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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 there new feedbacks that allow for more warming to occur?
Methane’s important. The warming effect on global surface temperatures is a wider impact, at 0.6 degrees Celsius, since 1750, according the Intergovernmental Panel on Climate Change report. That’s roughly half carbon dioxide’s impact.
In the 1980s, methane experienced a strong increase and then stabilized. It started rising again in 2007 from the tropics to the subtropics.
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 can be found in both natural sources (roughly 40%) and human activity (about 60%), which includes natural gas leaks that are mostly methane.
Annually, approximately 600 million tonnes of carbon dioxide are released. Wetlands release around 200 million tonnes annually, cattle emit 115million tonnes, landfills 70million, and fires emit 30 million. Around 120 million tonnes of fossil fuel emissions are produced annually by the oil, gas, and coal industries.
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 microbiologically similar stomachs of ruminants, such as cows and sheep, are wetlands. These animals exhale 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’. Two stable, radioactive forms of carbon are used by archaeologists as dating tools.
Different methane sources have characteristic stable isotopic ‘signatures’, the ratio of carbon-12 to carbon-13. Biogenic methane has slightly more carbon-13 than methane derived from fossil fuels and fires.
Ice cores reveal that the carbon-13 content of atmospheric methane increased over two centuries, largely due to 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 is it that biological emissions from wetlands, farming, and other activities are rising, particularly in the tropics and the high north, where growth has been most rapid?
Rising temperatures lead to an increase in wetland emissions. Rainfall has also increased in important areas of the moisttropics in recent years, increasing both the amount of wetlands and supporting more grass, more sheep, and more goats. Is it possible that methane feedbacks are at work? With warming feeding more warming?
This research is being led by Scottish scientists. 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. Paul Palmer and Mark Lunt, both from Edinburgh University, have used satellites in order to view Africa from afar. Jo Smith and Pete Smith are the leaders in understanding global food systems in Aberdeen.
MOYA teams flew aircraft at low levels to measure emissions and isotopic signatures of wetlands, farming, and fires in Senegal, Uganda. Zambia, Bolivia, Arctic Scandinavia, and tracking large summer emissions from Siberian wetlands.
They discovered some massive emissions. The Bangweulu wetlands of Zambia, where Dr Livingstone died, may produce half the amount of methane as Britain and possibly more than Bolivian Amazonia. While we do not know whether biogenic emissions are increasing as a result of climate warming feedbackss, it appears likely.
The obvious response is to reduce fossil fuel emissions. Leakages can be found easily, but long-term cuts will require reduced consumption, especially because pumping and shipping liquefied gas long distances emits both methane & 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. It is possible to remove methane from coal mine vents. It is not possible to end coal use completely without political dynamite. Despite the pollution it causes, coal provides jobs. Scotland still fumes over Thatcher’s closures. That’s the debate now in South Africa, bravely trying to create new employment in renewables.
It is difficult to reduce farming emissions. It may be possible to remove methane from dairy barns. Cattle are vital food sources in many tropical countries, and they are culturally vital. India has by far the most cows, while pasturelands feed Africa’s growing human populations. With increased crop cultivation and fertiliser use, and increased emissions, giving up cattle would accelerate crop farming and deforestation.
However, there are still many things that can be done. The widespread burning of crop-waste in large quantities can cause methane to be released into the atmosphere, which can lead to air pollution. It is possible to reduce burning. Thin soil covers can reduce the amount of greenhouse gases released from tropical landfills. Africa’s agricultural emission growth is a result of rising obesity and fast-growing populations. These are stabilized by education for women, social support and good governance.
Is there an emergency methane situation? Yes. The Global Methane Pledge could succeed if there was strong community support in tropical countries as well.
Professor Euan Nisbet led the UK MOYA ‘Global Methane Budget’ consortium. His lab at Royal Holloway University of London was funded by NERC (European Commission) and UN Environment Programme.
