Read our COVID-19 research and news.

Some ships, such as this ferry off Corsica, keep their engines running in port, releasing both air and water pollutants.

PASCAL POCHARD-CASABIANCA/AFP via Getty Images

Shipping rule cleans the air but dirties the water

In an unwelcome twist, a global effort to curb pollution from the heavy fuel oil burned by most big ships appears to be encouraging water pollution instead. A 2020 regulation aimed at cutting sulfur emissions from ship exhaust is prompting many owners to install scrubbing systems that capture pollutants in water and then dump some or all of the waste into the sea.

Some 4300 scrubber-equipped ships are already releasing at least 10 gigatons of such wastewater each year, often in ports and sometimes near sensitive coral reefs, researchers reported last month in the first effort to quantify and map the releases worldwide. The shipping industry says pollutants in the waste don't exceed national and international limits, and that there's no evidence of harm. But some researchers fear scrubber water, which includes toxic metals such as copper and carcinogenic compounds called polycyclic aromatic hydrocarbons, poses a rapidly growing threat, and they want to see such systems outlawed.

“There's definitely reason for concern,” says Ida-Maja Hassellöv, a maritime environmental scientist at the Chalmers University of Technology who studies the issue. “A ban of the scrubbers is most urgent.”

The emerging debate is the result of a 2020 regulation put into place by the International Maritime Organization (IMO), an arm of the United Nations that works with 174 member states to develop common rules for international shipping. By banning the use of sulfur-heavy fuel oil, the rule intended to reduce pollutants that contribute to acid rain and smog. IMO estimated the rule would slash sulfur emissions by 77% and prevent tens of thousands of premature deaths from air pollution in ports and coastal communities.

But cleaner fuel can cost up to 50% more than the sulfur-rich kind, and the rule allows ship owners to continue to burn the cheaper fuel if they install scrubbers. In 2015, fewer than 250 ships had scrubbers (often to comply with local regulations); last year, that number grew to more than 4300, according to industry figures.

A scrubber system sends exhaust through a meters-tall metal cylinder, where it is sprayed with seawater or freshwater, depending on the type, at rates comparable to gushing fire hydrants, to capture pollutants. In the most popular systems, called open loop scrubbers, seawater is discharged to the ocean after little or no treatment. Other systems retain sludge for disposal on land and release much smaller (but more concentrated) amounts while at sea.

To come up with its estimate of annual discharges, a group led by environmental policy researcher Bryan Comer of the International Council on Clean Transportation (ICCT), a nonprofit think tank, analyzed roughly 3600 scrubber-equipped ships. The 10 gigatons a year they calculated is likely an underestimate, Comer says, because more ships are adding scrubbers and many have discharge rates higher than the IMO estimates used in the study.

The ICCT study, released on 29 April, also examined the routes taken by the ships in 2019 and found that scrubber discharge is concentrated where shipping traffic is dense, such as the North Sea and the Straits of Malacca. But it also spans the exclusive economic zones of many nations, which extend 370 kilometers out to sea. “Our mapping shocked even me,” Comer says, because it wasn't obvious that so many kinds of ships, operating all over the world, would opt to install exhaust scrubbers.

Researchers are particularly worried about discharges in areas that IMO has designated as ecologically sensitive. The Great Barrier Reef, for example, receives about 32 million tons of scrubber effluent per year because it's near a major shipping route for coal. Ships also release scrubber water around the Galápagos Islands.

Ports see substantial discharges, too. Cruise ships dominate those releases, contributing some 96% of discharges in seven of the 10 most discharge-rich ports. Cruise ships typically need to burn fuel in port to continue to operate their casinos, heated pools, air conditioning, and other amenities. Most ports have shallow water, so pollutants are less diluted and can accumulate more rapidly.

Smoke in the water

Vessels equipped with exhaust scrubbers often discharge the resulting wastewater while in port. Six of the 10 ports that received the most scrubber water are shown below.

Southampton, U.K. Barcelona, Spain Civitavecchia, Italy Sydney, Australia Vancouver, Canada George Town, Cayman Islands Scrubber water discharge at ports in millions tons per year 0–1 1–2.5 2.5–7 7–42
(MAP) OSIPOVA ET AL., GLOBAL SCRUBBER WASHWATER DISCHARGES UNDER IMO'S 2020 FUEL SULFUR LIMIT, INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION, 2021; ADAPTED BY N. DESAI/SCIENCE; (PHOTO) PASCAL POCHARD-CASABIANCA/AFP/GETTY IMAGES

Industry organizations, including one called the Clean Shipping Alliance 2020, say the discharge figures are misleading. They argue, for example, that it is the waste's possible toxicity that matters, not its volume.

So far, few researchers have tested scrubber water on marine life. One laboratory study, published last month in Environmental Science & Technology, found that samples from three North Sea ships harmed the development of a common copepod (Calanus helgolandicus), a tiny crustacean that is a key part of Atlantic Ocean food webs. At very low doses, young copepods stopped molting, and the animals died at rates three times that found in the wild. Such impacts could be “a big deal” for food webs in the real world, says co-author Peter Thor, a marine ecologist now with the Swedish University of Agricultural Sciences.

“We were surprised that we saw effects at such low concentrations,” says Kerstin Magnusson, a co-author and a marine ecotoxicologist with the Swedish Environmental Research Institute. None of the measured pollutants alone occurred at what seemed to be a harmful concentration; instead, the mixture could be to blame, and it's possible that scrubbers generate new compounds.

Next, the researchers, who are participating in a €7.5 million European effort to study shipping pollution called EMERGE, would like to study how scrubber water affects fish larvae, which are likely more sensitive than copepods to scrubber pollutants.

But shippers have become hesitant to share samples and data with scientists. “We're reluctant to give it to organizations which we know have already an established agenda,” says Mike Kaczmarek, chairman of the Clean Shipping Alliance 2020. The group will work with “science-based organizations where we think that [the data will] get an objective treatment.” He points to studies that predict no harm from scrubber discharges, and says that, so far, there's no evidence of actual damage. “Show me the harm,” Kaczmarek says. “There isn't any.” Comer, however, says those studies have methodological lapses that make it impossible to know whether they used samples that meet existing guidelines.

The ultimate solution, he and others say, is to require ships to use the cleanest fuel, called marine gas oil. In the meantime, 16 countries as well as some localities have banned the most common scrubbers. Those bans have reduced discharges by 4%, Comer says. “There needs to be a global fix on top of that,” he says. “That's going to take a long time,” he concedes, given that it can take years for nations to agree on new shipping rules, especially when they increase costs.