By: Nicole Gotthardt & Justin Freiberg

Background image at the top of this article belongs to Leonidikan/Dreamstime.com

Until recently, climate change action has been associated with one particular greenhouse gas, carbon dioxide (CO₂). More and more, action-oriented climate groups are turning their focus to super pollutants that have an outsized impact on warming in the near term. Methane (CH4) and F-gases are often spotlighted for their strong warming potential, but there’s another powerful pollutant driving rapid climate change that deserves attention: black carbon (BC). With its significant—yet difficult-to-quantify—warming impact, BC is both a fascinating and vexing pollutant to confront. 

BC is a tiny particle that forms from the incomplete combustion of fossil fuels or biomass, such as wood or waste. However, unlike other greenhouse gases, BC is neither a specific molecule nor even a gas. BC is an operational term that encompasses multiple strongly light-absorbing compounds, including elemental carbon and light-absorbing organic carbon, also known as brown carbon.

BC is emitted from various sources, including inefficient biomass cookstoves, diesel and two-stroke engines, open-air-vented coal furnaces, wildfires, brick kilns, energy production, and agriculture residue and waste burning. In 2019 alone, an estimated 5.8 million tons of BC were emitted.^[1] While the 5.8 million tons of BC is modest when compared to the over 40 billion tons of global annual emissions of CO₂, BC’s strong warming effect creates an annual impact equivalent to over 8 billion tons of CO₂. The exact impact of BC is tough to quantify, as we’ll explain later.

Black carbon emissions are not solely a climate issue—they are also a major health threat. When inhaled, small particles of BC can penetrate deep into the lungs and facilitate the transport of toxic compounds into the bloodstream. As a component of fine particulate matter (PM_2.5), long-term exposure to BC particles causes major negative health effects, such as heart disease, lung cancer, asthma, lower respiratory infections, and adverse birth outcomes.^[2]

Black carbon shares similarities with other super pollutants: it contributes significantly to warming and has a short atmospheric lifetime. Yet, unlike gases like CH₄ or CO₂, BC is an aerosol—a solid particle suspended in air. As an aerosol, BC behaves differently in the atmosphere. Rather than lingering for years or centuries like other greenhouse gases (CO₂ can last for hundreds to thousands of years, whereas CH₄ remains for around a decade),^[3] black carbon settles out of the atmosphere quickly through rain or by gravity, remaining in the air for only 4 to 12 days.^[4]

Another important difference is how BC interacts with energy in the atmosphere. Unlike other greenhouse gases, which trap the infrared radiation that is re-emitted by the Earth, BC directly absorbs sunlight and releases it as heat, warming the air and surfaces in regions where it is concentrated. BC also indirectly contributes to warming by altering cloud properties and decreasing the albedo, or reflectivity, of the surfaces it lands on.

Alongside warming, black carbon emissions have far-reaching impacts that endanger vulnerable communities and ecosystems. Through interactions with the atmosphere—such as dimming the Earth’s surface, reducing patterns of evaporation that feed cloud formation, and acting as cloud condensation nuclei—BC particles disrupt normal weather patterns that are vital for agriculture and livelihoods (e.g., the Indian Monsoon). When particles settle on plant leaves, they heat the surface, damaging cells and inhibiting carbon sequestration through photosynthesis.

The challenges in quantifying the radiative forcing of black carbon:

Even with a short atmospheric lifetime, BC is a significant contributor to climate change. Studies estimate black carbon has a direct global radiative forcing of around +0.40 to +1.1W/m², making it either the second or third most impactful warming agent after CO₂ and CH₄.^[5]

Quantifying the actual climate impact of BC is complex and depends on several factors that are poorly understood: atmospheric lifetime, the presence of co-emitted pollutants, where and at what altitude it is emitted, how it interacts with clouds, and how it ages and mixes with other aerosols in the atmosphere. 

