Scientists have discovered why atmospheric methane jumped in 2020

​In 2020, the concentration of methane in the atmosphere increased by 15.1 parts per billion (ppb). This is the largest increase since atmospheric measurements began in the 1980s.. This dramatic increase is explained in the new study published in Nature, led by Professor Shushi Peng of Peking University in China. With a team of scientists from CEA, UVSQ and CNRS working at Laboratory of Climate and Environmental Sciences (LSCE) in France and American and Norwegian co-authors, work has been carried out using inventories to assess methane emissions from fossil fuels and agriculture, and with ecosystem models to predict emissions related to areas damp and fires. These results are supplemented by inverse modeling of regional emissions based on measurements of atmospheric concentrations.

Two factors explain the sharp increase in atmospheric methane in 2020. We have combined different methods to understand this phenomenon, which constitutes a life-size experiment and which sheds new light on the global methane balance. explains Philippe Ciais, researcher at the CEA who co-directed the study at the LSCE.

First, the presence of hydroxyl (OH) radicals, the main culprits for removing methane from the atmosphere, decreased in 2020. More methane therefore remained in the atmosphere. The decrease in OH is mainly explained by a decrease in nitrogen oxide (NOx) emissions induced by the temporary reduction of emissions in polluted regions, during the confinements of the COVID-19 pandemic. This is therefore one of the main reasons explaining the abnormally high increase in the concentration of methane in the atmosphere and contributing about half of it.

In 2020, researchers also found an increase in natural methane emissions from wetlands. This is explained by the wetter and warmer conditions observed in the high northern latitudes and in the tropics of the northern hemisphere. This effect explains the second half of the increase in methane.

This is potentially two bad news for climate change », declares Marielle Saunois, expert in the methane cycle at the LSCE.

First, the efforts made under the Paris Agreement to reduce the use of fossil fuels, together with the adoption of measures to reduce air pollution, will probably reduce NOx emissions in the ‘coming. In view of what was observed in 2020, the reduction of NOx could in turn accelerate the rise in the concentration of methane. We will therefore have to adopt even more restrictive mitigation measures on methane.

Additionally, according to Ben Poulter, a researcher at NASA’s Goddard Space Flight Center and co-author of the study, ” the year 2020 indicates that climate change is already impacting wetlands, where changes in rainfall and warming trends are increasing methane emissions that could amplify global warming in the future “.

The study showed that emissions from wetlands are very sensitive to climate variability. Thus, increased methane emissions in regions containing tropical and northern wetlands where precipitation is expected to increase in the future could amplify global warming.

Key points

  • In 2020, during the periods of containment of the COVID-19 pandemic, the slowdown in human activities had been accompanied by a reduction in methane emissions in the fossil fuel sector, as reported by theInternational Energy Agency (IEA). At the same time, emissions from the agriculture and waste sector remained stable. Overall, compared to the year 2019, the study showed that anthropogenic methane emissions decreased by 1.2 Tg CH4 yr-1. Therefore, anthropogenic emissions alone would contribute to a slowing down and not an acceleration of the concentration of methane, as observed in the atmosphere.
  • The year 2020 was relatively wet, except in North America and the Amazon. The study showed that emissions from forest fires decreased by 6.5 Tg CH4 yr-1 compared to 2019. However, 2019 had been a particularly extreme year in terms of fires, with an excess burnt surfaces and emissions, particularly in Amazonia and Australia.
  • Due to a warmer and wetter climate over wetlands in 2020, simulations of two wetland ecosystem models predict an increase in wetland emissions of 6.0 ± 2.3 Tg CH4 yr-1 , mainly in the boreal region of North America, western and eastern Siberia, and the northern tropics. The higher wetland emissions simulated by the two models, using different climate datasets, explain about 42% of the anomalous increase in atmospheric methane concentration in 2020, compared to 2019.
  • The biggest challenge was to quantify in 2020 the changes in atmospheric hydroxyl radicals (OH), the main destroyer of methane in the atmosphere, which removes about 85% of global methane emissions each year. These extraordinarily reactive radicals are present in minute quantities in the atmosphere. Their lifetime is less than one second and it is impossible to measure their concentration directly. Didier Hauglustaine, research director at LSCE, simulated the changes of OH in the atmosphere using the 3D digital chemistry-transport model INCA for which anthropogenic emissions of carbon monoxide, hydrocarbons and NOx were prescribed. in 2020. These emissions affect the production and disappearance of OH in the atmosphere. Carbon monoxide and NOx emissions from anthropogenic combustion processes have been inferred from fossil CO2 emissions estimated daily since 2019 by the Carbon Monitor project ( Wildfire emissions are from the Global Fire Database ( GFED4) Overall, surface NOx emissions from anthropogenic combustion fell by 6% in 2020, with the largest reductions occurring in the first half of the year, when containment measures have been imposed in many countries in the northern hemisphere (e.g. January to March for China and April to June for the United States, Europe, India, etc.). aircraft NOx emissions into the upper atmosphere virtually ceased during the peak of the pandemic.
  • From the results of the INCA atmospheric chemistry model, the researchers obtained a 1.6% decrease in tropospheric OH, mainly due to the reduction in NOx pollution (Uncertainty 1.6 ± 0.2) . The result of this simulation is supported by an independent estimate using atmospheric measurements of three fluorinated gases removed from the atmosphere by hydroxyl radicals. Although the magnitude of the decrease in OH in 2020 estimated by the two independent approaches seems small at first glance, it actually translates into an anomalous increase in the methane concentration of 7.5±0.8 Tg CH4 an- 1 in 2020, which explains about half of the observed growth rate in the atmosphere.
  • However, emission estimates from statistical inventories and ecosystem models for wetlands, as well as the weakening of the atmospheric methane sink due to the decrease in OH did not allow to close the methane budget, leaving a missing source of more than 5.3 Tg CH4 yr-1 to explain the observed increase in atmospheric methane in 2020, even taking into account the high value of the estimated increase for emissions in wetlands.
  • The reconciliation was carried out using observations of atmospheric methane from surface measurement stations and the GOSAT satellite. This approach, called “inverse modelling”, establishes a reconciliation between inventories and atmospheric measurements, using a 3D model of atmospheric chemistry-transport. The results of the inverse modeling show that 47 ± 16% of the record methane growth rate in 2020 is explained by higher natural emissions, mainly from wetlands – although higher emissions from Arctic soils could also have been explained. occur ; and 53 ± 10% by reduction of methane removal by OH.
  • For 2021, other elements of explanation could explain the persistent anomaly of the increase in the concentration of methane. Indeed, the decrease in OH could a priori be less in 2021 due to the rebound in emissions, and probably also in 2022.
  • Methane emissions related to wetlands can increase rapidly during warm and humid years, as observed in 2020. These natural emissions are not reported by national inventories which only cover emissions from human activities, such as agriculture, waste and fossil fuels. The positive feedback between wetland methane releases and climate is still poorly understood and could be a barrier to global warming mitigation in the Paris Agreement.
  • In addition, improving air quality with a reduction in NOx emissions could increase the lifetime of methane in the atmosphere: further reductions in methane emissions would therefore be needed to achieve the target of Paris Agreement. To implement the goals of the Global Commitment on Methane, launched at COP26 in November 2021 by 150 countries that pledged to collectively reduce methane emissions by 2030 by at least 30% compared at 2020 levels, therefore, trends in anthropogenic emissions of NOx and other pollutants that alter the lifetime of atmospheric methane will also need to be taken into account.

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Scientists have discovered why atmospheric methane jumped in 2020


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