More than half of methane growth can be attributed to lower hydroxyl sink
Photo: iStock Low nitrogen oxide pollution and warming wetlands likely drove global methane emissions to record high levels in 2020, according to a new study.
Global methane emissions reached roughly 15 parts per billion (ppb) in 2020 from 9.9 ppb in 2019, the study published in the journal Nature noted.
This increase occurred despite the COVID-19 lockdown, which brought the world to a standstill. In 2020, methane emissions from human activities decreased by 1.2 teragrams (Tg) per year, the study showed.
It was expected that the overall atmospheric methane growth rate would slow down, Shushi Peng, assistant professor with Peking University and lead author of the study, told Down To Earth. “However, observations from US National Oceanic and Atmospheric Administration Global Monitoring Laboratory (NOAA/GML) and Integrated Carbon Observation System (ICOS) both show the growth rate in 2020 is the highest during the period 1984–2020.”
“The study helps unravel a puzzle concerning why globally methane increased when many other greenhouse gases like carbon dioxide decreased during 2020,” Vinayak Sinha, associate professor at the Indian Institute of Science Education and Research, Mohali, told Down To Earth. He was not involved in the study.
Methane has more than 80 times the warming power of carbon dioxide over the first 20 years of its lifetime in the atmosphere.
The common sources of methane are oil and natural gas systems, agricultural activities, coal mining and wastes, according to the United States Environmental Protection Agency (EPA). Nitrogen oxide enters the atmosphere from exhaust gases of cars and trucks as well as electrical power generation plants.
Nitrogen oxide (NOx) can impact methane levels. In the troposphere — the upper part of the atmosphere — NOx combines with ozone to form hydroxyl radicals, Sinha explained.
These radicals, in turn, remove 85 per cent of methane annually from the atmosphere, Philippe Ciais, researcher of the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) and an author of the study, wrote on Twitter.
Hydroxyl radicals in the troposphere react with methane to form water and CO2. Less nitrogen oxide pollution means less hydroxyl and more methane, Ciais tweeted.
Ciais and researchers from China, Europe and the United States collected information on greenhouse gas emissions from coal mining, oil and gas production, agriculture and waste.
The team gathered data on emissions from fire and agricultural activities. They also estimated wetland methane and hydroxyl emissions using models.
Methane emissions from oil and natural gas decreased by 3.1 Tg per year compared to 2019. Contributions from coal mining decreased by 1.3 Tg per year, showed the study.
Fire emissions, too, dropped by 6.5 Tg per year. Globally, fire emissions appear to have fallen in 2020 compared to 2019, the researchers wrote in the study.
Further, contributions from the agricultural sector went up by 1.6 Tg per year. Wetland emissions rose by 6.0 Tg per year.
Northern Eurasia experienced warmer-than-usual conditions from early spring to late summer, the researchers wrote in the study.
Also, 2020 was wetter than usual. Precipitation over global wetlands showed a 2-11 per cent annual increase relative to 2019, the findings stated.
Water‐logged soils make conditions ripe for soil microorganisms, allowing them to produce more methane.
Nitrogen oxide levels fell by 6 per cent in 2020 from 2019, mainly from March-May. During this time, many countries in the northern hemisphere imposed COVID-19 lockdown measures.
This decrease was seven times more than the trend from 2005-2019. The contribution of hydroxyl radicals in removing methane decreased by roughly 7.5 Tg per year.
Roughly 53 per cent of the methane growth can be attributed to lower hydroxyl sink, and the remaining 47 per cent from natural sources, predominately wetlands, the findings stated.
“The results have significant implications for our ability to reliably predict methane changes in a future world with lower anthropogenic emissions of pollutants like nitrogen oxides and also if we have a wetter world,” Sinha explained.
The researchers call for more ambitious efforts to reduce methane emissions to achieve the Paris Agreement target of restricting global warming to 1.5 degrees Celsius above pre-industrial levels.
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