To complicate matters further, BC is co-emitted with other pollutants, including CO2, carbon monoxide (CO), nitrous oxides (NO_x), sulfur dioxide (SO₂), and volatile organic compounds (VOC). Some of these co-pollutants, particularly SO₂ and VOCs, happen to be cooling because they reflect sunlight rather than absorb it. This means they can partially or fully offset the warming caused by BC in certain contexts. 

It is important to note that while global radiative forcing gives us a broad sense of the impact on climate change, it does not capture BC’s intense localized effects. Because BC is a short-lived aerosol, its warming influence is concentrated in specific areas, creating “hot spots.” When assessing the true warming impact of BC, it is important to focus on its regional impacts rather than highly uncertain global estimates. Localized efforts of BC mitigation will supply the largest benefits for local warming, as well as the health of communities.

How is Global Warming Potential (GWP) applied to black carbon?

Just as it is difficult to precisely quantify the direct radiative forcing of BC, estimating its GWP is also a challenge. Current best GWP calculations estimate that over a 20-year time frame, one ton of BC has approximately 1,600 times the warming impact of one ton of CO2. Over 100-year period, that number drops to around 450 times, which is still a staggering number.^[6]  

While GWP is widely used in climate policy to compare the warming effects of different greenhouse gases, it is not an accurate indicator for black carbon. GWP relies on several assumptions that do not apply to this non-gas, short-lived pollutant. For one, GWP assumes that compared emissions that produce radiative forcing are evenly spread across the globe. This assumption does not hold for BC because, as a short-lived and regionally concentrated aerosol, it has a highly localized warming impact.

As a measurement standardly calculated with gases, GWP also assumes that each ton of a pollutant has the same warming effect. Particles behave differently. Each BC particle has a unique warming impact due to its specific shape, residence in the atmosphere, altitude, and interactions with clouds and other aerosols. Therefore, no two particles of BC can have the same warming impact as measured by GWP.

Finally, GWP overestimates the overall warming effect of short-lived climate pollutants. GWP ignores the fact that the radiative forcing of a short-lived climate pollutant at the end of the integrated time period is much weaker than a pollutant that decays more slowly over time, such as CO₂. Capturing the true warming impact of black carbon requires the use of an alternative emissions metric—an important focus for future research. 

Since black carbon is not a gas, and it is difficult to accurately quantify its warming impact, it remains mostly absent from international-scale climate policy. As of 2025, only nine countries have set explicit BC reduction targets in their Nationally Determined Contributions (NDCs) to fulfill their Paris Agreement commitments, while seven others consider BC in their overall climate goals.^[7]

While it’s challenging to quantify BC’s warming impact on a global scale, its impact is understood in specific regions. In the Arctic, BC definitively contributes to increased ice melt and accelerated warming. BC emitted in the Arctic (60°-80°N) warms the surface nearly five times more than emissions at mid latitudes (28°-60°N).^[8] This strong warming impact drives Arctic amplification—a positive feedback loop of ice melt that causes the region to warm at a rate three to four times faster than the global rate. When dark BC particles settle on the bright ice and snow of the Arctic, the surfaces absorb additional heat, causing the ice to melt faster. As more ice melts, dark land and ocean surfaces with low albedo become exposed, leading to increased Arctic warming and triggering additional ice melt. Black carbon deposition on snow has already contributed up to 39 percent of total glacier melting,^[10] and emissions are linked with 20 percent of the snow and ice loss over the 20th century.^[11] Beyond the Arctic, BC emissions have a direct impact on glacier melt in the Himalayas, Alps, Tibetan Plateau, and the Rockies. 

The first step in addressing BC is understanding how it behaves once released into the atmosphere. The next—and more critical—step is preventing those emissions before they occur. Mitigating emissions requires a clear understanding of the wide range of sources that produce BC, as well as the multitude of strategies to reduce them.

While all BC is formed through incomplete combustion, its sources differ greatly in scale and location. Some are at the industry scale—at coal plants, oil refineries, or factories—while others are at the household level—in individual cookstoves, diesel generators, or vehicles. 

