Warming global temperatures are providing ideal conditions for microbes to produce methane in freshwater wetlands. With that rise comes the threat to the carbon-sink status of these critical habitats.
Methane, a potent greenhouse gas, has contributed to about 30% of the anthropogenic rise in global temperatures since the Industrial Revolution kicked off in 1760.
Once in the atmosphere this gas is 80 times more effective at retaining heat over a 20-year period than carbon dioxide, says hydrogeologist, Associate Professor Bryce Kelly of the University of New South Wales.
Around 60% of methane emissions come from human activity. Using fossil fuels — ‘natural gas’ is mostly methane, landfill sites and agriculture. The remainder is from natural sources. The waterlogged soils of wetlands, inundated with water for at least part of the year, are the world’s largest natural source of methane.
Archaea — tiny methane factories
It’s all down to the tiniest and most ancient of organisms. Methane-making (‘methanogenic) ‘archaea’ are found in freshwater wetlands, peatlands, rice and ruminant (cattle) agriculture, landfills and wastewater, peatlands and termites.
Anywhere organic matter is decomposing in the absence of oxygen. The warmer it gets, the faster these microbes grow and divide, and the more methane they produce. It’s not quite as one-sided as it sounds, though. A warmer world is also good for the ‘methanotrophs’, methane-eating bacteria, that benefit from some of that methane production.
Luke Jeffrey up a flooded Melaleuca quinquenervia measuring tree stem methane fluxes towards the canopy. For many years, they measured CH4 fluxes from below 2m, but they are now scaling to new heights. Credit: Luke Jeffrey
‘Archaea’ means ‘ancient life’. They look like small bacteria (less than I micron, 1 millionth of a metre) but have different cells walls. Archaea and some bacteria make methane from carbon dioxide and hydrogen. These organisms have been around at least 2.5 billion years and archaea include species found in the most extreme environments on earth, ‘extremophiles’, loving intense cold and heat, salty, acidic, alkaline, just about any conditions imaginable and often in combination.
Wetlands: critical carbon sinks, and methane producers
Wetlands cover around 6% of the earth’s surface, existing anywhere there are waterlogged soils. Most methane comes from freshwater wetlands, relatively little out of saltwater ecosystems, such as mangroves.
“Tides deliver sulphate twice a day to mangrove soils, says wetland biogeochemist, Dr Luke Jeffrey of Southern Cross University. Microbes adapted to use sulphur compounds outcompete methane-producers, meaning that mangroves produce very little methane, when you compare them with freshwater, forests and wetland ecosystems.”
“Mangroves and salt marshes, so called ‘blue carbon’ are fantastic because they do capture lots of carbon and store that in their soil.”
Wetlands in general are critical carbon sinks, storing carbon and buffering against climate change as plants grow and sediment, rich in organic material, gets deposited through tidal action and runoff. The Ramsar Convention on Wetlands of International Importance describes wetlands as the most important carbon sinks on Earth. A ‘sink’ means more carbon is stored than is lost, in this case through methanogenesis.
That may be changing.
As organic matter is broken down in wetlands, the methane that archaea produce is lost to the atmosphere in three main ways. Two of them are straightforward: diffusion through soil and water, and release from plant surfaces.
Wetland tree stems host both methanogenic archaea and methanotrophic bacteria in and on their three-dimensional bark, but more gas is produced than consumed.
“In paperback, for instance, we estimate that about a third of the methane that would otherwise have made its way to the atmosphere is actually being consumed by methanotrophs that live in bark,” says Jeffrey.
The third way is bubbles, often seen popping through the swamp water’s surface. Eerie ‘will-o’-the-wisps’, those flickering, hovering lights seen in marshes at night, are the bubbling gas spontaneously catching fire. Sometimes ‘rotten-eggs’ are also smelt as traces of hydrogen sulphide gas hitch a ride.
Biogeochemist, Dr Ralf Aben of Radboud University recently estimated that such bubbling, which he and his coauthors call ‘ebullition’, could increase between 6 and twenty percent per degree of global warming.
Increased methane emissions have already been reported. Global average wetland emissions increased by around 8-10m (million) tonnes each year between 2007 and 2021, compare to a 2000-2006 baseline writes climatologist Professor Zhen Zhang of the Chinese Academy of Sciences and his international team of collaborators.
These emissions have since accelerated, with an estimated 14-26m tonnes released in 2020 and 13-23m tonnes in 2021. Tropical wetlands contributed much of that increase. Zhang says projected increases in biogenic methane could offset reductions in fossil fuel emissions.
Geoscientist, Dr Mark Lunt, from the University of Edinburgh agrees, “Emissions from a wetland-climate methane feedback have the potential to offset any reductions made through agreements like the Global Methane Pledge”.
Why wetlands are giving off more methane is unclear. Jeffrey says it could be due to direct heating as temperatures rise, or the expansion of these ecosystems as more soil gets waterlogged with increasing in rainfall. Warmer air holds more moisture which leads to more rain.
Upland forests may offset some of that methane rise. These forests account for about 80% of all those on Earth, says Jeffrey, and could be a net sink for methane.
Clarifying ‘upland’, he says that just means well-drained, not on top of a mountain.
These forests, when on aerobic (aerated), well-drained soil, contain methanotrophic bacteria. But where’s the methane?
“Some studies suggest it’s produced within wet wood, or possibly rotting heartwood of bigger, older trees, where methanogens have been found. Others have suggested that perhaps these big, old upland trees can draw methane from deeper in the groundwater table, and maybe it is being transported from the subsurface soil.”
“Even though the surface of these soils appears to be a methane sink, perhaps some of that methane is bypassing oxidation in the soil, and coming up through the tree transpiration system,” says Jeffrey.
Biogeochemist Professor Vincent Gauci of the University of Birmingham calculated that upland tropical, temperate and boreal — the cold temperate birch, poplar and conifer forests south of the Arctic Circle, could take up 25 to 50m tonnes of methane globally.
He and his international team of researchers estimated that total global woody surface area could be around 143 million km2, approximately equal to the global land surface area (149 million km2). That’s a lot of methanotrophs!
Finding solutions to rising wetland methane
Planting more trees would be a good start. But draining wetlands is completely illogical, says Jeffrey, because there’s so many other important ecosystem functions other than just the net carbon balance.
“Givers of life, vegetation, biodiversity, they help with floods and nutrient runoff and things like that.”
Perhaps the biggest global issue with wetlands is anthropogenic disturbance, mainly excess nutrients causing eutrophication, he says.
Nutrients and runoff increase production of algae and other organisms, which boosts microbial methane production. “We’ve doubled the global available nitrogen budget through fertiliser production, so we’ve added a lot of extra nutrients into the global nutrient cycle.”
But there is still hope in this battle to deal with rising wetland methane.
“I’m always hopeful”, says Jeffrey. “I think there’s lots of people doing lots of good things out there. They often don’t always make the headlines, because often we only hear the bad, bad stories. But I’m hopeful that people will never give up on trying to find solutions to do things better.”