It is little known but omnipresent, saturating the atmosphere of many Asian countries, invading the oceans and melting the glaciers of the Himalayas and the Arctic ice floe: “black carbon”. But then, what exactly is this carbon soot? “It is 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.
What are the health risks of “black carbon”?
This soot only remains in the atmosphere for a dozen days but it strongly affects the health of populations exposed to it, particularly in South Asia and Africa. “These are fine particles which can penetrate quite deeply into the lungs” and “fine particles in general are linked to certain cancers and heart diseases”, underlines Bertrand Bessagnet, air quality manager at ICIMOD (Centre international integrated mountain development) in Kathmandu (Nepal) and author of a thesis on carbon aerosols.
Inserm researchers showed that this pollutant was associated with a 30% increase in the risk of lung cancer, in a study published in 2021.
What are the consequences of “black carbon” on glaciers?
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 Program (UNEP). “It has to do with 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, the “black carbon” is deposited at the top of the glaciers of the Himalayas, and all the way to the poles. Once covered with soot, these normally white surfaces lose their “albedo effect”, that is to say their ability to reflect solar radiation.
In the French Alps, “black carbon”, combined with dust from the Sahara, accelerates the melting of snow: it has reduced the snow cover period by 17 days on average over the last 40 years. It also brings forward the spring meltwater peak, according to a study published in 2021 in Nature Communications. “It only takes small quantities of black carbon for there to be an impact on the color of the snow and therefore on the melting,” explains Marie Dumont, head of the snow study center (CNRS/Météo-France) and co-author of the study. This early melting “disturbs the balance of fragile mountain ecosystems” and can also impact agriculture and hydroelectric production, she points out.
And on the oceans?
Emitted in particular by cruise ship engines, black carbon also has a “disproportionate impact” on the melting of the Arctic Ocean sea ice, according to Sian Prior, advisor to the Clean Arctic Alliance (CAA). This gathering of 23 NGOs is calling for the adoption of binding regulations in order to drastically reduce “black carbon” emissions from ships sailing in the Arctic. Black carbon also contaminates all of the world’s oceans. The quantity that flows there each year “is two to ten times higher than the mass of plastic that reaches the ocean”, underlines Xavier Mari.
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Its massive presence modifies, according to the researcher, “the efficiency of the biological carbon pump”, a mechanism which contributes to storing, in 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. “We find it everywhere, even in babies who have just been born, because it passes through the placenta,” emphasizes Xavier Mari.
