The environmental effects of shipping include air pollution, water pollution, noise pollution, and oil pollution. Ships are responsible for more than 18% of nitrogen oxide pollution and 3% of greenhouse gas emissions.

Although maritime transport is the most energy-efficient way to move large amounts of cargo over long distances, the large scale of the industry means it has a major impact on the environment. The growing amount of shipping each year reduces the benefits of improvements in efficiency, such as slower ship speeds. The amount of cargo moved by sea has increased by an average of 4% each year since the 1990s and has grown five times larger since the 1970s.

The fact that shipping receives many tax benefits has helped increase emissions over time.

Ballast water

Ships releasing ballast water can harm the ocean environment. Cruise ships, large tankers, and ships that carry large amounts of goods use large amounts of ballast water. This water is usually taken in from coastal areas in one location after ships release wastewater or unload goods, and then released at the next port where more cargo is loaded. Ballast water often includes a variety of living things, such as plants, animals, viruses, and bacteria. These materials may contain non-native, harmful species that can cause major harm to ocean ecosystems and lead to health issues for people.

Sound pollution

Noise pollution from ships and other human activities has become more common in recent years. The sounds made by ships can travel far across the ocean. Marine animals that use sound for navigation, communication, and finding food can be harmed by this noise.

The Convention on the Conservation of Migratory Species has recognized ocean noise as a possible danger to marine life. Disrupting whales' ability to communicate with each other is a serious problem that threatens their survival. A Discovery Channel article titled Sonic Sea Journeys: Deep into the Ocean reported that very loud noises from commercial ships, oil and gas exploration, naval sonar exercises, and other sources have changed the ocean's quiet sound environment. This change makes it harder for whales and other sea creatures to live and survive. Whales are beginning to react to these noises in ways that can be dangerous to them.

Although sonar is used by both military and civilian groups, it is harming marine life. Katie Moore, Director of the IFAW Animal Rescue Program, explained, "There are different ways that sounds can affect animals. There is a constant background noise level that keeps increasing, which interferes with how animals communicate and move. There is also a sudden, strong impact from loud sounds that can cause physical harm or force animals to flee in fear."

Wildlife collisions

Marine mammals, such as whales and manatees, are in danger of being hit by ships, which can cause injury or death. For example, a ship traveling at 15 knots has a 79% chance of killing a whale. Ship collisions may be a major reason for the decline in numbers of whale sharks.

A well-known example is the endangered North Atlantic right whale, of which 400 or fewer remain. The biggest threat to these whales is injury from being hit by ships. From 1970 to 1999, 35.5% of recorded deaths were caused by ship collisions. From 1999 to 2003, an average of one case of death or serious injury from ship strikes occurred each year. From 2004 to 2006, this number rose to 2.6 cases per year. Deaths from collisions have become a threat to the survival of the species. In 2008, the United States' National Marine Fisheries Service (NMFS) and National Oceanic and Atmospheric Administration (NOAA) set rules to slow down ships near North Atlantic right whales, but these rules ended in 2013. In 2017, a rare event happened, causing the deaths of 17 North Atlantic right whales, mainly from ship strikes and being caught in fishing nets.

Atmospheric pollution

Exhaust gases from ships are a major source of air pollution, including both common pollutants and greenhouse gases.

Air pollution from ships comes from diesel engines that burn fuel oil with high sulfur content, called bunker oil. This process produces sulfur dioxide, nitrogen oxide, and tiny particles, along with carbon monoxide, carbon dioxide, and hydrocarbons. These substances can form aerosols and other chemicals in the air, such as HCHO and ozone. The U.S. Environmental Protection Agency (EPA) has classified diesel exhaust as a likely cause of cancer in humans. The EPA also notes that emissions from marine diesel engines contribute to poor air quality, including high levels of ozone and carbon monoxide, as well as health issues linked to tiny particles in the air, reduced visibility, acid rain, and changes in water quality. In 2000, large marine diesel engines in the United States were responsible for about 1.6% of mobile source nitrogen oxide emissions and 2.8% of mobile source particulate emissions. In some ports, these contributions may be higher. Ultra-low-sulfur diesel (ULSD) is a type of fuel with much less sulfur. By 2006, most diesel fuel in Europe and North America was ULSD, but bunker oil is still used. Large marine engines can switch between fuel types by opening or closing valves on different fuel tanks.