Primary sources of BC also differ by region. In Asia and Africa, 60 to 80 percent of anthropogenic BC emissions stem from the burning of solid fuels like wood for household energy. Europe and North America differ, with diesel engines dominating BC emissions at approximately 70 percent. Globally, more than three-quarters originate in Asia, Africa, and Latin America.^[11] Countries in these high-emitting regions lack stringent air pollution regulation and primarily rely on open biomass burning and the continued use of traditional fuels (e.g.,  wood, coal, and kerosene) for everyday cooking, heating, and lighting. 
The following sections highlight key emitting sectors—residential energy, transportation and shipping, and industry—and explore solutions to reduce emissions from each. This is not a comprehensive review of all sources or strategies, but rather a highlight of major sources and interventions that could drive meaningful reductions in black carbon. Figure 2 spotlights the four largest black carbon-emitting sectors (excluding forest fires), providing the major sources, mitigation priority areas, and example solutions.

Residential Energy: Clean Cookstoves

Around 2.5 billion people worldwide still rely on open fires or inefficient stoves fueled by kerosene, biomass, or coal for cooking, lighting, and heating. These practices produce BC and severely impact health, especially for women who cook indoors. 

Cleaner alternatives—such as improved-efficiency cookstoves and modern fuels such as electricity and biogas—are readily available, but often inaccessible in lower-income nations due to cost and access. In response, a range of organizations are working to expand access to clean cooking solutions. A notable company—Burn Manufacturing (BURN)—works at the intersection of health, climate, and gender equity by providing clean cookstove technologies to communities in Africa. To lower the cost of clean cooking alternatives, BURN participates in the carbon markets as a clean cookstoves project developer, selling carbon credits verified by Gold Standard and Metered and & Measured Energy Cooking Device (MMECD) methodologies. 

Clean cookstove efforts are a popular voluntary carbon market (VCM) project type, representing around 15 percent of all carbon credits issued from May to November 2023.^[12] However, clean cookstove credits in the voluntary carbon market (VCM) have come under scrutiny due to claims of widespread overcrediting, where projects credit for more emission reductions than actually occur. Investigations have shown that some methodologies rely on unrealistic usage assumptions or inflated baseline emissions, leading to the issuance of millions of carbon credits without real climate benefits. In 2024, a peer-reviewed study found that sampled projects were, on average, 9.2 times overcredited.^[13] The same study found Gold Standard’s metered methodology was only 1.5 times over-credited, suggesting clean cookstove projects still have the potential to deliver meaningful BC mitigation and health benefits through the VCM.

On- and Off- Road Transportation

While stringent emission controls in the United States (U.S.) and Europe have helped reduce BC emissions, heavy-duty diesel vehicles are expected to remain the primary transportation-related BC source in 2050. To make matters worse, the global use of vehicles is predicted to triple by 2050. Diesel engines, especially in heavy-duty vehicles, generate particulate matter (PM), including BC, which leads to negative health impacts for nearby communities. Off-road vehicles—such as back-up diesel generators, road paving equipment, construction equipment, and cranes—are also a key source of BC emissions.

Diesel Particulate Filters (DPFs) are among the most effective technological solutions for reducing BC and PM emissions from the transportation sector. Widely adopted in the U.S. and Europe, DPFs have significantly lowered particulate emissions in urban areas by capturing and burning exhaust soot from diesel engines. In countries with the most concentrated transportation emissions, such as China and India, these controls have been historically absent, allowing the widespread use of heavy-polluting vehicles. Recently, both nations adopted regulations aligned with European emissions standards. India introduced Bharat Stage VI in 2020, and China implemented VI standards in 2023 to limit PM and nitrogen from light- and heavy-duty vehicles. While these regulations make important progress, they primarily apply to new vehicles and do not address legacy fleets, which will continue to emit disproportionately high levels of BC.

