Environmental impact of shipping
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The environmental impact of shipping include air pollution, water pollution, acoustic, and oil pollution.[1] Ships are responsible for more than 3% of greenhouse gas emissions and 18% of nitrogen oxides pollution.[2][3]
Although maritime transport is the most energy-efficient method to move a given mass of cargo a given distance, the sheer size of the industry means that it has a significant effect on the environment.[4] The annual increasing amount of shipping overwhelms gains in efficiency, such as from slow-steaming. The growth in tonne-kilometers of sea shipment has averaged 4 percent yearly since the 1990s,[5] and it has grown by a factor of 5 since the 1970s.[citation needed]
The fact that shipping enjoys substantial tax privileges has contributed to the growing emissions.[6][7][8]
Ballast water
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Ballast water discharges by ships can have a negative impact on the marine environment.[1] Cruise ships, large tankers, and bulk cargo carriers use a huge amount of ballast water, which is often taken on in the coastal waters in one region after ships discharge wastewater or unload cargo, and discharged at the next port of call, wherever more cargo is loaded.[9] Ballast water discharge typically contains a variety of biological materials, including plants, animals, viruses, and bacteria. These materials often include non-native, nuisance, invasive, exotic species that can cause extensive ecological and economic damage to aquatic ecosystems along with serious human health problems.
Sound pollution
[edit]Noise pollution caused by shipping and other human enterprises has increased in recent history.[10] The noise produced by ships can travel long distances, and marine species who may rely on sound for their orientation, communication, and feeding, can be harmed by this sound pollution.[11][12]
The Convention on the Conservation of Migratory Species has identified ocean noise as a potential threat to marine life.[13] The disruption of whales' ability to communicate with one another is an extreme threat and is affecting their ability to survive. According to a Discovery Channel article on Sonic Sea Journeys Deep into the Ocean over the last century, extremely loud noise from commercial ships, oil and gas exploration, naval sonar exercises and other sources has transformed the ocean's delicate acoustic habitat, challenging the ability of whales and other marine life to prosper and ultimately to survive. Whales are starting to react to this in ways that are life-threatening. Despite sonar's military and civilian applications, it is destroying marine life. According to IFAW Animal Rescue Program Director Katie Moore, "There's different ways that sounds can affect animals. There's that underlying ambient noise level that's rising, and rising, and rising that interferes with communication and their movement patterns. And then there's the more acute kind of traumatic impact of sound, that's causing physical damage or a really strong behavioral response. It's fight or flight".[14]
Wildlife collisions
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Marine mammals, such as whales and manatees, risk being struck by ships, causing injury and death.[1] For example, a collision with a ship traveling at only 15 knots has a 79% chance of being lethal to a whale.[15] Ship collisions may be one of the leading causes of population decline for whale sharks.[16]
One notable example of the impact of ship collisions is the endangered North Atlantic right whale, of which 400 or fewer remain.[17] The greatest danger to the North Atlantic right whale is injury sustained from ship strikes.[15] Between 1970 and 1999, 35.5% of recorded deaths were attributed to collisions.[18] From 1999 to 2003, incidents of mortality and serious injury attributed to ship strikes averaged one per year. From 2004 to 2006, that number increased to 2.6.[19] Deaths from collisions has become an extinction threat.[20] The United States' National Marine Fisheries Service (NMFS) and National Oceanic and Atmospheric Administration (NOAA) introduced vessel speed restrictions to reduce ship collisions with North Atlantic right whales in 2008, which expired in 2013.[21] However, in 2017 an unprecedented mortality event occurred, resulting in the deaths of 17 North Atlantic right whales caused primarily from ship-strikes and entanglement in fishing gear.[17]
Atmospheric pollution
[edit]Exhaust gases from ships are a significant source of air pollution, both for conventional pollutants and greenhouse gases.[1]
Conventional pollutants
[edit]Air pollution from ships is generated by diesel engines that burn high sulfur content fuel oil, also known as bunker oil, producing sulfur dioxide, nitrogen oxide and particulate, in addition to carbon monoxide, carbon dioxide, and hydrocarbons which again leads to the formation of aerosols and secondary chemicals reactions including formations of HCHO[22] and ozone in the atmosphere.[1] Diesel exhaust has been classified by the U.S. Environmental Protection Agency (EPA) as a likely human carcinogen. The agency recognizes that these emissions from marine diesel engines contribute to ozone and carbon monoxide nonattainment (i.e., failure to meet air quality standards), as well as adverse health effects associated with ambient concentrations of particulate matter and visibility, haze, acid deposition, and eutrophication and nitrification of water.[23] EPA estimates that large marine diesel engines accounted for about 1.6 percent of mobile source nitrogen oxide emissions and 2.8 percent of mobile source particulate emissions in the United States in 2000. Contributions of marine diesel engines can be higher on a port-specific basis. Ultra-low-sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents. As of 2006, almost all of the petroleum-based diesel fuel available in Europe and North America is of a ULSD type. However, bunker oil is still available, and large marine engines are able to switch between the two types simply by opening and closing the respective valves from two different on-board fuel tanks.
