Reducing air pollution may inadvertently increase natural methane emissions from wetlands, a new study suggests. Researchers say the decline in global sulphur emissions as a result of clean air policies – coupled with the warming and fertilisation effects of increasing carbon dioxide levels in the atmosphere – is increasing emissions from areas like peatlands and swamps.
As a result, the study found that natural wetlands could emit an additional 20 to 34 million tonnes of methane a year.
“Well-meaning policies aimed at reducing atmospheric sulphur appear to be having the unintended consequence of lifting this sulphur ‘lid’ on wetland methane production,” says senior author of the study, Professor Vincent Gauci from the University of Birmingham.
“This coupled with increased CO2 means we have a double whammy effect that pushes emissions much higher.”
How does cutting sulphur emissions increase methane from wetlands?
Wetlands make up around 6 per cent of the planet’s surface and are the largest natural source of methane. Previous research has shown that methane emissions from these waterlogged soils have risen faster this century than even the most pessimistic climate predictions.
Methane is thought to be responsible for around 30 per cent of all human-caused global warming since the Industrial Revolution. Most emissions of this potent greenhouse gas come from human activities like the fossil fuel industry, landfill sites and agriculture.
But research has shown that policies to clean up our air – vital to reducing the long term health and climate effects of pollution – may also be increasing natural emissions. Sulphur in particular has a very specific effect.
“How has this happened? Put simply, sulphur provides the conditions for one set of bacteria to outmuscle another set of microbes that produce methane when they compete over the limited food available in wetlands,” Professor Gauci explains.
He says that conditions of acid rain sulphur pollution during the past century have been enough to reduce wetland methane emissions by up to 8 per cent.
“Now that clean air policies have been introduced, the unfortunate consequence of reducing sulphur deposition, which does have important and welcome effects for the world’s ecosystems, is that we will need to work much harder than we thought to stay within the safe climate limits set out in the Paris Agreement.”
The ‘complexity’ of our global climate system
The study is the latest to suggest that reductions in atmospheric sulphur may be driving global warming at a faster-than-anticipated rate.
Anti-pollution regulations for shipping were introduced in 2020, aimed at reducing sulphur dioxide and fine particles that are harmful to human health.
This reduction in atmospheric pollution over the ocean has been implicated in a kind of inadvertent geoengineering effect that led to more warming than expected. This phenomenon has come to be known as ‘termination shock’ but is still an area of deep scientific uncertainty.
A recent analysis by renowned climate scientist Professor James Hansen and colleagues concluded that both the impact of recent cuts in sun-blocking shipping pollution and the sensitivity of the climate to increasing fossil fuel use is greater than initially thought.
Lead author of the paper Lu Shen from Peking University says the wetland study also points to the “complexity” of the global climate system. It highlights why understanding these relationships is vitally important for scientists and policymakers.
“Representation of these complex biogeochemical interactions has not previously been well integrated into estimates of future methane emissions. We show that it is essential to consider these feedbacks to get a true understanding of the likely future of this important greenhouse gas.”
Professor Gauci adds that, while we do an excellent job of working out many aspects of climate change, there are still certain biogeochemical effects that haven’t been properly quantified.
Incorporating these known interactions, like that of reducing sulphur emissions, into how we understand our warming world is vital as these unexpected effects can reinforce or amplify otherwise considered climatically driven processes such as increasing emissions.
How can we mitigate the impact of rising wetland emissions?
Returning to a time when the atmosphere was even more heavily polluted by shipping fuel and particulates isn’t the answer. So what can we do to counteract increasing natural emissions?
Professor Gauci says people’s first response is usually to ask what can we change about wetlands to stop them from emitting more methane.
“That is absolutely what we shouldn’t be doing because they’re mostly natural ecosystems and they’re essentially doing what they have always done,” he explains, “they’re just responding naturally as other ecosystems would respond to changes in the atmospheric composition.”
With two-thirds of the methane emitted into the atmosphere coming from human sources, the answer lies in how much of the global methane budget our own emissions take up.
At COP26 in Glasgow in 2021, more than 150 nations signed up to the Global Methane Pledge. This international commitment aims to reduce human-caused emissions of methane by 30 per cent from a 2020 baseline, by 2030.
With 8 to 15 per cent of the allowable space for methane emissions taken up by natural wetlands, Professor Gauci says we need to have an analogous reduction in emissions through policymaking like this.
“The infrastructure for this sort of policy framework is there. It just needs to be enhanced.”
Solutions for the removal of methane from the atmosphere also exist – and they aren’t all via complicated technology. Previously, soil was thought to be the only terrestrial methane sink. But another study published last July also led by researchers from the University of Bristol found that tree bark plays a vitally important role.
“So if you were to plant more trees, you could increase removal of methane from the atmosphere,” Professor Gauci explains.
“That’s actually a win-win for biodiversity and for climate.”