Shipping: Maritime Industry in the Arctic

While urban areas experience the highest BC concentrations from transportation, emissions are alarmingly high in the Arctic, where pollution from international shipping disproportionately accelerates the melting of snow and ice. While shipping only accounts for around five percent of the BC emissions in the Arctic,^[14] BC emitted from vessels persists at lower levels of the atmosphere and has an outsized warming impact. After heating the air for under two weeks, particles land on snow and ice, and BC’s warming impact is seven to ten times greater.^[15] Unlike other BC sources that have been steadily declining in the Arctic, BC emissions from shipping continue to increase. Between 2015 and 2019, they rose by 85 percent.^[16] This increase is largely driven by melting ice exposing new travel routes through Arctic territories, leading to another positive feedback loop of increased pollution and warming. 

Transitioning to different fuel sources for this increasing shipping traffic holds some potential solutions to the BC issue. Many vessels in the Arctic (75%) still run on heavy fuel oil (HFO), a thick, tar-like waste substance from the oil and gas refining process.^[16] Burning HFO releases black carbon, PM, CO₂, NO_x, and SO₂.

International policy plays a critical role in accelerating the shift to cleaner fuels. Alternative fuels—such as liquefied natural gas (LNG), marine diesel oil (MDO) blended with drop-in biodiesel, and methane—emit significantly less BC and PM than HFO. Starting in 2024, the International Maritime Organization, the main international regulating body for maritime transport, established a ban on HFO use in vessels in the Arctic. However, this rule includes exemptions until 2029, allowing an estimated 74 percent of the HFO-fueled fleet to continue operating in the Arctic.^[16]

Industry: Brick Kilns in South Asia

BC emissions from the industrial sector are heavily concentrated in energy-intensive processes that rely on coal. Brick kilns are particularly intense emitters, where the combination of coal or biomass feedstocks and inefficient technologies drives large amounts of BC, PM, CO₂, and SO₂ emissions. Concentrated in South Asia, brick kilns are a crucial aspect of the economy and produce a majority of bricks for the rapidly growing construction industry in the region. Outdated and inefficient brick production methods have led to severe air pollution, significantly worsening health outcomes in the region. Respiratory illnesses—particularly pneumonia, the leading cause of death among children in Bangladesh—are especially prevalent.^[17]

BC emissions in brick kilns can be abated through known technological modifications and operational changes to improve kiln efficiency. These technologies, such as zig-zag or vertical shaft kilns, can cut coal use by nearly 25 percent, offering a promising path toward cleaner brick production.^[17] Some governments have demonstrated agreement with these infrastructure changes, with India’s Ministry of Environment, Forest, and Climate Change mandating in 2023 that all new kilns adopt these improved technologies or switch to cleaner fuels. Although the technological solutions are readily available and can be deployed today, progress has been stifled by high upfront costs, limited access to financing, and a shortage of skilled labor to implement and operate newer systems.

Black carbon mitigation efforts have been ongoing for decades, driven by a wide range of actors across multiple sectors and regions. The number of groups that have contributed meaningfully to this work is too large to list comprehensively in this article. Here, we highlight a few organizations to demonstrate the diversity of approaches and collaborations at play:

Many solutions that reduce BC emissions—such as DPFs and switching to cleaner fuels—are technologically mature and readily available for deployment. However, BC mitigation still faces major barriers to financing, particularly in high-emitting, resource-constrained countries. Despite the significant climate and health impacts of BC, it remains largely invisible in international climate and development finance, mirroring its absence in international climate goals. Between 2018 and 2022, BC-specific projects made up a mere 0.1 percent of the total $18 million in outdoor air quality funding.^[11] These funds are allocated from a very small pool of international funders, with only eight donors identified as of 2022.^[11] BC’s exclusion from carbon markets—largely due to the difficulty of quantifying its precise warming impact—further limits access to finance. However, opportunities exist to fund BC mitigation through health-focused initiatives, especially in the household energy and transportation sectors. 

Confronting black carbon offers a promising opportunity to deliver immediate benefits for both climate and health, and it deserves far greater attention. Stay tuned for upcoming CC Lab blog posts that will dive deeper into key BC sources and emerging solutions that will accelerate mitigation action.