What is black carbon, and what does it mean for climate change?

Antarctica, the vast and frozen continent that holds much of the world’s freshwater, appears to the imagination as an unchanging giant. Indeed, most of this remote territory remains free of human habitation and landscape changes. 

But even here, human pollution can be detected. Researchers have found that Antarctica is melting each summer earlier than the one before, due to the black carbon emitted by researchers and a growing number of tourists – and it’s darkening the continent’s snowpack. In research published this February in Nature Communications, it was estimated that each of the 53,000 average annual tourists who visited the Antarctic Peninsula between 2016 and 2020 contributed to the melting of up to 83 tons of snow.

“Recently, tourist and research activities in Antarctica have grown, particularly in the Antarctic Peninsula, one of Earth’s most rapidly warming regions,” said Alessandro Damiani, an assistant professor at Chiba University’s Center for Environmental Remote Sensing and an author of the report, in an email. “We found that the snow around research facilities and tourist-landing sites is darker than elsewhere in the continent.”

But what is black carbon? And why is it causing so much Antarctic snow to melt?

Made up of pure carbon, black carbon is a major component of fine particulate matter (PM2.5) and soot. It is often created from the incomplete combustion of fossil fuels or biological material, such as from forest fires. In Antarctica, black carbon is usually emitted from sources like ships, aircraft or power generators. 

The pollutant was long infamous for its role in urban air pollution since the Industrial Era. But it was first discovered to have global atmospheric importance during the 1950s when it was found within Arctic Haze aerosols – the reddish-brown haze seen over the Arctic caused by air pollution – and on the snow, thus contributing to the Arctic warming faster than any other region on the planet.

The ability of a surface to deflect solar radiation, a characteristic called ‘albedo,’ is important for reducing heat on the planet. Snow, ice and clouds are surfaces with a high albedo – in other words, they reflect a lot of the sunlight that hits the Earth back into space. Sea ice in polar regions such as the Antarctic and Arctic are crucial for reflecting sunlight and keeping the planet cool. 

But black carbon, which can absorb a high amount of solar radiation, reduces albedo. When black carbon particulates are emitted, travel through the air and become deposited in the Arctic, the surface of the snow darkens and then reflects less radiation. Even a little darkening can spur further drops in albedo, as the snow’s darkened surface absorbs more of the sun’s heat and melts. Water has a lower albedo than snow, so more heat is absorbed from sunlight, leading to even more melting.

“Antarctica is the cleanest place in the world. Indeed, the concentration of black carbon in Antarctic snow is usually below 1 nanogram per gram – at least one order of magnitude lower than that of other places,” said Damiani. “However, black carbon concentrations can quickly rise exponentially in restricted areas, downwind of research facilities and airports.”

The poles are not the only snow-covered areas impacted by black carbon. Glaciers on the Himalayan Mountains feed some of Asia’s mightiest and most important rivers. But even here black carbon has been found to melt snowpack. For example, a study found that the pollutant was responsible for an estimated 39 percent of a Nepalese glacier’s shrinkage. The melting of Himalayan glaciers due to overall climate change already threatens the freshwater and food supplies of billions of people in Asia.

But even more than snow-based black carbon, airborne black carbon particles have an even greater role in heating the planet as they directly absorb sunlight and keep it in the air. It is even thought that these can influence cloud formation patterns, which might then affect precipitation and climate change. 

Radiative forcing, the change in the amount of solar radiation received and reflected by the Earth’s atmosphere, is often used in science to measure how much a gas or particle heats the planet. It is measured in watts per meter^-2, with a positive number meaning a rising temperature. Although estimates vary, a 2013 study found that black carbon emissions since the Industrial Era have resulted in a positive radiative forcing of 1.1 watts per meter^-2 – second only to carbon dioxide. In other words, through existing in the atmosphere, on ice and in clouds, black carbon emissions have caused the planet to absorb more solar radiation and reflect less back into space, thus heating the planet.