In 2016, the International Maritime Organization (IMO) introduced new rules to reduce sulfur emissions from large ships, which took effect in 2020.

Marine shipping contributes 18 to 30% of global nitrogen oxide emissions and 9% of sulfur oxide emissions. Sulfur in the air can cause acid rain, which harms crops and buildings. Breathing in sulfur can lead to breathing problems and increase the risk of heart attacks. Irene Blooming, a spokesperson for the European environmental group Seas at Risk, explains that fuel used in oil tankers and container ships has high sulfur content and is cheaper than fuel used for land vehicles. "A ship releases about 50 times more sulfur than a truck for every ton of cargo it carries," she says.

In cities like Long Beach, Los Angeles, Houston, Galveston, and Pittsburgh in the United States, heavy shipping traffic has led to serious air quality issues. Increased trade between the United States and China has raised the number of ships traveling across the Pacific, worsening environmental problems. To support China’s economic growth, large amounts of grain are being shipped to the country, and this trend is expected to continue.

Nitrogen oxide emissions depend mainly on the temperature of the engine’s combustion process. Since air is mostly nitrogen, some of it reacts with oxygen during burning. Higher combustion temperatures produce more nitrogen oxides, but lower temperatures result in more unburned or partially burned hydrocarbons (soot). This creates a balance between nitrogen oxides and soot.

To reduce nitrogen oxide emissions, exhaust gases can be passed through a catalytic converter or treated with diesel exhaust fluid, which uses a urea solution to convert nitrogen oxides into nitrogen, carbon dioxide, and water. However, these methods add cost and weight to ships. The urea used in diesel exhaust fluid often comes from fossil fuels, so it is not carbon neutral.

Another method uses wet scrubbers, which spray seawater through the exhaust system to remove sulfur oxides, soot, and nitrogen oxides. This process leaves behind a sludge containing soot and acidic compounds. This sludge can be treated on the ship or dumped into the ocean, but it can harm marine life, especially near the shore.

A recent study found that using alternatives like ultra-low-sulfur diesel or liquefied natural gas (LNG) does not significantly reduce carbon dioxide emissions. However, LNG can release methane, a powerful greenhouse gas, during its use. Methane breaks down slowly in the environment.

In some inland waterways, flue gas scrubbing is used when sulfur cannot be fully removed from fuel before burning. This process adds weight and cost to ships and creates waste, such as calcium sulfate, which must be disposed of. Producing the materials used in scrubbing, like calcium hydroxide, also releases carbon dioxide.

Local governments are increasingly addressing the impact of ships on air quality when they are docked. For example, in Baton Rouge and New Orleans, Louisiana, large marine diesel engines are thought to contribute 7% of nitrogen oxide emissions. In Santa Barbara, California, ships account for about 37% of nitrogen oxide emissions, and this number is expected to rise to 61% by 2015. Cruise ships, though a small part of the global shipping fleet, can have a large local impact in coastal areas they visit repeatedly. Shipboard incinerators also burn waste, producing ash and potentially harmful emissions.

In 2005, the MARPOL Annex VI regulations were introduced to reduce pollution from ships. Cruise ships now use cameras and measuring devices to monitor emissions and sometimes use clean-burning gas turbines in sensitive areas.

Maritime transport is responsible for about 3% of all greenhouse gas emissions, mostly carbon dioxide. According to the World Bank, in 2022, the shipping industry was the sixth-largest source of global greenhouse gas emissions, between Japan and Germany.

Reducing carbon emissions from shipping is a key goal. The industry aims to cut emissions to net-zero by around 2050.

Oil spills

Oil spills are the most well-known type of pollution caused by ships. The shipping industry, which is important for the economy, contributes a lot to oil pollution in the oceans. Transporting oil by sea is very risky. Soto-Onate & Caballero (2017) say that oil spills are a major harmful effect on the environment that is not directly paid for by the companies causing them. Oil spills can be either accidental or intentional.

