Reducing car activity helps reduce air pollution. But antagonistically, it also contributes to increasing the problematic concentration of methane, as shown by a study published in the journal Nature.
Reducing automotive activity: a double-edged sword
Does reducing car activity reduce air pollution? While it doesn't magically make all pollutants disappear from the atmosphere, data such as those from Airparif have proven during the major lockdown of 2020 that the sharp drop in road traffic had a clear influence on the concentration of nitrogen oxides (NOx). Counterintuitively, however, researchers have observed an increase in the concentration of methane in the atmosphere at several points around the world where transport activity had significantly reduced that year.
It is a study published in the journal Nature by Professor Shushi Peng of Peking University that helps us understand why. According to the study's data, the concentration of nitrogen oxide has decreased in the atmosphere during periods of confinement. This decrease in the concentration of nitrogen oxide has contributed to increasing that of methane: nitrogen oxide emissions lead to the presence in the air of “hydroxyl radicals” (OH), which neutralize in particular the methane present. When these nitrogen oxide emissions decrease, there would therefore be fewer hydroxyl radicals and therefore more methane.
The mysterious increase in methane in 2020
Researchers were surprised by a sharp increase in the concentration of methane (CH4, the same molecule as natural gas) in the atmosphere in 2020, despite the lockdowns associated with the Covid-19 pandemic. The slowdown in economic activity has resulted in a drop in methane emissions linked to the fossil fuel industry.
In a study published in the journal Nature, led by Professor Shushi Peng of Peking University, an international team of scientists revealed the key to the enigma, which can be explained by two phenomena. The first factor is a lower presence in 2020 of hydroxyl radicals (OH), the main ones responsible for eliminating methane in the atmosphere. These OH, present in infinitesimal quantities and with a lifespan of less than a second, are “the Pac-Man of the atmosphere: as soon as they see something, they eat it and they disappear”, explains French researcher Philippe Ciais.
Bad news for the climate
The second factor identified is linked to an increase in natural methane emissions from wetlands. Microorganisms present in marshes or peat bogs naturally produce this gas. These areas are “very sensitive to climate,” says Marielle Saunois, from the Laboratory of Climate and Environmental Sciences (LSCE). “Microorganisms will produce more methane in warmer and more humid climates.” In 2020, wetter and warmer conditions in the high northern latitudes and in the tropics of the Northern Hemisphere therefore explain a greater production of methane.
These findings point to a possible “runaway” warming, which could in turn produce more harmful emissions, fueling a vicious circle. The study also highlights the paradoxical effect of reducing pollution. “When you reduce NOx emissions by about 20%, you can make methane increase twice as fast, and that surprised us a lot,” notes Philippe Ciais.
Methane: a major challenge in the fight against global warming
Emission control programs are therefore likely to increase methane in the future. “So if we want to meet the objectives of the Paris Agreement, we will have to act even faster and even harder to reduce sources of methane,” argues Philippe Ciais.
Methane is the subject of particular attention in the fight against climate change because, even though it has a much shorter lifespan in the atmosphere (about ten years) than that of CO2, its warming power is much greater: 28 times more over 100 years. This gas is responsible for about a third of global warming since the Industrial Revolution. Its emissions are mainly (60%) linked to human activity, on which it is therefore possible to act: fossil fuel industry, agriculture, livestock farming, waste, etc. The rest comes from wetlands.
Further research and challenges ahead
An enigma remains, because the concentration of methane continued to increase in 2021. But researchers already have some leads: “it would seem, according to very preliminary results, that we still have a problem with OH,” says Philippe Ciais. The phenomenon could be explained by the only partial resumption of air transport – whose role in the production of NOx at high altitude is very important – and by still low transport activity in the United States and India.
In summary, while reducing automobile activity helps reduce air pollution, it also poses unexpected challenges for global warming. Efforts to balance NOx and methane emissions will need to be carefully calibrated to avoid unintended climate consequences.
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