Puzzle solved.
Scientists have finally been able to explain why methane, a powerful greenhouse gas, mysteriously increased in the atmosphere in 2020. And the news is not good: paradoxically, it is the drop in emissions of a polluting gas which leads, in part, to the rise in methane.
We take stock of this study, published Wednesday in the journal Nature.
Why this study?
In 2020, the planet was mostly confined due to the Covid-19 pandemic.
The slowdown in economic activity then resulted in a drop in methane (CH4) emissions linked to the fossil fuel industry.
And yet… Despite this fall in the diffusion of CH4 into the atmosphere, the concentration of methane exploded that year (+50% compared to 2019).
Scientists therefore wanted to understand this paradox.
What did the researchers find?
In this study, led by Professor Shushi Peng of Peking University, an international team of scientists unveils the key to the enigma.
Two phenomena explain the counter-intuitive rise in methane in 2020.
A pollutant less present.
The first factor is a reduced presence in 2020 of hydroxyl radicals (OH), the main responsible for the elimination of methane.
"These are the Pac-Man of the atmosphere: as soon as they see something they eat it and they disappear," Philippe Ciais, a French researcher from the Commissariat for Atomic Energy and Alternative Energies, told AFP ( CEA) and co-director of the study at the Laboratory of Climate and Environmental Sciences (LSCE).
These OH are linked to emissions of nitrogen oxide (NOx), a pollutant resulting in particular from transport.
However, NOx emissions decreased during the confinements.
And, therefore, those of OH too.
Conclusion: when pollution decreases, methane increases.
“When we reduce NOx emissions by about 20%, we can increase methane twice as fast and that surprised us a lot,” said Philippe Ciais.
A more humid and warm climate.
A second factor explains the rise in CH4 two years ago: the increase in natural methane emissions from wetlands under the effect of global warming.
The micro-organisms present in swamps or peat bogs naturally produce this gas.
And in 2020, wetter and warmer conditions were seen in northern high latitudes and in the Northern Hemisphere tropics, the study authors note.
And how to explain the rise in methane in 2021?
The rise in methane continued in 2021 and even reached a record level.
Can the two phenomena observed in the study explain, again, this increase?
The enigma remains.
But the researchers have some clues:
“It would seem, according to very preliminary results, that we still have a problem at the OH level”, argued 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 weak transport activity in the United States and India.
How to fight against this phenomenon?
The paradoxical effect pointed out by this study is problematic since we now know that programs to combat emissions risk increasing methane in the future.
Even if it has a lifespan in the atmosphere much shorter (about ten years) than that of CO2, the warming power of CH4 is much higher: 28 times more over 100 years.
“So if we want to meet the objectives of the Paris agreement, we will have to act even faster and even stronger to reduce the sources of methane”, warned Philippe Ciais.