Accidental spills happen because of things like ship collisions, fires, or running aground. Many factors can cause these accidents. These include human issues, such as how well the crew is trained or how the shipping company manages the ship. Ship-related issues, like the condition of the ship’s equipment, and environmental challenges, such as hard-to-navigate areas, also play a role.

Intentional spills often happen during normal operations. These are not studied or reported as much as accidental spills, but they cause more pollution. They usually involve not properly handling oily waste or water, such as illegally cleaning a ship’s tanks at sea. This helps ship operators save money by not following international rules. It is also common to classify small spills caused by human mistakes as intentional.

Oil spills can happen anywhere in the world. However, studies show that oil spills are more common in areas with heavy shipping traffic, such as major shipping routes, coastlines, and near ports or oil facilities. This is because there is a higher chance of accidents in these places. Intentional spills often happen in groups and may occur in areas outside national control to avoid being noticed or because ports lack proper facilities to handle waste. Areas with war or political problems also experience more oil spills.

Even though the number of oil spills has decreased over the years, oil pollution from ships is still a risk to the environment. Major accidental spills, like the Exxon Valdez (1989) and the Sanchi oil spill (2018), have caused serious harm to marine life. It is estimated that intentional oil spills make up about 45% of marine oil pollution, while accidents account for about 8%.

Several trends could increase the risk of oil spills again. These include more oil being transported and the use of larger tankers. Climate change may also lead to more accidents because of stronger and more frequent storms and melting ice. Additionally, the use of "shadow fleets" — ships that are old, poorly maintained, and owned anonymously — and unsafe transfers of oil between ships increase the risk of spills.

Oil spills are very harmful to marine ecosystems and wildlife. Crude oil contains substances called polycyclic aromatic hydrocarbons (PAHs), which are toxic to marine life. These substances are hard to clean and can stay in the ocean and sediment for many years. PAHs can harm the development, reproduction, and ability of marine life to fight diseases. Animals such as fish, seabirds, mammals, invertebrates, and coral reefs are affected. The impact of a spill depends on factors like the type of ship, the amount of oil spilled, the location, weather, and the time of year. Measuring the exact effects of oil spills and the long-term consequences, especially for intentional spills, is difficult.

Oil spills are managed by a system with many levels of organizations. This system has evolved over time in three phases. The first phase (1950s–1970s) started with the creation of the International Convention for the Prevention of Pollution at Sea by Oil (OILPOL54), which introduced rules and measures to prevent pollution. The second phase (1980s–1990s) improved these rules, and the third phase (21st century) added new ones. Major oil spills, like the Torrey Canyon (1967), Exxon Valdez (1989), Erika (1999), and Prestige (2002), led to changes in the system.

Oil spill management involves many groups. At the international level, the United Nations agencies International Maritime Organization (IMO) and International Labor Organization (ILO) play important roles. The IMO, for example, helps create rules like the MARPOL 73/78 convention and provides guidance on responding to oil spills.

At the national and regional levels, governments and organizations also contribute. Countries like the United States and the European Union have pushed for stronger international rules. Governments are responsible for enforcing international laws within their own countries. Specific duties apply to the countries that register ships (flag states), the countries where ships stop (port states), and the countries along coastlines (coastal states).

Regional groups, such as the European Maritime Safety Agency (EMSA), the Helsinki Commission (HELCOM), and the Northwest Pacific Action Plan Marine Environment Emergency Preparedness and Response Regional Activity Centre (NOWPAP MERRAC), help with monitoring, training, and emergency planning.

Non-government groups, like the shipping industry, ports, and organizations such as Sea Alarm and the World Wildlife Fund (WWF), also help manage oil spills. They contribute through research, regulations, and monitoring efforts.

Many rules and measures exist to prevent and manage oil spills. These include laws, technical solutions, and systems that encourage or require certain actions. International agreements like OILPOL54 and MARPOL 73/78 are key. Other agreements, such as the International Convention for the Safety of Life at Sea (SOLAS74) and the International Convention on Maritime Search and Rescue (SAR), also help prevent spills.

