CO2 emissions are significantly lower for maritime transport modes per tonne than for land or air based travels, and therefore shipping is often considered to be a relatively clean transport mode (Jägerbrand 2019). However, cargo ships are by far one of the biggest components of the modern shipping process, accounting for 70% of the total value of international trade (Jägerbrand 2019). This is especially true in the context of modern demands for the fast and efficient movement of goods (Han 2010). Speed is not a strong suite for cargo ships, and prioritizing speed comes at the cost of efficiency and impacts.
Due to the large loads that cargo ships carry, small optimizations on a relatively small fleet size can make a huge difference. Thus, many researchers feel optimizing maritime trade is a great way to reduce emissions with relatively efficient engineering efforts. Below are three key areas of maritime shipping impact: water discharges, physical impacts, and air emissions.
Water discharges encompasses spill and operational discharges of oil and cargo, wastewater, marine litter, nonindigenous species, and antifouling paints.
Likely due to stricter regulations, the number and volume of large spills from tankers since the 1980s has decreased (Jägerbrand 2019). However, there are doubts around this data as there are increasing dumps of "other substances", which often include discharges of oil.
Operational discharges (e.g. release of bilge water, lubrication of propeller shaft bearings, illegal cleaning of tanks) have remained quite high and have likely increased - contributing over 70% of total shipping related oil discharge (Jägerbrand 2019). Outside of oil and related substances, 78,500 tons of potentially hazardous dry bulk cargo enter the marine environment by operational discharges annually (Jägerbrand 2019).
All of these substance impacts have the ability to change the community composition and be toxic to the environment and ecosystems they enter. Ingestion or inhalation of petroleum components can affect digestive, respiratory, and circulatory systems of marine life (Jägerbrand 2019). These effects can be immediately toxic and kill or can have life long impacts.
Certain environments and life are particularly vulnerable. Coastal wetlands susceptible due to their low-oxygen environments with slow decomposition (Jägerbrand 2019). This is especially concerning as 80% of the fleets are harbored or near the coast, where these wetlands are located (Han 2010). Certain species such as sea birds, sea otters, seals, and fish have particular weaknesses to operational discharges. These toxic discharges can be directly toxic, destroy natural lipids - reduce their natural defenses, cause swelling, decrease filtering capacity, induce liver damage, hinder growth, etc. (Jägerbrand 2019).
Physical impacts from maritime transportation include noise, artificial light, wildlife collisions, shoreline erosion and resuspension of sediments, and grounding.
Cargo ships have a variety of mechanical systems that release noises anywhere between 10 Hz and 1 kHz. These noises can obstruct the natural soundscape. Because the range of frequencies is so wide, they are likely to overlap with the frequencies recognized by a wide array of organisms; this results in interference with navigation, communication, mating, habitat selection, predator avoidance, and the detection of prey (Jägerbrand 2019).
Light sources above the water can cast into the water. These can confuse creatures who use light for guidance or signaling. For example, seabirds are attracted to the light and can become disoriented while navigated. Sea turtle hatchlings can be adversely affected by the light productions (Jägerbrand 2019).
Ships can physically run into marine life while traveling, large whales are particularly vulnerable.
Speed is the biggest determiner of the probability of a collision taking place as well as the severity of the impact (Jägerbrand 2019). This is a challenge as shipping speed can need to be increased to match modern fast shipping demands. However, other factors can increase risks such as forces such as ship design. The propeller suction effect can draw creatures, often whales, towards the hull. Certain ship designs are also hard for animals to detect and avoid (Jägerbrand 2019).
To reduce chances of impact shipping speed should be delayed and a traffic separation scheme should be implemented during the wintering season. The separation scheme reduces time in areas with a high risk of humpback whale collisions, reducing impacts by 95% (Jägerbrand 2019).
Ships increase the wave energy of the waters surrounding them. This is most hazardous near shores. The increased energy can damage oyster reefs, benthic invertebrates, fish, and birds nesting in the shoreline area (Jägerbrand 2019). The shoreline's fine sediments can be transported to deeper waters, affecting the bottom habitats of both environments (Jägerbrand 2019).
Reducing speed can help to minimize these effects (Han 2010).
Over 8600 large shipwrecks lie on the seafloor, predominantly in shallow areas. Ship's are full of hazardous chemicals that will leak into the environment (Jägerbrand 2019). However, with proper preparation and planning, shipwrecks can be used to create artificial reefs that may improve the local ecosystem and protect from erosion (Jägerbrand 2019). A poorly designed reef, on the other hand, may interfere surrounding natural reefs.
Half of a ship's operating expense is general fuel. To reduce costs and increase fuel economy, ship's use degraded, residue heavy fuel oil (Han 2010). This fuel is high in levels of asphalt, carbon residues, and sulfur and metallic compounds. When burned, the ships emit black smoke, particulate matter, nitrogen oxides, unburned hydrocarbons, sulfur oxides, carbon monoxide, and carbon dioxide (Jägerbrand 2019, Han 2010.
All of these emissions deplete the ozone layer, enhance the greenhouse effect, and can produce acid rain (Jägerbrand 2019, Han 2010). Since, as stated earlier, most ship emissions occur near the coasts. This means that emissions not only influence marine conditions and marine life, but also influence soils, rivers, and lakes (Han 2010).
Ship contributions as a percent of global emissions is expected to rise to more than 30% for nitrogenous oxides, 18% for sulfur, and 3% for carbon dioxide by 2050 if trends continue. Emissions for fine particles are also expected to double (Han 2010).
To reduce emissions and fuel consumption, ships can run at lower speeds where they consume less fuel. Higher speeds correlate with an exponential use of fuel for a linear increase in velocity travelled.
However, this is a difficult thing to achieve in the current environment. Most ports are locally owned and ports don't want to voluntarily lower their speeds out of concern for a competitive disadvantage (Han 2010). To achieve this goal, there are going to have to be more systemic changes and more pressures from big players. If speeds can be reduced, ship designs should be modified to be at their lowest drag coefficient at these lower speeds.
More ports need to be electrified or have an option for shore-based power supplies. Currently, most vessels burn fuel using auxiliary or main engines while at port to provide heating, cooling, and electricity (Han 2010).
Longer term, there should be a move to more efficient fuels - with a goal of moving off fossil fuels. There once again need to be fundamental changes around shipping culture. Expecting fast, peak-time efficiency shipping all the time is not environmentally sustainable.