In 2016, the IMO adopted new sulfur-emissions regulations for implementation by larger ships beginning in January 2020.[24][25][26]
Of total global air emissions, marine shipping accounts for 18 to 30 percent of the nitrogen oxides and 9% of the sulfur oxides.[3][27] Sulfur in the air creates acid rain which damages crops and buildings. When inhaled, sulfur is known to cause respiratory problems and even increases the risk of a heart attack.[28] According to Irene Blooming, a spokeswoman for the European environmental coalition Seas at Risk, the fuel used in oil tankers and container ships is high in sulfur and cheaper to buy compared to the fuel used for domestic land use. "A ship lets out around 50 times more sulfur than a lorry per tonne of cargo carried."[28]
Cities in the United States like Long Beach, Los Angeles, Houston, Galveston, and Pittsburgh see some of the heaviest shipping traffic, which has left local officials desperately trying to clean up the air.[29] Increasing trade between the United States and China is helping to increase the number of vessels navigating the Pacific and is exacerbating multiple environmental problems. To maintain the level of growth China is experiencing, large amounts of grain are being shipped to China. The numbers of shipments are expected to continue increasing.[30]
In contrast to sulfur emissions (which depend on the fuel used), nitrous oxide emissions are primarily a function of combustion temperature. As air contains over 70% nitrogen by volume, some of it will react with oxygen during combustion. Given that those reactions are endothermic, a higher amount of nitrous oxides will be produced at higher combustion temperatures. However, other pollutants, particularly unburned or partially burnt hydrocarbons (also known as hyperfine particulates or soot), will be more common at lower combustion temperatures, so there is a trade-off between nitrogen oxides and soot.
Other than replacing ambient air with pure oxygen or some other oxidizing agent, the only ways to significantly reduce the nitrogen oxide emissions are via passing flue gasses through a catalytic converter and/or diesel exhaust fluid treatment, whereby an aqueous solution of urea reacts with the nitrous oxides in the flue gas to produce nitrogen, carbon dioxide and water. However, both those options add cost and weight. Furthermore, the urea in diesel exhaust fluid is usually derived from fossil fuels, and therefore it is not carbon neutral.
A third option entails the use of wet scrubbers that essentially spray seawater through the exhaust column as it is pumped through a chamber. Depending on the detailed engineering-design attributes of the wet scrubber, these devices can wash out the sulfur oxides, soot and nitrogen oxides from the engine exhaust, thus leaving a sludge that contains soot and various acidic compounds (or neutralized compounds, if alkaline substances are mixed in with the scrubbing liquid beforehand).[31] This material can then be either treated via an on-board device (closed-loop system), or it can simply be dumped overboard (open-loop system). The discharged material has been shown to harm marine life, especially in nearshore settings.[32]
In a recent study, the future of ship emissions has been investigated and reported that the growth of carbon dioxide emissions do not change with most common alternatives such as ultra-low-sulfur diesel (ULSD) or liquefied natural gas (LNG) as well as growing volume of methane emission due to methane slip through the LNG supply-chain.[33] Methane is a much more powerful greenhouse gas than carbon dioxide per unit volume, and is only slowly broken down in the environment by various chemical, photochemical and biological processes.