Wastewater

Blackwater is wastewater from toilets and medical areas that may include harmful germs, viruses, parasites, and nutrients. If not properly treated, this waste can pollute fish and shellfish habitats, risking public health. Nutrients like nitrogen and phosphorus in sewage can cause too much algae to grow, which uses oxygen in the water and harms fish and other sea life.

Greywater is wastewater from sinks, showers, kitchens, laundry, and cleaning on ships. It may contain bacteria, detergents, oil, grease, metals, chemicals, nutrients, food waste, and medical waste. Tests by the EPA and Alaska found that untreated greywater from cruise ships can have high levels of bacteria, sometimes higher than untreated household waste. Greywater can harm the environment because of its high nutrient and oxygen-demanding content. It is the largest type of liquid waste from cruise ships, making up 90 to 95 percent of total waste. Each person on a cruise ship produces about 110 to 320 liters of greywater daily, meaning a 3,000-person ship creates 330,000 to 960,000 liters daily.

A large cruise ship with 3,000 passengers and crew produces about 55,000 to 110,000 liters of blackwater waste daily. The cruise industry releases 970,000 liters of greywater and 110,000 liters of blackwater into the ocean each day.

MARPOL Annex IV, effective since September 2003, limits the discharge of untreated sewage to ships traveling at least 4 knots and more than 12 nautical miles from land, ensuring no visible solids or color changes in the water. Some regions have stricter rules.

Modern cruise ships often use special treatment systems, like membrane bioreactors (e.g., G&O, Zenon, or Rochem), to clean blackwater and greywater. These systems produce very clean water that can be reused in ship machinery.

Solid waste

Solid waste on ships includes items such as glass, paper, cardboard, aluminum and steel cans, and plastics. This waste can be non-hazardous or hazardous. When solid waste enters the ocean, it becomes marine debris and can harm marine life, humans, coastal communities, and industries that rely on ocean waters. Cruise ships manage solid waste through methods like reducing waste at the source, minimizing waste, and recycling. However, about 75% of solid waste on cruise ships is burned on board, and the ash is often released into the ocean, though some is taken ashore for disposal or recycling. Marine animals such as whales, fish, sea turtles, and birds can be injured or killed by entanglement with plastics or other waste from cruise ships. On average, each passenger on a cruise ship produces at least two pounds of non-hazardous solid waste daily. Large cruise ships carry thousands of passengers, so the amount of waste generated each day is very large. For a large cruise ship, about 8 tons of solid waste are created during a one-week trip. It is estimated that 24% of all solid waste from ships worldwide comes from cruise ships. Most cruise ship waste is processed on board (burned, pulped, or ground) before being released into the ocean. When waste must be removed from the ship (such as glass or aluminum that cannot be burned), cruise ships may overwhelm port facilities, which are often not prepared to handle large passenger vessels.

Plastic pollution is a major environmental problem, with over 75% of marine debris made of plastic. About 20% of global plastic pollution comes from ocean sources, and shipping is a major cause. Plastic pollution from shipping is divided into two types: direct discharge and indirect pollution. Direct discharge includes illegal dumping of plastic waste into the ocean. Indirect pollution includes lost cargo and improper waste management, such as discarding food packaging and other plastic items into the ocean. Containers can be lost due to extreme weather, human error, or poor securing practices, leading to large amounts of debris in the ocean. Some ships break international rules by directly dumping plastic into the sea. Improper waste management during normal operations causes plastic items to be thrown into the ocean.

Plastic pollution in the ocean is categorized by size: macroplastic (larger than 20 mm), mesoplastic (5–20 mm), microplastic (smaller than 5 mm), and nanoplastic (smaller than 1000 nm). These pollutants harm the environment and marine life. Large plastics, such as lost cargo, can entangle or be eaten by marine animals, harming their movement, breathing, and feeding. Over time, larger plastic debris breaks into smaller pieces, becoming microplastics that spread throughout the ocean. Microplastics are also found in cruise ship wastewater from laundry and cosmetics. These microplastics absorb harmful chemicals and carry them through the ocean. By 2014, over 5 trillion plastic particles were floating on the ocean’s surface, weighing nearly 270,000 tons. When marine animals eat these particles, the toxins can build up in their bodies, harming ocean ecosystems and possibly affecting human health.

