Exploding craters and overflowing landfills are unexpected sources of methane
A new methane-monitoring satellite is turning a powerful eye on methane from oil and gas, helping spot rogue emissions. But the scale of the methane challenge goes way beyond fossil fuels.
Agriculture is the biggest anthropogenic source of methane worldwide, closely followed by leaks from oil and gas fields, and remains a major climate blindspot. And while 60% of global methane emissions comes from human activities, the remaining 40% comes from natural sources, including permafrost and wetlands, which are thawing rapidly and becoming increasingly waterlogged due to rising temperatures.
Methane fuels 20-30% of the heating the planet has experienced to date. Although shorter-lived in the atmosphere than carbon dioxide (CO2), methane has a global warming impact more than 80 times higher than CO2 over a 20-year period.
Here are some of the world's biggest hidden sources of this highly potent greenhouse gas.
Permafrost
In Russia's Yamal and Gyda peninsulas, mysterious giant craters have been appearing in the north Siberian permafrost. Elevated levels of methane have been detected in the water at the base of the craters. One theory suggests that methane may be bubbling up from deep pockets of gas where permafrost is thawing, from methane-producing bacteria, or from the ice itself. As the gas builds up under ice cover, it eventually ruptures explosively, hurling ice and earth for hundreds of metres and leaving massive scars behind. And while the precise reasons these craters are appearing in the Russian Arctic are not fully understood, what is clear is that, worldwide, the permafrost thaw resulting from climate change could become a huge source of methane.
Glacial meltwater
Glaciers around the world are melting rapidly due to rising temperatures and this is revealing unknown environments – and hidden methane stocks – that have remained hidden for thousands of years. A 2023 study by the University of Copenhagen found that methane concentrations in the meltwater of three glaciers in the Yukon territory of north-west Canada were up to 250 times higher than those in the atmosphere. This surprised scientists as it was previously thought that glacial methane emissions require oxygen-free environments, such as vast ice sheets. "The release of methane under ice is more comprehensive and much more widespread than we thought," write the authors. It is unclear what the global effect of this melting will be, they warn.
Hydropower
Hydroelectric dams and their reservoirs are one of the biggest sources of methane escaping from water, releasing the equivalent of almost one billion tonnes of CO2 each year. The methane comes from the decomposing matter at the bottom of reservoirs which is released when the water cascades through turbines that generate electricity. UK start-up Bluemethane is working to capture these methane bubbles to use as biogas for electricity generation and heating, and as fuel for vehicles.
Polluted rivers
Freshwater ecosystems such as rivers and lakes account for almost half of global methane emissions. A 2020 analysis of the rivers snaking through the New Territories, one of Hong Kong's lushest areas, revealed that the waters were supersaturated with high concentrations of methane, carbon dioxide and nitrous oxide. The more polluted the river was, the higher its emissions, the scientists found. Large amounts of carbon and nitrogen end up in rivers worldwide via pesticide runoff and these are broken down via anaerobic or aerobic respiration, releasing carbon dioxide, methane and nitrous oxide.
Cows
The methane emitted in cow burps often makes headlines, but despite being a well-known source of methane it has so far proved tricky to reduce. Agriculture – from rice paddies to livestock – makes up the largest human source of methane emissions on the planet, according to the International Energy Agency. And within farming, cattle are arguably the biggest offenders, with one Californian feedlot producing more methane than the biggest oil and gas fields in the state.
It's a difficult source of methane to address. Not only are governments reluctant to tell people what to eat, or farmers how to farm, but public datasets of livestock facilities are hard to come by, so knowing where to direct the gaze of remote, methane-tracking sensors or satellites is fraught. Many livestock operations are also dispersed over large areas, or packed into places where farming might not be the main emissions source, analysts point out.
Wetlands
Wetlands are the world's largest natural source of methane. As climate change leads to rising temperatures and erratic rainfall, these waterlogged soils are releasing methane into the atmosphere more rapidly. 2024 analysis by the Department of Energy's Lawrence Berkeley National Laboratory found that wetland methane emissions across the Boreal-Arctic region have increased 9% over the past two decades. The scientists found that between 2002 and 2021 wetlands in these regions released an average of 20 teragrams – or 20 trillion kg – of methane per year - equivalent to the weight of about 55 Empire State Buildings.
Landfill sites
Waste is the third large source of methane globally, after agriculture and energy, according to the International Energy Agency. Organic matter rotting in landfill sites emits large amounts of methane. A 2022 study revealed that a landfill site in Mumbai released about 9.8 tonnes of methane per hour, or 85,000 tonnes per year. Composting leftover scraps instead of sending them to landfill can help reduce the amount of methane released into the atmosphere.
Wildfires
As wildfires grow in frequency and intensity around the globe, tracking their greenhouse gas emissions – including methane – is becoming ever more important. These fires are a major source of methane pollutants. The amount of methane emitted from the US' 20 biggest fires in 2020 alone was seven times more than wildfires over the past 19 years.
And, methane continues to be emitted long after the flames die out, Nasa researchers in Alaska found. Methane hotspots were 29% more likely to occur in tundra that had been scorched by wildfire than nearby unburned areas.
Methane has long lived in the shadow of CO2 – but its impact on global temperatures is such that it is vital we understand it better.
