Governments are poised this week to begin discussing rules to curb black-carbon pollution from ships, after nearly seven years of preparation. The sooty emissions, which are produced by diesel engines, warm the climate and harm human health.

At a meeting in London, a panel of the United Nations International Maritime Organization (IMO) is expected to agree on measurement techniques to gather data that could support eventual regulations. That is the second step in a three-step process begun in 2011. Agreeing on a definition for black carbon took four years; the final step, writing rules, could take a few more.

Reducing the amount of black carbon emitted by ships could have a significant impact on the climate. The pollutant, a melange of particles and oil droplets that come in many shapes and sizes, is the second-largest driver of global warming — behind only carbon dioxide. Diesel engines, such as those in ships, account for around one-fifth of the world’s black-carbon emissions, according to a study published in 20131).

The pollution is also dangerous when inhaled, in part because black-carbon particles collect other contaminants — such as sulfuric acid and heavy metals — as they travel through the atmosphere.

Advocates are pushing the IMO to speed up its negotiations, which involve more than 170 countries. “We really only have 90 minutes per year where we are actively discussing the topic, so it’s easy to delay and to stall,” says Bryan Comer, a senior researcher at the International Council on Clean Transportation, a non-profit research group in Washington DC.

Although global black-carbon emissions from diesel engines on land are roughly 20 times higher than those from ships’ engines, the health and environmental impacts of shipping pollution hits many busy ports and coastal areas disproportionately hard, says Daniel Lack, an independent consultant in Brisbane, Australia. “When you concentrate all of these ships into specific areas, all of a sudden they become one of the most dominant sources of pollution.”

One area of special concern is the rapidly melting Arctic. The region’s shipping traffic is projected to increase in the coming decades, as sea ice recedes — a thaw that could be exacerbated by particles of black carbon, which hasten melting when they land on snow and ice.

Measuring black-carbon emissions is not a trivial task, Lack says. The most accurate, and expensive, technology fires a laser pulse through exhaust samples in a tube. Black-carbon particles absorb and then release the energy from the pulse, creating a pressure wave whose strength is equivalent to the amount of light that was absorbed. The shipping industry is pushing for a cheaper, but less accurate, method that draws exhaust through a filter; measurements of the reflectivity of that filter before and after use are then used to determine how much pollution a ship emitted.

Both approaches could serve a purpose as the IMO moves forward with regulations, Comer says. But he adds that many of the regulatory actions that the organization could pursue to reduce black-carbon emissions do not require regular measurements from ships. Shifting from ‘heavy’ fuel oil to cleaner types — similar to those used in trucks — would reduce ships’ black-carbon output by 35–80%, depending on the engine. And installing filters on the vessels’ exhaust systems would cut emissions by at least 85%.

The shipping industry is under pressure to curb other types of pollution. The United States, Canada and the European Union already require ships to use lower-sulfur fuels in some coastal zones. And in 2016, the IMO agreed to reduce the sulfur content in all shipping fuels from 3.5% to 0.5% by 2020. That is good news for public health, but it could inadvertently exacerbate global warming, says James Corbett, an engineer at the University of Delaware’s School of Marine Science and Policy in Newark.

In a study published in Nature Communications on 6 February2, Corbett and his colleagues found that the IMO sulfur standard could reduce global cardiovascular and lung-cancer deaths attributable to fine particulate matter by 2.6%, and the incidence of childhood asthma by 3.6%. But the new standard could accelerate climate change by decreasing the number of bright, sulfur-containing particles in the atmosphere that cool the planet by reflecting sunlight back into space. The researchers estimate that this effect would increase the human contribution to warming by around 3%.

“We’re talking some big numbers,” says Corbett.

For Comer, that is all the more reason to press forward with black-carbon regulations. “It’s frustrating,” he says. “We already know how to control [black-carbon] emissions, but we’re stuck going through the three-step process."