Dual role as air pollutant and warming agent makes black carbon a neglected player in policy

Increasing amounts of black carbon in the atmosphere are leading to extreme rainfall in Asian countries such as India and China, suggested a policy brief from the Center for Study of Science, Technology and Policy (CSTEP) and Clean Air Fund. Black carbon emission is also affecting the rainfall pattern as it shifts the rain clouds northwards.

Black carbon, a short-lived climate pollutant, is also considered to be the second major component responsible for global warming, after carbon dioxide (CO₂). It is emitted in the form of soot from incomplete combustion of biomass, fossil fuels and waste and is a significant part of fine particulate matter, PM_2.5. It is also a short-lived climate forcer (SLCF), which means it remains in the atmosphere for a shorter time than CO₂ but has a high potential to warm the atmosphere. Black carbon has an estimated lifespan of a few weeks but a warming impact of up to 1,500 times stronger than CO₂ per unit of mass.

The report highlights the role black carbon plays in speeding up the melting of snow and ice in the Arctic, the Himalayas and the Andes, disrupting monsoon patterns in West Africa and India, and worsening the effects of dangerous heat waves.

Despite its impact, black carbon is often missing from climate change discussions because of disputes among countries over black carbon-rich sources and uncertainties about its contribution to warming. Its dual nature, as a climate and air pollutant, has resulted in neither of these fields taking full ownership and it remains largely absent from mainstream climate and health agendas. However, as research and studies identify black carbon’s impact on global warming, snow and ice melt, monsoon and weather patterns, flood risk, heat stress and public health, the demand to include it in climate action endeavours is increasing.

The policy brief was released on the sidelines of the 28th UN climate change summit, COP28, held in Dubai in December 2023.

“Air quality regulations do not explicitly target black carbon and so black carbon emissions are not directly controlled in either climate frameworks or through air quality regulations. Through this policy brief, we hope to highlight the dual nature of black carbon both for air pollution (people breathe in black carbon/soot emitted by diesel engines, wood/dung cookstoves, kerosene lamps, etc.) and for climate (faster melting of glaciers and sea ice),” R. Subramanian, head of Air Quality at CSTEP and one of the authors of the policy brief told Mongabay-India.

Affecting the monsoon

Black carbon significantly contributes to regional precipitation changes and the slowdown of the hydrological cycle, leading to drier conditions, the report highlights. Its impact on global hydrological sensitivity is nearly twice as much as that of carbon dioxide (CO₂).

Black carbon emission is also a reason for extreme rainfall events leading to flooding in China and India. Black carbon emissions are also predicted to be behind the early onset of monsoon.

Read more: Scientists unpack the influence of black carbon aerosols on rainfall in northeast India

“Black carbon can affect the atmospheric thermodynamics and thus impact the convective potential and dictate the seasonal trends in wetting and drying of rainfall over India. Due to its absorbing nature, it acts in two fold reduces the solar radiation reaching the surface and warming the atmosphere simultaneously. As such it affects both surface temperature and atmosphere temperature and thus monsoon circulation,” says Chandan Sarangi, assistant professor at Indian Institute of Technology, Madras. Sarangi is an earth system scientist and has worked extensively on aerosol-cloud-climate interactions.

However, the brief suggests that the exact impact of black carbon on regional precipitation patterns requires focussed research because of many complexities such as local energy balances, moisture changes and cloud effects.

“Black carbon can affect the precipitation; the deposited black carbon can accelerate snow and glacier melt. It can also affect air quality and further human health. Actually, the High Mountain Asia (HMA) is a hotspot region for studying the black carbon impacts of the climate system (including snow),” says Dalei Hao, an earth scientist at Pacific Northwest National Laboratory.

Sarangi and Hao both were not part of the policy brief.

Forest fire a big contributor

Hao expresses his concern about the increasing role of forest fires in black carbon emission. He also suggests further investigations into this area.

CSTEP’s Subramanian puts wildfire’s contribution, in global black carbon emission, higher than transportation. “In urban areas, transportation is often seen as the major source of black carbon emissions. On-road motor vehicle diesel emissions are estimated to contribute about one-quarter and residential biomass fuel about 35% of anthropogenic black carbon – which all told is two-thirds of global black carbon. Wildfires contribution varies by year but generally is about one-third of global black carbon (so substantially higher than transportation),” he says.

Studies conducted in Uttarakhand’s Bhagirath-Kharak Glacier and Gangotri Glacier Valley have suggested a greater role of wildfire and biomass burning in the high rate of black carbon influx in Central Himalaya.

“In India, we need to quantify the emission of black carbon and thus their contribution from forest fires. This is not well quantified, but could be significant for regions like mountains where the local emissions and transport from industry and traffic is less,” says Sarangi.

Need for more research

The policy brief recommends more research in the field of black carbon and aerosol to investigate remaining uncertainties in understanding of this short-lived climate pollutant (SLPC). It also calls upon granting bodies to finance further scientific research.

“There are large uncertainties on the impacts of black carbon on our climate systems. More field measurements and model developments are needed to better quantify and understand the black carbon’s impacts. This review paper shows that there are large differences between the observations and simulations of the impacts of black carbon in the snow and ice, and a deep understanding of the lifetime processes of black carbon in the climate system is urgently needed,” Hao says.

Read more: Soaring temperatures, black carbon impact glaciers in Drass region of Ladakh

“While the climate impact of black carbon is clear cut after it drops out of the air on to snow and ice, the effects are less clear (with greater uncertainty) while black carbon is being transported through the atmosphere, which is why not much climate action has been directed at this warming agent,” says Subramanian. “Yes, some scientific aspects of black carbon are still uncertain. As one example, when black carbon is suspended in the atmosphere, the warming effect depends on how much light it absorbs (which is then released as heat and warms the surrounding air) and how long the aerosol stays suspended in (and warming) the atmosphere,” he adds.

“We need much more certainty in the emission inventories of black carbon in India at high spatial resolution and at monthly scale for improved quantification of its climatic impacts,” says Sarangi. Dust is a natural aerosol and is emitted in large quantities to the atmosphere. Along with black carbon and brown carbon, dust is also an absorbing aerosols which can affect climate warming and changes in a similar manner. Hence, more research on the relative role of black carbon (human made) and dust (natural) is essential for India’s future policies, he explains.

What India needs to do

Talking about efforts to tackle the increasing issue of black carbon, Subramanian says, “As we show in the policy brief, solutions exist and are financially feasible, but need to be scaled up. This will significantly improve air quality especially in developing countries of the Global South and also yield significant climate benefits – two wins for the price of one.”

He also suggests that India should include black carbon in its National Clean Air Programme (NCAP 2.0) and Nationally Determined Contributions (NDCs) to address its polluting and warming aspects.

The policy brief also suggests that donor agencies and philanthropies should come forward to support decarbonisation efforts in black carbon-rich sectors.

Banner image: A wildfire in Assam’s Kaziranga National Park. Wildfire and biomass burning play a great role in the high rate of black carbon influx. Photo by gnozef/