In inland-waters-based applications where sulfur cannot (fully) be removed from the fuel before combustion (desulfurization), flue gas scrubbing is commonly employed. However, this would add weight and cost on ships and produce a further waste stream (usually calcium sulfate if flue gases are scrubbed by being passed through calcium hydroxide solution) which would have to be disposed of, adding yet further cost. In addition, calcium hydroxide commonly being produced by calcination of calcium carbonate releases yet more carbon dioxide into the atmosphere. While this stream is comparatively small in relation to carbon-dioxide emissions caused by combustion of fossil fuels, it needs to be taken into account as well, as part of a complete life-cycle assessment.[citation needed]
Localized air pollution
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One source of environmental stresses on maritime vessels recently has come from states and localities, as they assess the contribution of commercial marine vessels to regional air quality problems when ships are docked at port.[34] For instance, large marine diesel engines are believed to contribute 7 percent of mobile source nitrogen oxide emissions in Baton Rouge and New Orleans, Louisiana. Ships can also have a significant impact in areas without large commercial ports: they contribute about 37 percent of total area nitrogen oxide emissions in the Santa Barbara, California area, and that percentage is expected to increase in future years[35].[23] Again, there is little cruise-industry specific data on this issue. They comprise only a small fraction of the world shipping fleet, but cruise ship emissions may exert significant impacts on a local scale in specific coastal areas that are visited repeatedly. Shipboard incinerators also burn large volumes of garbage, plastics, and other waste, producing ash that must be disposed of. Incinerators may release toxic emissions as well.
In 2005, MARPOL Annex VI came into force to combat this problem. As such cruise ships now employ CCTV monitoring on the smokestacks as well as recorded measuring via opacity meter while some are also using clean burning gas turbines for electrical loads and propulsion in sensitive areas.
Greenhouse gas emissions
[edit]Maritime transport accounts for about 3% of all greenhouse gas emissions, primarily carbon dioxide.[36] According to the World Bank, in 2022, the shipping industry's 3% of global greenhouse gas emissions make it "the sixth largest greenhouse gas emitter worldwide, ranking between Japan and Germany."[37][38][39]
| Year | Tankers | Dry bulk and general cargo | Container | Other |
|---|---|---|---|---|
| 2012 | 25.04% | 28.57% | 27.80% | 18.59% |
| 2013 | 24.61% | 28.77% | 27.47% | 19.15% |
| 2014 | 24.50% | 28.87% | 27.18% | 19.45% |
| 2015 | 25.03% | 28.42% | 26.99% | 19.56% |
| 2016 | 25.31% | 28.33% | 26.83% | 19.53% |
| 2017 | 25.62% | 28.06% | 26.91% | 19.41% |
| 2018 | 25.76% | 27.42% | 27.09% | 19.73% |
| 2019 | 26.41% | 27.22% | 25.84% | 20.53% |
| 2020 | 27.38% | 28.13% | 25.35% | 19.14% |
| 2021 | 26.71% | 28.28% | 26.13% | 18.88% |
| 2022 | 27.28% | 27.56% | 25.35% | 19.81% |
| 2023 | 28.55% | 27.52% | 24.03% | 19.90% |
| The group "other" includes vehicles and roll-on/roll-off ships, passenger ships, offshore ships and service and miscellaneous ships. | ||||
Decarbonization of shipping
[edit]The decarbonization of shipping is an ongoing goal to reduce greenhouse gas emissions from shipping to net-zero by or around 2050, championed by the International Maritime Organization (IMO).[41] As of 2025 maritime transport emits 3% of global greenhouse gas (GHG) but that could rise to 10% by 2050.[42] The IMO has an initial strategy that includes lowering, or limiting, the combustion of fossil fuels for power and propulsion to limit emission of carbon dioxide (CO2); development of alternative fuels such as green ammonia, hydrogen, and biofuels to reduce reliance on fossil fuels, and adopting digital technologies to increase vessel efficiency; and reducing emissions by 40% by 2030.[43][44] However in 2025 proposed IMO regulations aiming to reduce the GHG intensity of ship fuel, and the world's first global mandatory charge on GHG emissions (the IMO Net-Zero Framework), were blocked by the United States and Saudi Arabia.[45]
There is a trend in shipping to transition to Liquid Natural Gas (LNG) with claims of associated GHG reductions.[46][47] Environmental groups (e.g., Stand.earth and the Clean Arctic Alliance[48]) and some climate scientists warn that LNG could be a "climate shipwreck."[49] The biggest concern is unburned methane escaping into the atmosphere referred to as methane slip.[50] One study[49] by Stand.earth states Methane is roughly 80 times more potent than CO2 as a greenhouse gas over a 20-year period.
