Peng Shushi's research team published the latest research in Nature to reveal the mechanism of the surge in atmospheric methane concentration in 2020

Methane is an important greenhouse gas
.
Since the Industrial Revolution, atmospheric methane concentrations have increased 2-fold, accounting for one-third
of all radiative forcing of greenhouse gases.
To achieve the greenhouse gas reduction targets of the Paris Agreement, it is urgent to limit global temperature rise to 2°C, reduce anthropogenic methane emissions and reduce atmospheric methane
concentrations.
Atmospheric methane concentrations have shown an accelerated growth trend in the past 15 years, and in 2020 even reached the maximum
recorded by site observations since 1984.
If atmospheric methane concentrations continue to accelerate in the future, it will pose serious challenges
to reducing emissions and achieving the Paris Agreement's temperature control goals.
Therefore, there is a need to answer the international question
of why atmospheric methane concentrations have accelerated in the near future.

The growth rate of atmospheric methane concentration is mainly controlled by anthropogenic source emissions, natural source emissions and atmospheric sinks, however, due to the uncertainty of these three estimates, it is still difficult to attribute the growth rate of atmospheric methane concentration, especially the contribution of atmospheric sinks is the most challenging
.
In 2020, due to the spread of the new crown epidemic and the implementation of epidemic prevention measures in various countries, anthropogenic methane emissions decreased, but atmospheric methane concentrations soared, providing the perfect "experiment"
to control anthropogenic emissions to study atmospheric methane concentrations.

Fig.
1 Observation curve
of atmospheric methane concentration.
Gray is the observation, orange is the trend line, and brown is the growth rate

In the face of the above challenges and opportunities, the team of Peng Shushi, a researcher at the School of Urban and Environmental Sciences of Peking University, cooperated with a number of domestic and foreign research institutions to explore the mystery
of the surge in atmospheric methane concentration in 2020 based on multi-source data such as atmospheric methane concentration observation, energy and agricultural inventory data, meteorological and remote sensing data, using wetland process models, atmospheric chemical transport models and atmospheric inversion models, combined with "bottom-up" and "top-down" greenhouse gas source and sink assessment methods.

The study found that anthropogenic methane emissions decreased by 1.
2 million tonnes and fire methane emissions by 6.
5 million tonnes
in 2020 compared to 2019.
Warmer and wetter climates increased methane emissions from wetlands by 2.
5-9.
4 million tonnes in 2020, mainly from high-latitude wetlands and northern tropical wetlands
.
However, the maximum increase in wetland emissions (9.
4 million tonnes) is still not enough to explain the surge in atmospheric methane concentrations in 2020, suggesting that methane atmospheric sinks may decrease
in 2020.

Atmospheric methane is removed
primarily by reacting with OH radicals in the atmospheric troposphere.
OH radical lifetime is extremely short and difficult to measure directly, which is the most difficult problem
to study the change of atmospheric methane sink.
The study found that the concentration of tropospheric OH radicals simulated by atmospheric chemical transport models decreased by an average of 1.
6% due to the reduction of nitrogen oxides emitted by the new crown epidemic in 2020, which coincided
with another change in OH radicals based on the inversion of three fluorinated gas concentrations in the atmosphere.
A decrease in atmospheric OH radical concentrations in 2020 resulted in a 7.
5 million tonnes
reduction in atmospheric methane sinks.

The study further found that the spatial pattern of methane emissions changes derived from the "top-down" assessment method was consistent with the "bottom-up" results, supporting that the increase in natural emissions was dominated
by the increase in wetland emissions.
Thus, half of the acceleration of atmospheric methane concentration growth in 2020 was attributed to an increase in natural emissions dominated by wetlands, and the other half to a decrease
in tropospheric OH radical concentrations.

The study provides new insights into understanding the global methane budget, reveals that wetland methane emissions cannot be ignored to achieve global temperature control goals, and recognizes that future methane reduction plans need to also consider changes in atmospheric methane lifetime caused by changes in anthropogenic pollutant emission trends such as nitrogen oxides, providing a scientific basis
for achieving the goals of the Paris Agreement and global methane emission reduction commitments.

Figure 2 Attribution
to the surge in atmospheric methane concentrations in 2020.
a.
attribution of accelerated growth in atmospheric methane concentrations in 2020 compared to 2019; b.
Spatial patterns of changes in methane emissions based on "top-down" atmospheric inversion methods; c.
Spatial patterns of dominant sources of change in methane emissions in 2020; d.
Spatial patterns of changes in methane emissions as estimated by the "bottom-up" approach

The findings, titled "Wetland emission and atmospheric sink changes explain methane growth in 2020," were published in Nature on December 15
, 2022.
Peng Shushi is the first author and co-corresponding author, and Dr.
Lin Xin of the French Climate and Environmental Science Laboratory is the co-corresponding author
.
At the same time, the journal Nature published a review article entitled "Cause of the 2020 surge in atmospheric methane clarified" by Professor George H.
Allen, which highly praised
the study.
The research was supported
by the National Natural Science Foundation of China (41722101, 41830643).