Efforts to reduce plastic pollution from shipping involve cooperation between international groups, governments, businesses, and the public. However, current solutions face challenges. Key policies include the International Convention for the Prevention of Pollution from Ships (MARPOL), which bans the direct dumping of plastic waste into the ocean. The United Nations Agenda 21 also highlights the need to control shipping pollution. Many countries monitor marine litter to collect data on floating debris, seafloor waste, and plastic in marine life. These programs help create better policies. Regional efforts include the Paris Memorandum of Understanding (MoU), which uses port inspections to enforce environmental rules. The EU’s Marine Strategy Framework Directive (MSFD) requires reducing ship-based litter and supports ocean protection efforts. In the Arctic, the Arctic Monitoring and Assessment Program (AMAP) focuses on microplastic pollution.

Although MARPOL requires ports to provide waste disposal services, many ports need improvements to better handle plastic waste recycling. The MSFD also struggles to monitor small and medium-sized plastic pollution, and more research is needed on how plastic affects marine life. Improving plastic product designs, such as creating biodegradable materials, could help reduce environmental harm.

Bilge water

On a ship, oil can leak from engine and machinery areas or during engine maintenance and mix with water in the bilge, which is the lowest part of the ship's hull. Although bilge water is filtered and cleaned before being released, even small amounts of oil can harm fish or cause long-term health problems. Bilge water may also contain solid waste and pollutants that use up oxygen in water, along with oil and other chemicals. A large cruise ship typically produces about 8 tonnes of oily bilge water every 24 hours of operation. To keep the ship stable and prevent dangerous oil vapors in these areas, the bilge spaces must be cleaned and emptied regularly. Before the bilge can be emptied and the water released, oil must be removed from the bilge water. The removed oil can then be reused, burned, or taken to a port for disposal. If the oil separator, which is used to remove oil, is broken or intentionally bypassed, oily bilge water could be released directly into the ocean, harming marine life.

Some shipping companies, including large cruise lines, have sometimes broken rules by illegally bypassing the onboard oil separator and releasing untreated oily wastewater. In the United States, these violations, often using a tool called a "magic pipe," have led to legal action and heavy fines. However, in other countries, enforcement of these rules has been inconsistent.

International regulation

Major international agreements, such as the Marine Pollution Treaty in Honolulu and the UN Convention on the Law of the Sea, address issues like pollution from ships and the protection of marine life. From the 1950s to the 1980s, maritime rules were mostly decided by the International Maritime Organization (IMO), a global group. However, after the 1980s, the European Union (EU) and its member countries began taking a bigger role in setting maritime rules, partly because people were unhappy with how poorly the IMO enforced its own rules. This led to a new type of cooperation between the EU, the IMO, and other groups, called "polycentric governance." The EU has been especially active in shaping and supporting IMO rules. Four key examples of this include: creating special areas under IMO rules, adopting the Paris Memorandum of Understanding (MoU) on Port State Control, developing EU shipping policies, and using market-based initiatives by ports and cargo owners.

Although many rules have been created over time, many current rules are seen as not strong enough. These rules often focus on technical details, like safety and pollution control, but do not address deeper problems, such as why some ships do not follow rules, the lack of rewards for following rules, or the difficulty of enforcing rules. When major groups and agreements work well together, it helps create effective environmental rules. However, when major groups and agreements have conflicts, it creates problems like unclear rules, poor enforcement, and areas of the ocean where rules are not followed.

