A spike in the greenhouse gas methane in 2020 may be explained in part by a drop in nitrogen oxide emissions during the covid-19 pandemic
14 December 2022
A mysterious rise in methane levels in the atmosphere in 2020 may be partly explained by a drop in emissions of nitrogen oxide in the early stages of the covid-19 pandemic.
Methane, a potent greenhouse gas, is responsible for one-fifth of the atmospheric warming linked with human activity. It can be emitted by the production and transport of coal, natural gas and oil, or from biological sources, such as livestock.
Levels of methane in the atmosphere have been rising since 2007, but in 2020 they made their biggest annual jump since records began in 1983.
This rise was puzzling, as fossil fuel use dropped in 2020 due to a slowdown in human activity caused by the covid-19 pandemic. The International Energy Agency (IEA) said that the fossil fuel sector released less methane into the atmosphere that year than in 2019.
To investigate where the gas had come from, Shushi Peng at Peking University in Beijing, China, and his colleagues analysed the greenhouse gas inventories countries provided to the UN in 2020. These inventories included emissions data linked with each country’s agriculture, waste and fossil fuel industries.
The researchers found that global production of natural gas decreased by 3.8 per cent in 2020 and that global oil production fell by 7.9 per cent compared to the previous year. They used this information to estimate that methane emissions from these sectors would have dropped by 3.1 million tonnes compared to the previous year.
The researchers also found that methane emissions from the global waste sector decreased slightly in 2020 compared to the previous year, while those from the agriculture industry increased.
Peng says this suggests that the pandemic didn’t have as big an impact on the release of methane due to human activities as expected.
Methane is also produced in wetlands by microbes that live in low-oxygen environments. To estimate wetland methane emissions, the researchers devised a model using climate data that describes the effects of temperature and rain on wetland emissions.
They found that warmer and wetter weather in the northern hemisphere led to wetlands in that region producing 6 million tonnes more methane in 2020 compared to 2019. Wetter weather increases the area of wetlands and warmer weather increases the activity of methane-producing microbes.
But this rise in wetland emissions only accounted for about 47 per cent of the global rise in methane levels, says Peng.
The team then looked at the concentration of molecules called hydroxyl radicals in the atmosphere. Hydroxyl radicals remove about 85 per cent of methane in the atmosphere by reacting with the compound to form carbon dioxide and water.
Hydroxyl radical concentrations in the atmosphere are difficult to measure as they have a lifetime of under a second, says Peng. But they are strongly linked to the concentration of nitrogen oxides, which are produced by burning fossil fuels, so the team used data on global fossil fuel combustion to estimate hydroxyl radical levels.
The researchers estimated that hydroxyl radical concentration dropped by 1.6 per cent in 2020. The reduction compared with 2019 was as much as 6 per cent in April, May and June, when many countries in the northern hemisphere instigated lockdowns to suppress the spread of covid-19.
This drop in hydroxyl radical concentration may account for the other 53 per cent of the methane emission rise in 2020, says Peng.
He says the findings mean that methane reduction targets need to be made more ambitious to take into account that moves to reduce nitrogen oxide emissions – by cutting fossil fuels – will lead to methane remaining in the atmosphere for longer, because there are less hydroxyl radicals to react with it.
“This study underscores the need for urgent and rapid methane emissions reductions,” says Amy Townsend-Small at the University of Cincinnati, Ohio. “The findings suggest we could be heading toward even faster rates of increase in atmospheric methane.
“Because of the high short-term global warming potential of methane, this could cause even faster warming, leading to other tipping points like ice sheet destabilisation and sea level rise, and storm intensification,” she says.
Journal reference: Nature, DOI: 10.1038/s41586-022-05447-w
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