Gerard Rocher-Ros researches the water bodies' emissions of greenhouse gases.
Photo Credit: Mattias Pettersson
Scientific Frontline: "At a Glance" Summary
- Main Discovery: The unprecedented surge in atmospheric methane during the early 2020s was primarily driven by a temporary decline in hydroxyl (\(\mathrm{OH}^\bullet\)) radicals, which reduced the atmosphere's ability to break down the gas, coupled with increased natural emissions from wetlands due to wetter climate conditions.
- Methodology: Researchers synthesized data from satellite observations, ground-based measurements, and atmospheric chemistry datasets with advanced computer models to isolate variables, specifically integrating novel estimates for monthly methane emissions from running waters and wetlands.
- Key Data: The reduction in \(\mathrm{OH}^\bullet\) radicals during 2020–2021 accounted for approximately 80% of the year-to-year variation in methane growth, while the extended La Niña period (2020–2023) caused significant emission spikes in tropical Africa, Southeast Asia, and the Arctic.
- Significance: The study resolves the anomaly of the 2020s methane spike and demonstrates a complex feedback loop where reduced air pollution (specifically nitrogen oxides from transport) inadvertently extended methane’s atmospheric lifetime by limiting \(\mathrm{OH}^\bullet\) radical formation.
- Future Application: Global climate strategies must now incorporate the trade-offs between air quality improvements and methane persistence, necessitating upgraded monitoring systems for tropical and northern wetland emissions to correct predictive model deficiencies.
- Branch of Science: Atmospheric Chemistry and Biogeochemistry
- Additional Detail: The findings expose critical weaknesses in current climate models, which significantly underestimated the sensitivity of wetland and riverine ecosystems to climate variability and precipitation changes.
Methane rose at an unprecedent rate in the early 2020s. A new international study published in Science, with contributions from Umeå University, shows that this surge was driven primarily by a temporary weakening of the atmosphere’s ability to remove methane, combined with climate-driven increases in natural emissions in Africa, Asia, and the Arctic.
Methane is the second most important human-driven greenhouse gas after carbon dioxide. In the early 2020s, its levels in the atmosphere increased sharply, reaching a peak that researchers can now explain.
The atmosphere contains hydroxyl (\(\mathrm{OH}^\bullet\)) radicals that act as the main “cleaning agent”, breaking down methane. During the covid-19 lockdowns, emissions of nitrogen oxides and other air pollutants from transportation decreased. These pollutants are needed to form $\mathrm{OH}^\bullet$ radicals through chemical reactions involving sunlight, ozone, and water vapor.
When \(\mathrm{OH}^\bullet\) levels dropped, the atmosphere became less effective at removing methane, allowing it to accumulate faster.
Analyzing satellite observations, ground-based measurements, atmospheric chemistry data, and using advanced computer models, the researchers found a sharp decline in \(\mathrm{OH}^\bullet\) radicals during 2020–2021. This explains around 80 percent of the year-to-year variation in methane concentration growth. Fossil fuel emissions and wildfires only played a minor role.
La Niña affected methane levels
Gerard Rocher-Ros, Assistant Professor at the Department of Ecology, Environment and Geoscience at Umeå University and IceLab, contributed to the study by estimating monthly methane emissions from running waters.
“This study was a great puzzle, where scientists modelling methane fluxes from different sources and atmospheric models each brought one piece, and we had to figure out how to fit them together,” he says.
At the same time as levels of \(\mathrm{OH}^\bullet\) radicals declined, climate variability strongly amplified methane emissions from natural sources. An extended La Niña period from 2020 to 2023 brought wetter-than-average conditions across much of the tropics, expanding flooded areas and increasing methane emissions from wetlands and inland waters, which are the largest single methane source around the world at present.
The largest increases occurred in tropical Africa and Southeast Asia, while Arctic freshwaters also showed significant growth.
Weaknesses in current models
The findings expose important weaknesses in current methane emission models, many of which underestimated wetland emissions during this period.
“Our current models for methane in rivers are still primitive compared to other ecosystems. My group is working on newer approaches that hopefully can help advance science in this field, starting with the Arctic, where emissions are increasing fast,” says Gerard Rocher-Ros.
The publication in Science clarifies why the atmospheric methane burden rose so rapidly – and why it has recently slowed down a little bit. It also underscores that future methane trends will depend not only on emission controls, but also on air quality policies and climate-driven changes in the natural methane cycle.
“In particular, we should better monitor and understand how tropical and northern wetland emissions of methane respond to the Earth's climate, which becomes warmer and wetter,” says Philippe Ciais, lead author of the study from the Laboratoire des Sciences du Climat et de l’Environnement (LSCE) in France.
Reference material: What Is: El Niño, La Niña, and a Climate in Flux
Published in journal: Science
Title: Why methane surged in the atmosphere during the early 2020s
Authors: P. Ciais, Y. Zhu, Y. Cai, X. Lan, S. E. Michel, B. Zheng, Y. Zhao, D. A. Hauglustaine, X. Lin, Y. Zhang, S. Sun, X. Tian, M. Zhao, Y. Wang, J. Chang, X. Dou, Z. Liu, R. Andrew, C. A. Quinn, B. Poulter, Z. Ouyang, W. Yuan, K. Yuan, Q. Zhu, F. Li, N. Pan, H. Tian, X. Yu, G. Rocher-Ros, M. S. Johnson, M. Li, M. Li, D. Feng, P. Raymond, X. Yang, J. G. Canadell, R. B. Jackson, X. Yu, Y. Li, M. Saunois, P. Bousquet, and S. Peng
Source/Credit: Umeå University | Sara-Lena Brännström
Reference Number: chm020926_01
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