Consequences for health

Black carbon’s impacts on human health may well be as consequential and overlooked as its implications for the climate. Particulate matter with black carbon contains carcinogens that can be inhaled by people, and air pollution has been linked to pneumonia, heart disease, stroke and cancer. An estimated 8 million people die every year due to outdoor and indoor air pollution.

Indeed, soot was one of the first pollutants to be controlled in developed countries. Before the mid-20th century, major cities in Europe and North America were notorious for their heavy soot pollution, mostly from burning coal, and the health effects were disastrous. 

This culminated in the Great Smog of 1952, in which a heavy blanket of smog enveloped London for four days and killed at least 4,000 people. This led to the Clean Air Acts of 1956 and 1968 in the U.K., which regulated the fuels used by residents and factories and drastically reduced soot concentrations in the country. The U.S. soon followed suit with its 1963 Clean Air Act.

Nowadays, developing countries have become the largest source of black carbon. China and India alone account for around a quarter of these emissions, mainly because of residential combustion of solid fuels and coal-dominated industry. 

Burning solid fuels for heat is a key cause of indoor black carbon emissions, whereas vegetation fires and traffic are major outdoor sources. Northern India, eastern China and equatorial Africa are global emissions hotspots for both these indoor and outdoor emissions. The World Health Organization estimates that 2.6 billion people, mostly living in developing countries, still suffer from indoor air pollution by using unsafe cooking methods.  

Solutions

Overall, reducing black carbon emissions globally could save millions of lives each year and help slow climate change. In fact, lowering these emissions might be one of the easiest short-term ways to combat global warming: doing so by using available strategies could already cut warming by 0.2 degrees Celsius by 2050. And, the effects of tamping down black carbon emissions would appear quicker and more locally (in the form of cleaner air) than decreasing carbon dioxide.

The sources of black carbon are wide-ranging, so there are many ways to approach the goal of cutting global emissions. For example, slash-and-burn agriculture is a globally widespread practice in which land is cleared for growing crops through use of fire, and the burning of the biomass releases black carbon. An alternative could be the method of slash-and-char, in which woody biomass is turned into charcoal using low-intensity, covered fires, as native Amazonians had done for millennia.

Another major source of black carbon emissions is the use of certain solid biomass fuels (like agricultural waste or wood) in traditional stoves for heating and cooking, which are used by many around the world. Safer and less-polluting options include cookstoves that use cleaner fuels like liquefied petroleum gas, briquettes or pellets with little moisture content or ones designed for more safety, such as those with forced air fans or gasifiers.

Overall, switching to more efficient stoves or fuel would cut fuel costs, the time that would otherwise be spent collecting wood for fuel and deforestation. The lower black carbon emissions would also result in a lower health impact, which would particularly benefit women, who often spend more time indoors near stoves. But this would require having safer cookstoves and fuels that are affordable on the market and educating people about these alternatives.

In urban areas, traffic is a major source of black carbon emissions, mainly from diesel vehicles such as trucks, construction equipment or farm vehicles. Setting vehicle emissions standards, retrofitting older vehicles with diesel particulate filters and providing subsidies to encourage people to replace old and heavy-emitting vehicles with newer, cleaner ones are a few ways to lower traffic-related black carbon. The challenge here though, of course, is the cost of change and maintenance and the capacity for enforcement. 

As for Antarctica, Damiani noted that the energy demands of generators, vehicles and aircraft in the region – as well as the lack of solar energy options during the continent’s long, dark winter – means that completely eliminating black carbon emissions remains unforeseeable for now, especially with the expected rise of research activity and tourism.

But he added that positive steps have been taken. Many ships passing through Antarctica already use a less-polluting marine diesel fuel, and research stations now operate under strict environmental regulations. Future vessels in the region could adopt even cleaner engines or fuel, or possibly transition toward hybrid or electric ships. 

“Both Antarctic tour operators and government research agencies are already doing their best to limit emissions,” he said.