"It's the oldest pollutant in the world. The first human who domesticated fire in his cave, he breathed black carbon," smiles Xavier Mari, biogeochemist, research director at the Research Institute for Development (IRD) in Bangkok.
Contributing to atmospheric pollution, "black carbon" is formed during the incomplete combustion of biomass (wood, green waste) or fossil fuels (coal, oil, gas).
Coming from domestic heating and cooking (43% of global emissions), transport (23%) and industry (11%), its emissions have increased tenfold since the start of the industrial revolution. They have declined slightly since a peak in the early 2010s.
This soot only remains in the atmosphere for about a dozen days but it seriously affects the health of populations exposed to it, particularly in South Asia and Africa.
"These are fine particles that can penetrate quite deeply into the lungs" and "fine particles in general are linked to certain cancers and heart diseases", underlines Bertrand Bessagnet, head of air quality at ICIMOD (International Centre for Integrated Mountain Development) in Kathmandu (Nepal) and author of a thesis on carbon aerosols.
Inserm researchers have shown that this pollutant was associated with a 30% increase in the risk of lung cancer, in a study published in 2021.
1.500 times more powerful than CO2
Harmful to health, black carbon also has a warming power up to 1.500 times greater than CO2, according to the Climate and Clean Air Coalition (CCAC) of the United Nations Environment Programme (UNEP).
"It's linked to the color of the particle. It's black, so it absorbs light and stores it in the form of heat," explains Xavier Mari.
Carried by the wind, "black carbon" is deposited on the top of the glaciers of the Himalayas, and all the way to the poles. Once covered in soot, these normally white surfaces lose their "albedo effect", that is to say their capacity to reflect solar radiation.
In the French Alps, "black carbon", combined with dust from the Sahara, accelerates snow melting: it has reduced the snowfall period by an average of 17 days over the last 40 years. It also advances the spring meltwater peak, according to a study published in 2021 in Nature Communications.
"Small quantities of black carbon are enough to have an impact on the colour of the snow and therefore on melting," explains Marie Dumont, head of the snow study centre (CNRS/Météo France) and co-author of the study.
These early melts "disrupt the balance of mountain ecosystems which are fragile" and can also impact agriculture and hydroelectric production, she points out.
A note of "hope" however: black carbon emissions have declined in Europe since the 2000s, which has slightly offset the effect of global warming on snow melt.
"We find them everywhere"
Emitted in particular by cruise ship engines, black carbon also has a "disproportionate impact" on the melting of the Arctic Ocean ice cap, according to Sian Prior, advisor to the Clean Arctic Alliance (CAA). This gathering of 23 NGOs is calling for the adoption of binding regulations to drastically reduce "black carbon" emissions from ships sailing in the Arctic.
Soot carbon also contaminates all of the world's oceans. The amount that flows into them each year "is two to ten times higher than the mass of plastic that reaches the ocean," Mr. Mari points out.
Its massive presence modifies, according to the researcher, "the efficiency of the biological carbon pump", a mechanism which contributes to storing, over the long term, the vast quantities of CO2 absorbed each year by the oceans.
Soot carbon also enters the food chain, from zooplankton to mammals. "It's everywhere, even in newborn babies, because it passes through the placenta," Mari says.