New global synthesis sharpens the picture of black carbon’s role on climate

The review brings together experts from across the world in atmospheric observations, emission inventories and modelling, under the lead of scientists from Stockholm University, including Professor Örjan Gustafsson from the Department of Environmental Science, and highlights persistent gaps that could affect our understanding of near-term climate forcing.

A major source of uncertainty

Black carbon has long been recognised as a strong absorber of sunlight, but exactly how much it warms the climate remains uncertain.

“Black carbon is one of the most complex entities when it comes to climate effects,” says Gustafsson.

The review presents growing evidence that models may not fully capture its behaviour. “Observations, relative to models, find higher solar light absorption, less rain-out and higher concentrations of soot aerosols in the atmosphere,” he says.

Black carbon is also unevenly distributed across the globe. In some regions, emissions are dominated by biomass burning; in others, fossil fuel combustion plays a larger role. This uneven distribution complicates efforts to measure and mitigate its effects.

A pollutant that changes as it travels

But the deeper difficulty is that black carbon does not stay the same.

Once released, particles begin to evolve mixing with other compounds, changing structure, and often becoming more efficient at absorbing sunlight. These processes are not always fully captured in models and are thought to contribute to differences between observations and simulations. In some regions, models underestimate how much sunlight black carbon absorbs by as much as a factor of two.

From the atmosphere to the Arctic

Black carbon’s climate effects also depend on where it ends up.

When deposited on snow and ice, it darkens surfaces and accelerates melting, a process with particular consequences in sensitive regions such as the Arctic. In the atmosphere, its climate effects remain difficult to represent in models. “It is very well established that black carbon on snow and ice have bigger effect than in the atmosphere. Same goes with a higher effect of black carbon above clouds than below,” says Gustafsson.

Fast action, uneven progress

There are, however, signs of progress. In parts of Europe, North America and East Asia, black carbon emissions have declined in recent decades, linked to air quality regulations and cleaner technologies.

Because black carbon is short-lived, reducing emissions can lead to relatively rapid benefits both for the climate and for human health.

At the same time, its overall climate impact remains debated. “Based on the latest IPCC report, the net climate warming effect of soot is not so large,” says Gustafsson.

Yet the new findings may shift that perspective. “One implication of these review findings of a larger climate effect of black carbon is that there is a larger impetus and pay-off for mitigating soot emissions” he adds. “In addition to reducing climate warming, it would also improve the quality of the air for billions of people, which is a great co-benefit for health.”

A clearer picture but not the full story

For researchers, the message is clear: black carbon remains one of the most complex and uncertain components of the climate system.

For policymakers, the message is more immediate. Unlike carbon dioxide, it does not linger for centuries. It is fleeting but powerful meaning that cutting emissions now could have a rapid effect.

Black carbon may be visible in the world around us but at the planetary scale, its role has remained partly obscured. This new synthesis brings that role into sharper focus, even as it reminds us how much there is still to understand.

About the study

Alongside Gustafsson, co-authors affiliated with Stockholm University include Sean Clarke, Manoj Remani and Hari Ram, both affiliated with the Department of Environmental Science and the Bolin Centre for Climate Research.

For more than two decades, Stockholm University scientists have operated strategically located observatories on remote ocean islands, capturing polluted air masses transported over long distances. These measurements have helped position the research groups at the forefront of efforts to understand how air pollution interacts with the climate system.

Read the review in Nature Reviews Earth & Environment