In 2020, the concentration of methane in the atmosphere jumped like never before. At least since the start of measurements in the 1980s The enigma was well worth an international team of researchers*, including French people from Laboratory of Climate and Environmental Sciences (LSCE)**, gets on it.
Methane is a powerful greenhouse gas. Although short-lived – only ten years in the atmosphere – it is the second most abundant greenhouse gas in the atmosphere, behind CO2 and has a much higher warming potential than this last. “Methane is responsible for 20% of global warming,” summarizes Philippe Ciais, researcher at LSCE.
A 50% increase between 2019 and 2020
After a little over a year of work, these scientists have just published their investigation in the scientific journal Nature. Spoiler: the conclusions are hardly encouraging. They tend to show how difficult the fight against climate change will be and reinforce this hypothesis. of a runaway climate. Clearly: this domino effect, feared by climatologists, which causes the increase in temperatures to lead to natural feedbacks, which in turn contribute to climate change.
First of all, Marielle Saunois, LSCE teacher-researcher who contributed to the study, begins by precisely posing the enigma of the start. “The concentration of methane has been increasing since it was measured and for a long time before that. In 2021, we exceeded 1,900 parts per billion (PPB). In other words, we have, in the atmosphere, 1,900 molecules of methane for one billion molecules of air. “Small quantity therefore, but big consequence.
It is therefore not so much the fact that the concentration of methane has continued to increase in 2020 that has intrigued the researchers. But much more the significant jump recorded that year, yet marked by the Covid-19 and the massive confinements. Global CO2 emissions had also fallen that year. “Those of methane have increased by just over 15 PPB against 10 PPB a year earlier”, continues Marielle Saunois. A 50% increase.
White cabbage on anthropogenic emissions
To unravel the mystery, this team of researchers reviewed, one by one, the different sources of methane emissions. Marielle Saunois begins by distinguishing the sources of anthropogenic emissions, that is to say those caused by human activities. “In this category, the main sources of emissions are linked to fossil fuels (extraction, distribution, consumption), but also agriculture (mainly the farts and burps of ruminants) or the treatment of waste”, recalls the researcher. If we add forest fires, which also emit their share of methane, these anthropogenic sources represent approximately 60% of total methane emissions each year. Did they explode in 2020? Not at all. They even fell slightly that year compared to 2019, which is explained by a slight decrease in activity in the fossil fuel sector. As for methane emissions linked to forest fires, “again, they were lower in 2020 than in 2019, a very dry year marked by major fires”, recalls Marielle Saunois.
In short, the key to the enigma is not there. Researchers then have two other major avenues to explore. The first is that of natural methane emissions. The remaining 40%. In large part, they are caused by micro-organisms in wetlands that produce methane to feed themselves,” explains Marielle Saunois. The second track is no longer about methane emissions, but about their nibbling. Be careful, you will have to hold on a little: once emitted into the atmosphere, the methane will be largely oxidized by photochemical reactions, in particular thanks in particular to hydroxyl radicals (OH), present in infinitesimal quantities in the atmosphere*** but which, despite everything, ensures 85% of the destruction of methane”. If this gas only has a lifespan of ten years, then these gluttons are the ones to thank.
Less pollution in cities… but more methane?
Was the year 2020 then marked by anomalies on one or the other of these two tracks? In other words, have there been increased emissions of natural methane? Or a decrease in the amount of OH in the atmosphere? Both, answers Philippe Ciais. Let’s start with OH radicals. “This chemical species is produced by pollutant emissions, in particular nitrogen oxides (Nox)”, continues the LSCE researcher. However, these Nox, which have a large share of responsibility in the air pollution of large cities, are mainly emitted by combustion engine vehicles. Engines which have precisely turned much less in 2020 due to the pandemic. In other words: less NOx, less OH in the air and methane, which offers a little extra in the atmosphere. “The various models that we have used have led to the conclusion of a 1.6% drop in the concentration of OH in the atmosphere, indicates Philippe Ciais. It may seem weak, but it still explains half the riddle. »
The other therefore is an increase in methane emissions of natural origin. Philippe Ciais explains it by the particularly hot and humid conditions, in 2020, mainly in the northern hemisphere, rich in wetlands, whether in the boreal region of North America, in western and eastern Siberia, in the Northern Tropics. This is one of the lessons learned by the researchers from the simulation models they used: “They showed, a priori, that a warmer and more humid climate [que le changement climatique devrait rendre plus fréquent à l’avenir] is capable of triggering, very quickly, a surplus of methane emissions of natural origin”, explains Philippe Ciais. Surplus of methane which therefore aggravates global warming a little more.
Two bad news for climate change
This runaway effect is the first bad news to take away from this study. The second relates to OH radicals since the study tends to show that by tackling nitrogen oxide emissions, we then weaken the main methane cleaner. So to hell with the fight against air pollution? Nope ! Philippe Ciais and Marielle Saunois see only one solution to the problem: that of drastically reducing anthropogenic methane emissions. Either, all the same, 60% of the problem…