To make international shipping rules work better, countries need to work together more. If one country acts alone to reduce emissions, it might not help the environment overall. But if all countries agree on the same rules, emissions can decrease. However, even with the same rules, different ports may still face challenges because of local and industry-specific needs. These needs might require special solutions. The success of uniform rules also depends on systems that monitor, report, and check if rules are followed. These systems are not always used well, and better supervision and a global monitoring system are needed. Common problems include mistakes in paperwork and lack of information about cargo. For example, cruise ships are not regulated under the U.S. system that requires pollution control. In the Caribbean, many ports lack proper waste disposal, leading to waste being dumped at sea.

Because shipping is complex and hard to regulate, it is unlikely that a single, widely accepted plan to reduce greenhouse gas (GHG) emissions will be created soon. Efforts to agree on rules, like taxing shipping fuels, have failed to reach a solution. Conflicts between groups, like the United Nations Framework Convention on Climate Change (UNFCCC) and the IMO, also create problems. The UNFCCC believes developed countries should take more responsibility for reducing emissions, while the IMO believes all ships should be treated equally, regardless of where they are from. This disagreement makes it hard to create clear rules.

A 2016 study suggests that states, the shipping industry, and global groups should explore ways to reduce GHG emissions using economic incentives. These methods, called market-based mechanisms (MBMs), encourage better environmental practices by offering rewards. Examples include systems where companies can buy and sell pollution permits or pay taxes on fuel. These systems give companies a financial reason to improve energy efficiency. However, they can also reduce short-term profits. Some believe the EU’s system for reducing emissions could help the shipping industry without causing too much harm. Challenges include fairly distributing emissions, preventing pollution from moving to other areas, and handling the wide variety of ships. Other ideas include rewarding ships that use advanced technology to reduce emissions. However, these methods are not widely used and do not always focus on reducing GHG emissions.

Some people criticize these approaches. Lars Stemmler argues that focusing only on making shipping more efficient does not solve the bigger problems of climate change. Jason Monios says the shipping industry often follows a "business-as-usual" approach, assuming growth will continue and only solving small, manageable problems. However, climate change could cause major, unpredictable problems like hunger, war, and disease, requiring more radical changes. While some in the shipping industry talk about sustainability, their actions still mostly follow the old "business-as-usual" model, making it harder to create new rules and causing people to lose trust in the system. Finally, when MBMs are the main way to address these issues, they may not solve all the problems.

Issues by region

In 2025, about 80% of goods are moved across oceans. East Asia, especially, helps the global economy by sending 42% of all goods exported worldwide. The high amount of shipping in Asian ports has caused environmental problems. The large number of people living near Asia’s coasts also makes it harder to solve issues like air pollution, water pollution, and noise pollution.

In 2021, 430 million people lived near major Chinese port cities, such as Shanghai, Shenzhen, Tianjin, and Guangzhou. Higher population numbers in coastal areas mean pollution in these places needs special attention to protect global health. These areas release harmful gases like carbon dioxide, sulfur dioxide, and nitrogen oxides into the air. Problems like ocean acidification and eutrophication also affect the environment and sea life.

The world agrees that the negative effects of shipping on the environment must be addressed through teamwork. The Association of Southeast Asian Nations (ASEAN) has set a goal for its members to become carbon neutral by 2050.

• Cruise ship pollution in Europe
• EU efforts to reduce greenhouse gas emissions from shipping
• EU Sustainable Shipping Forum (ESSF)
• EC-IMO Energy Efficiency Project: A 4-year project to create Maritime Technology Cooperation Centres in 5 regions—Africa, Asia, the Caribbean, Latin America, and the Pacific. These centres will help less developed countries use low-carbon technologies and follow international energy efficiency rules (EEDI and SEEMP).
• MRV: Monitoring, reporting, and verifying CO₂ emissions from large ships using EU ports

• Merchant Shipping Act 1995
• Merchant Shipping (Pollution) Act 2006

It is expected that shipping traffic to and from the United States will double by 2020 compared to 2004. However, many shipping companies and port operators in North America (Canada and the United States) have joined the Green Marine Environmental Program to reduce environmental harm.

• Act to Prevent Pollution from Ships
• American Bureau of Shipping
• Cruise ship pollution in the United States
• National Oil and Hazardous Substances Contingency Plan
• Oil Pollution Act of 1990
• Regulation of ship pollution in the United States