What Is Black Carbon? Overview, Impacts, and Mitigation Strategies

Learn what we can do to address the second-leading cause of global warming.

By
David M. Kuchta
David Kuchta
David M. Kuchta
Writer
  • Wesleyan University, University of California, Berkeley

David Kuchta, Ph.D. has 10 years of experience in gardening and has read widely in environmental history and the energy transition. An environmental activist since the 1970s, he is also a historian, author, gardener, and educator. 

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Published October 26, 2021
Pedestrians walk through a cloud of dust and diesel exhaust from a transit bus near Yosemite Village, June 16, 2000 in Yosemite National Park, California.

David McNew/Newsmakers/Getty Images

Black carbon is one of the main components of soot, smoke, and smog. It's what's left behind from the incomplete burning of organic materials like firewood or fossil fuels.

In the right place, it's an important natural fertilizer in soils, the reason why people have practiced slash-and-burn agriculture for thousands of years. In the wrong place, black carbon settles deep in the lungs and leads to premature deaths, or settles on snow and increases the risk of catastrophic flooding. Left suspended in the atmosphere, it's the second leading contributor to global warming, after carbon dioxide.

Because of its disproportionate impact on disadvantaged communities, addressing the problem of black carbon is an environmental justice issue.

Sources of Black Carbon

Before the Industrial Age, fire was the main source of black carbon, whether naturally occurring or human-induced. As part of the natural carbon cycle, the burning of biomass produces more solid black carbon (biochar) than it produces airborne black carbon (soot). Fire primarily sequestered carbon in the soil rather than sending it into the atmosphere, and what was sent into the atmosphere was reabsorbed by plants.

Up to 40% of soil's organic carbon is black carbon, which acts to increase the soil's fertility. Even today, biochar is used to increase the fertility of soil degraded by intensive industrial agriculture.

The Industrial Age

With industrialization beginning in the late eighteenth century, coal (the dirtiest fossil fuel) replaced biofuels as the primary source of black carbon emissions. Atmospheric black carbon (soot) increased seven-fold, peaking in the early twentieth century.

The burning of biomass has continued, however, especially in rural areas of low-income countries, where two billion people worldwide rely on biomass—in the form of wood, dung, or crop residues—as their primary fuel for heating and cooking. Indeed, the burning of biomass doubled with the rapid growth in population in the twentieth century. Inefficient cookstoves are a primary source.

On a global scale, fossil fuels are the source of roughly twice the carbon emissions as biomass sources, which contribute an estimated 25% of all black carbon emissions. The contribution of each source to atmospheric black carbon varies depending on the industrialization and urbanization of the area, with biomass contributing more black carbon in rural regions and fossil fuels contributing more in urban areas.

A tugboat on the Thames near Tower Bridge in heavy smog, 1952.
The Great Smog of London in 1952 was largely black carbon.

Fox Photos/Hulton Archive/Getty Images

After fossil fuels and biomass, road dust is a third source of black carbon, particularly from vehicle exhaust and from brake and tire wear. Today, diesel exhaust emits more black carbon than any other single source, including 90% of the emissions from the transportation sector. A significant component of urban particulate matter (PM2.5), black carbon levels can be 50% to 200% higher near roadways. Around coal-fired power plants, soot that settles on or near roadways gets re-suspended in the air.

Dangers of Black Carbon

The impact of black carbon is as much a local problem as a global one. The effects depend on the source and location of emissions, with biomass sources of black carbon having localized affects on human health while fossil fuel sources can contribute to more global problems, such as increasing the risk of natural disasters and global warming.

Impacts on Human Health

While black carbon remains in the atmosphere for only a few days, its impact on human health can be enormous. In rural areas, household black carbon air pollution from cookstoves disproportionately affects women and young children, according to two studies. In urban areas, road dust, especially near coal plants and port facilities, carries similar risks with a significantly increased exposure to black carbon among low-income households and people of color. In one Detroit-area study, for example, near-road black carbon concentrations were 35%-40% higher in disadvantaged communities and communities of color than elsewhere.

Global Warming

Trucks are driven near the Ports of Long Beach and Los Angeles, the busiest port complex in the US
The Port of Long Beach, California.

David McNew/Getty Images

Black carbon has been identified as “the second most important” source of greenhouse gas emissions. Black carbon from fossil fuel sources have twice the global warming potential as black carbon from biomass sources. Because black carbon absorbs rather than reflects light, it prevents energy that would normally escape back into space from leaving Earth's atmosphere, thus contributing to global warming.

This is the case whether black carbon fall back to Earth's surface or is suspended in the atmosphere. Black carbon is especially potent when it falls on snow, causing the darkened snow to absorb more heat energy rather than reflect it back into space. According to recent research, black carbon is responsible for more than 50% of accelerating glacial and snowmelt. In polar regions, this is an immediate cause of sea-level rise.

Natural Disasters

In areas with year-round ice such as glaciers, the presence of black carbon increases flood risk. Glacial melt from the Himalayas increases the flood risk of 78 million people living in the basins of the Ganges and the Brahmaputra rivers. Black carbon has been linked to increased frequency of droughts in northern China and floods in southern China, as well as to increased intensity of tropical cyclones originating in the Arabian Sea.

Technological Solutions

A solar cooker in Zambia
A solar cooker in Zambia.

Tina Stallard/Getty Images

Black carbon is an environmental justice issue, as the risks primarily affect people who live in poverty, people in the developing world, and people of color all over the world. Importantly, ways of mitigating black carbon emissions already exist. Implemented, they can improve human health and reduce global warming by an estimated 0.2 degrees C by 2050.

Black carbon and carbon dioxide are often emitted during the same combustion processes (such as in the burning of diesel fuel), so many of the efforts to reduce CO2 emissions will have the effect of reducing black carbon as well. However, some mitigation efforts are especially significant for reducing levels of black carbon emissions.

  • Cleaner-burning cookstoves such as solar cookers have the potential to reduce rural black carbon emissions, slow deforestation, improve human health, and raise educational levels since children devote a significant amount of time to collecting firewood that cuts into their educational opportunities.
  • Regenerative agriculture involves the practice of maintaining soil health by returning carbon and other nutrients to the soil. Black carbon remains persistent and stable in soil for millennia, so returning it to the soil as biochar can also act as a form of carbon farming or “negative emissions.”
  • Hybrid and electric vehicles reduce levels of road dust by relying primarily on regenerative braking rather than friction braking, which produces an estimated 20% of the particulate matter stemming from road traffic.
  • Less traffic and cleaner traffic reduces exposure to black carbon. Low-emissions zones (LEZs) can also be effective: London's LEZ reduced black carbon by 40%-50%. Reduced diesel pollution from trucks can also improve health outcomes in low-income and disadvantaged communities; the Port of Long Beach, California, won the U.S. EPA's Environmental Justice Achievement Award for one such program.
  • Cleaner shipping. Because black carbon only remains suspended in the atmosphere for a few days, reducing ship emissions of black carbon in sensitive areas such as the polar regions has a significant impact on reducing snowmelt and sea-level rise.
View Article Sources

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