Countries are starting to take steps to cut human sources of methane emissions, but climate change is increasing emissions of the potent greenhouse gas from wetlands.
After years of taking a back seat to carbon dioxide, methane has seen a surge of attention as a potent greenhouse gas that must be reduced to meet climate targets. It is responsible for nearly a third of global warming so far. But just as many countries are starting to take steps to reduce methane emissions from fossil fuels and agriculture, climate change is causing methane emissions from wetlands and other hard-to-control natural sources to rise.
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| Wetlands have seen a mysterious spike in methane emissions over the past two decades blickwinkel/W. Pattyn/Alamy |
“We’re realising that the commitments being made [to reduce human-caused methane emissions] might need to be steeper than they already are to account for the wetland feedbacks,” says Benjamin Poulter at NASA.
Wetlands emit methane when microbes in them decompose organic matter in the absence of oxygen. The amount they emit depends on a complex set of factors, including temperature, water level and how much material there is to decompose. This makes wetland emissions the largest source of uncertainty in global methane estimates.
However, there is growing evidence they are a key source of the mysterious rise in methane seen since 2006; in the past two decades, methane in the atmosphere reached growth rates not seen since the 1980s, and a spike in 2021 was the largest increase on record.
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One such sign wetlands are involved is an increase in methane in the atmosphere from biological sources. Some models have also shown climate change boosting wetland emissions, as higher temperatures speed up microbes’ metabolisms and alter patterns of precipitation. But the strength of this wetland climate feedback remains unclear.
Now, Poulter and his colleagues have used 16 models of how methane is produced in the world’s wetlands to quantify changing emissions between 2000 and 2020 and understand what drove them.
They estimate that wetlands annually contributed between 134 million and 182 million tonnes of methane between 2010 and 2020, about a 4 per cent increase over the previous decade. This was mainly due to warming temperatures: for every 1°C rise in average temperatures over wetland, they saw a rise of about 4.6 million tonnes of methane. Rising CO2 boosts plant growth, which they found also played a role, as did changing patterns of precipitation.
“We’re only just getting to terms with the fact that current climate change is impacting wetland methane emissions,” says Poulter.
Even with this rise, emissions from wetlands were dwarfed by methane emissions due to fossil fuels, livestock agriculture, landfills and other direct human sources of methane, says Chris Wilson at the University of Leeds in the UK.
The most recent draft of the Global Methane Budget – an international collaboration that aims to regularly take stock of the planet’s methane emissions – attributes about 65 per cent of annual methane emissions between 2010 and 2019 to direct human sources. The rest are due to natural sources such as wetlands – although it attributes around a third of those “natural” emissions to human influence on those systems, for instance through warming or nutrient pollution.
The relatively small contribution from natural sources suggests a spike in methane from wetlands would not entirely cancel out efforts like the Global Methane Pledge, which has seen more than 150 countries promise to cut their methane emissions 30 per cent by 2030, says Wilson. However, if climate feedback continues to boost wetland methane emissions, we may need to cut even more. “We need to reach the targets that we’ve set and then plough on,” he says.
Drew Shindell at Duke University in North Carolina says the increased emissions from wetlands would make any methane emissions targets more difficult to achieve. However, he says it still only explains part of the rise in methane since 2006 and the spike since 2020. “That’s helping push everything up, but it’s not sufficient to explain what the atmosphere is showing.”
In a recent review, he and his colleagues attributed the sharp rise in methane since 2020 both to wetlands and to fossil fuels and agriculture. Another factor was the reduction in air pollution seen during the covid-19 pandemic – with that came a drop in reactive chemicals that break down methane in the air. “Most of the methane sources are going up,” he says.
Journal reference:
EGUsphere DOI: 10.5194/egusphere-2024-1584
This article has been shared from NewScientist under creative commons license. You may read the original article here.
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