In the year 1952, London disappeared from view. It was called the Great Smog, the worst air pollution the U.K. had ever seen. It killed thousands of people, shuttered above-ground public transport and sparked an unprecedented push for clean-air legislation in a country known for its toxic pea-soup fog caused by the airborne pollution associated with coal.

Air pollution is still a problem, not just in cities from Paris to Beijing but across the globe. And one of the main culprits is black carbon, particles in the air that arise from burning forests, diesel engines and coal pollution. But black carbon isn’t just a problem for those near forest fires or diesel-choked highways. According to the U.N.’s Climate & Clean Air Coalition …

Black carbon’s warming effect on the climate is as much as 1,500 times more intense than that of carbon dioxide (CO2).

While exact percentages on how much of planetary warming is caused by black carbon are not available, it’s widely considered to be the second-largest culprit when it comes to climate change, after CO2. A 2013 study of the substance found that black carbon is responsible for 1.1 watts of extra energy being stored in the atmosphere per meter of the Earth’s surface. By comparison, CO2 is responsible for an estimated 1.56 watts per square meter.

Black carbon is formed via an incomplete process of combustion. “Imagine a chemical reaction where you take an organic fuel — a fuel which has carbon and hydrogen — and you combine it with oxygen, and all that comes out is water and carbon dioxide,” explains Joshua Schwarz, a researcher and a physicist at the National Oceanic and Atmospheric Administration. The optimal level of combustion could be a gas stove: There’s a blue flame and no soot or other particles. But some processes use too much fuel for the amount of available air, meaning there are leftover carbon and hydrogen molecules. Black carbon is a component of such leftovers.

About half the planet’s black carbon is traced to burning fossil fuels, including coal, and the other half is from burning biomass like wildfires or agricultural razing. Apart from its polluting effects — the particles can settle in human lungs, causing respiratory diseases and other health problems — black carbon absorbs rays from the sun and converts it to heat, warming the planet with alarming efficiency. That’s particularly disastrous for icy landscapes like the Arctic and Antarctica: When black carbon settles on snow and ice, it heats the surface and melts them even faster. The Arctic is currently thought to be warming twice as fast as the global average.

Snow and glaciers have a particular vulnerability to black carbon, explains Örjan Gustafsson, a professor at the Department of Environmental Science and Analytical Chemistry at Stockholm University. “The effect of black carbon depends on how much darker the color black is relative to the ground color,” he says. Light-colored surfaces reflect light rather than absorbing it — until dark pollution molecules settle on them, meaning white snow and ice, and the air above them, are the most impacted by black carbon pollution. Gustafsson and his team have conducted 10-year studies on the impact of black carbon in the Arctic landscapes and the Himalaya-Tibetan Plateau, and the black carbon–induced glacier melts are most serious in areas like the Himalayas or in South America’s Andes, where glaciers are geographically close to large black carbon emissions. Those melting glaciers then contribute to rising sea levels.

There has been progress. In 2017, the Arctic Council — a federation including Canada, Denmark, Finland, Iceland, Norway, Russia, Sweden and the United States — pledged to reduce black carbon emissions by about a third by 2025, in the first-ever collective commitment to address the impacts of black carbon. They’ll aim to achieve that goal by adopting certain emissions standards for new diesel vehicles, upgrading old diesel vehicles and taxing pollution, among other measures.

Because black carbon restrictions address both climate change and pollution, they’re a key way for governments to make progress on multiple fronts at once. “You can win on climate and on health and air quality by addressing black carbon,” Schwarz says. But it’s not simple either, especially without further research into black carbon. “With CO2 molecules, each one is the same,” he says, “but with black carbon, each one looks a tiny bit different. Each one is mixed with other particles and has different properties. Every particle is unique, like how every snowflake is unique.”