Ammonia is an option for shippers, and when produced with renewable energy, it can reduce carbon dioxide emissions. The shipping sector makes up 3 percent of global carbon dioxide emissions. Ammonia produced using sustainable fuels is more expensive than LNG, but as the cost of producing clean energy falls, it is becoming more affordable. The International Maritime Organization wants the sector to be net-zero by 2050. [51]
Oil spills
[edit]Oil spills are the most commonly known form of environmental pollution by ships.[52]
Types and causes of oil spills
[edit]The economically important shipping industry[53][54][55][56] contributes heavily to the oceans' oil pollution.[52][55][56] The marine transportation of oil is particularly risky.[57][58][59][55] Soto-Onate & Caballero (2017), therefore, regard oil spills as a significant negative externality of the economy.[57] Oil spills can be categorized as accidental or intentional.[60][61][56]
Accidental oil spills
[edit]Accidental spills are the result of e.g. ship collisions, fires or groundings.[62][63][61][56][55][58][52][60] Various partly interrelated factors are underlying causes of these accidents. These include human factors such as discipline, and competence of the crew,[64][60][56][52][58] management by e.g. the shipping company,[58] ship factors such as technical installations,[58][52] and environmental factors like difficult to navigate areas.[58]
Intentional oil spills
[edit]Intentional spills can be operational.[61][62][60] Although less studied and publicized, they make up for more marine pollution.[57][56] They usually involve not properly managing oily residues or ballast and bilge water by e.g. illegally cleaning the ship's tanks at sea.[65][61][62][55][56][60] It is regarded as a way for ship operators to cut costs by not complying with international conventions.[60][56][52] Moreover, it is common to also classify minor spills caused by human error and negligence as intentional.[52][60]
Spatial distribution and frequency of oil spills
[edit]Oil spills are not restricted to certain regions but can occur globally.[66][67][63] Nevertheless, studies have shown that oil spill density is positively correlated with shipping density, i.e. spills more often occur where maritime traffic is intense – along major shipping lanes, coastlines, and close to ports or oil infrastructure.[60][56][59] This is related to a higher risk of accidents.[56] Intentional spills are, moreover, likely to occur in 'clusters'[60] and beyond national jurisdiction to avoid detection or because of inadequate reception facilities in ports. Lastly, zones characterized by war or political instability experience more oil spills.[67][56]
While scholars highlight the decrease of oil spills over the last decades,[61][67][55][57][56][58][59][60][63][64] oil pollution from shipping remains an environmental risk.[61][55][57][58][59][54] In the past decades several major accidental spills like the grounding of Exxon Valdez (1989), which despite considerable rescue efforts substantially harmed the marine environment off of Alaska,[68] or the Sanchi oil spill (2018) – "the most serious and most polluting oil tanker accident [in] the 21st century"[58] – occurred.[63] Concerning the number of intentional oil spills, a high number of unknown cases is expected.[60] It is estimated that intentional oil spills make up for 45%, while accidents only contribute to 8% of marine oil pollution.[56]
Currently, various trends could increase the risk of oil spills again. Among these are the growing oil trade and bigger tankers.[53][55][59][56] Even though not studied conclusively,[69] climate change might, moreover, make accidental spills more likely because of more intense and frequent storms at sea and melting ice.[70][67][53][54] Lastly, the growing problem of shadow fleets transporting oil on sub-standard, old, and anonymously owned vessels without proper safety, insurance and compliance standards, and engaging in dangerous ship-to-ship transfers exacerbate the risk of oil spills.[71][72][53][70]
Impact of oil spills
[edit]Oil spills are regarded as devastating and irreversible for marine ecosystems and biodiversity.[55][58][67][57] Polycyclic aromatic hydrocarbons (PAHs), which are in crude oil, are toxic for marine life. The difficult to clean PAHs can remain in the marine environment and sediment for years.[68] PAHs can hinder the marine life's development, reproduction, and resistance to diseases.[68] Affected are, among others, fish, seabirds, mammals, invertebrate communities, and reefs.[70][52]