Now it’s rising fast. But we don’t really understand why. Is this caused by increased human energy use – gas leaks and coal mining? Or are new feedbacks happening, with warming feeding more warming?
Methane’s important. Including wider impacts, its warming impact on global surface air temperature is about 0.6 degrees Celsius since the year 1750, according to the new Intergovernmental Panel on Climate Change report. That’s roughly half carbon dioxide’s impact.
Methane grew strongly in the 1980s, then stabilised. In 2007, it began rising again, led from the tropics and sub-tropics, and at times the high north, with the strongest growth on record in 2020.
Its new rise threatens the United Nations’ Paris Agreement. So, at Glasgow’s COP26 climate summit, over 100 countries agreed to the Global Methane Pledge, requiring 30 per cent cuts in emissions by 2030, to keep within reach the Paris goal of limiting warming to 1.5 degrees Celsius.
Methane’s lifetime in the air is about 9.1 years. It comes from both natural sources (roughly 40 per cent) like swamps, and human activity (about 60 per cent), such as leaks of natural gas, which is mostly methane.
Very roughly 600 million tonnes are emitted annually. Wetlands emit perhaps 200 million tonnes, cattle 115 million, landfills 70 million, fires 30 million. Fossil fuel emissions from the gas, coal and oil industries produce around 120 million tonnes annually.
Rotting vegetation in wetlands and peat bogs makes methane both in the tropics – especially the Amazon, Nile and Congo basins – and in Siberia and Canada, where beavers are magnificent methane machines, creating wetland and filling it with cut vegetation.
The stomachs of ruminants such as cows, sheep and deer are microbiologically like wetlands, so these animals breathe out methane. Other biologically-made, or ‘biogenic’, sources include landfills, sewage, and leaky biodigester facilities. ‘Pyrogenic’ sources are crop waste, grassland and forest fires, mostly human-lit.
A clue to why methane has been rising since 2007 comes from carbon’s ‘isotopes’. Carbon has two unradioactive, stable forms, or isotopes, carbon-12 and carbon-13, and radioactive carbon-14 which is used by archaeologists for dating.
Different methane sources have characteristic stable isotopic ‘signatures’, the ratio of carbon-12 to carbon-13. Methane from fossil fuels and fires has slightly more carbon-13, while biogenic methane has slightly less.
Ice cores show that for two centuries up to 2007, the carbon-13 in atmospheric methane was increasing, driven by fossil fuel emissions. But since 2007, when methane’s rise was renewed, the isotopic trend has reversed, now trending towards carbon-12.
That suggests biologically-made emissions – not fossil fuels – are driving recent growth, post 2007.
Why might biological emissions from wetlands and farming be rising, especially in the tropics where growth has been fastest, and also in the high north?
Wetland emissions increase with rising temperature. Rainfall too has increased recently in important parts of the moist tropics, both increasing wetlands and also supporting more grass and more cows, sheep and goats. So could methane feedbacks be at work, with warming feeding more warming?
Scottish scientists are leaders here. In the UK Natural Environment Research Council’s MOYA consortium, Carole Helfter and Ute Skiba, of the UK Centre for Ecology and Hydrology in Penicuik, have been working with Botswana’s Mangaliso Gondwe measuring emissions from the Okavango swamp. In Edinburgh University, Paul Palmer and Mark Lunt have used satellites to look widely over Africa. In Aberdeen, Pete Smith and Jo Smith are leaders in understanding global food systems.
MOYA teams have been flying aircraft at low level to measure emissions and isotopic signatures from wetlands, farming and fires in Senegal, Uganda, Zambia, Bolivia, and Arctic Scandinavia, tracking large summer emissions from Siberian wetlands.
They found some huge emissions. The Bangweulu wetlands in Zambia (where Dr Livingstone died) may make half as much methane as Britain, and Bolivian Amazonia perhaps more. We still do not know if biogenic emissions are increasing through climate warming feedbacks, but it looks likely.
The obvious response is to cut human fossil fuel emissions. Gas leaks are easily found but longer term cuts need reduced consumption, especially as pumping gas long distances or shipping liquefied natural gas emits both methane and carbon dioxide.
Hydrogen is an alternative, but it’s leaky, an indirect greenhouse gas, causes air pollution and affects ozone. Home heating by ‘non-greenhouse’ electricity may be better, from wind and solar (preferably on rooftops of warehouses, not arable fields).
Coal-mining countries like China, India, South Africa, Russia and Australia will suffer from climate change, but haven’t signed the Global Methane Pledge. Methane from coal mine vents can be removed. Fully ending coal use is necessary but political dynamite. Coal means jobs, despite the air pollution it causes. Scotland still fumes over Thatcher’s closures. That’s the debate now in South Africa, bravely trying to create new employment in renewables.
Cutting farming emissions is difficult. Methane removal may be feasible around dairy barns. But in many tropical nations, cattle are essential food sources and culturally vital. India has by far the most cows, while pasturelands feed Africa’s growing human populations. Giving up cattle would intensify crop farming and fertiliser use, with increased emissions and deforestation.
Nevertheless, much can be done. Widespread crop-waste fires emit methane and cause air pollution. Reducing burning is feasible. Emissions from tropical landfills can be cut by thin thin soil coverings. In Africa, agricultural emission growth reflects the demands of fast-growing populations and rising obesity. Female education, social support, and good governance all stabilise these.
Is there a methane emergency? Yes. But the Global Methane Pledge can succeed, given commitment and strong community support, within tropical countries as well as developed nations.
Professor Euan Nisbet led the UK MOYA ‘Global Methane Budget’ consortium. His lab at Royal Holloway, University of London, is funded by NERC, European Commission and UN Environment Programme