environmental impact of shipping in Europe. In 2022, cruise ships emitted more sulphur oxides and methane in European ports than in 2019, with emissions increasing by 9% and 500%, respectively. This is due to an increase in the number of cruise ships, the time spent around ports, and fuel consumption, which has risen by around a quarter since 2019.

A recent study by Transport & Environment found that cruise ships emitted as much sulphur oxide in 2022 as one billion cars, with emissions reaching 509 tonnes. This is despite the International Maritime Organization's sulphur cap, which came into effect in 2020, reducing the maximum sulphur content in fuels from 3.5% to 0.5%. Italy was the most cruise-ship polluted country in Europe in 2022, followed by Spain. Barcelona was the most polluted port in Europe, with cruise ships emitting almost three times more sulphur oxide than all the cars in the city.

Cruise ships also emit high levels of methane, a potent greenhouse gas. LNG-powered ships, while better in terms of air pollution, have been linked to significant climate damage due to methane leaks from their engines. In addition, scrubbers, which are used to reduce emissions, have been found to result in higher direct and life-cycle CO2 emissions, as well as higher emissions of climate-warming black carbon.

To reduce the environmental impact of cruise ships, the industry should abandon the use of scrubbers, use marine gas oil, test low or zero-emission fuels, and invest in zero-emission technologies. Policy changes, such as the European Union's Emissions Trading Scheme, which will charge ships for their carbon pollution, are also important in incentivising the industry to reduce emissions.

Ships use scrubbers that redirect pollution from air to water

In response to stricter regulations on ship air emissions, many shipowners have installed exhaust gas cleaning systems, or "scrubbers", to reduce air pollution. These scrubbers allow ships to continue using cheaper high-sulphur heavy fuel oil instead of more expensive low-sulphur fuels. While scrubbers are effective at reducing air emissions of sulphur dioxide (SO2), they redirect the sulphur and other contaminants from the exhaust gas—including carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals—into the water. This washwater or discharge water is highly acidic and has been linked to cancers and reproductive dysfunction in marine mammals.

The use of scrubbers has been increasing, with over 4,300 ships globally installing them by 2020, up from just three ships in 2008. This rapid increase has led to growing environmental concerns, with studies showing that the private economic benefits of installing scrubbers come at the expense of significant marine environmental damage. The International Council on Clean Transportation (ICCT) estimates that using scrubbers results in higher direct and life-cycle CO2 emissions than using marine gas oil, as well as higher particulate matter emissions, including climate-warming black carbon.

To address these concerns, some nations have taken unilateral action to restrict or prohibit scrubber discharges, particularly in ports and territorial waters. The International Maritime Organization (IMO) has also updated its guidelines for scrubbers, setting stricter limits for open-loop scrubbers on acidity and the discharge of certain chemicals. However, the IMO's guidelines have been criticized for not being sufficiently strengthened and for ignoring the cumulative effects of multiple ships discharging in heavily trafficked areas.

As a result, there are increasing calls for the shipping industry to transition away from scrubbers and towards the use of cleaner fuels and zero-emission technologies. This includes proposals from the European Parliament to add maritime transport to the Emissions Trading Scheme and to promote more sustainable fuels for ships. While the industry works towards achieving the ambition of becoming carbon neutral, it is essential to balance the benefits of cruising with the environmental impact of this extremely carbon-intensive mode of travel.

Air travel is the fastest-growing contributor to climate change

Air travel is a significant contributor to climate change. In 1960, only 100 million passengers travelled by air, but by 2019, this number had skyrocketed to 4.56 billion. This exponential growth in air travel has resulted in a corresponding increase in greenhouse gas emissions.

Before the pandemic, passenger air travel was responsible for the highest and fastest growth of individual emissions, despite significant improvements in aircraft and flight operations efficiency over the last six decades. The aviation industry's emissions grew at a slower pace than the industry's expansion until recently, when increasing air traffic caused a surge in the sector's contribution to global warming.

Air travel produces heat-trapping pollution, which not only contributes to climate change but also poses challenges for the aviation industry itself. As the planet warms, coastal airports are at risk of flooding, and rising temperatures increase turbulence, affecting flight safety and passenger comfort.

The impact of aviation on the climate is not limited to carbon dioxide emissions. Non-CO2 effects, such as aircraft contrails, also play a significant role. According to a 2021 analysis, aviation's climate impact accounted for 3.5% of total anthropogenic warming in 2011 and likely remained at that level in 2018.

With the window to limit global warming narrowing, reducing aviation's carbon footprint is critical. While completely decarbonizing aircraft may require technologies that are decades away, reducing the number of flights is one surefire way to curb emissions in the short term.

Aviation accounts for 3.5% of effective radiative forcing

Aviation and shipping are two of the most polluting modes of transport. While both have been responsible for the largest increase in emissions in the transport sector, aviation accounts for a greater proportion of effective radiative forcing.

Effective radiative forcing (ERF) is a measure of the change in the balance between incoming and outgoing energy in the Earth's climate system due to human activities. It takes into account not only the direct effects of greenhouse gases but also the indirect effects, such as changes in cloud cover and the formation of contrails.

The contribution of global aviation to ERF is complex and involves a range of factors. Aviation emissions include carbon dioxide (CO2), nitrogen oxides (NOx), water vapour, soot, and sulfate aerosols. The formation of contrails and the resulting cirrus cloudiness contribute the largest positive net ERF term, followed by CO2 and NOx emissions. The emission of sulfate aerosols yields a negative ERF term, as they have a cooling effect.

According to the data presented, aviation accounts for 3.5% of effective radiative forcing. This highlights the significant impact of aviation on climate change, especially when considering the various factors that contribute to aviation's ERF.

To address these environmental concerns, there have been growing calls for more sustainable alternatives. For instance, the International Council on Clean Transportation (ICCT) has proposed the use of low-sulfur marine gas oil and the testing of zero-life-cycle emission fuels for ships. Similarly, advancements in technology have led to plans for emissions-free cruise ships that utilize wind and battery power.

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