Global average atmospheric methane growth rates spiked between 2020 and 2022, and the culprits appear to be methane-excreting microbes that thrive in wet environments, both natural and man-made, according to new research conducted by a team of scientists from several countries
Their paper, published last month in PNAS (Proceedings of the National Academy of Sciences), delivered new insights into a dangerous ongoing trend.
“Methane emission rates continue to rise along the most extreme trajectory used in emission scenarios by the world’s leading climate scientists,” warned the Global Carbon Project at Stanford University in a September media release. The urgency owes to methane being a climate-buster about 84 times more potent than carbon dioxide over a 20-year span.
Annual growth in atmospheric methane began to climb steadily in the early 2000s, reaching roughly 5.3 parts per billion per year between 2008 and 2014, then surging to 9.2 parts per billion per year between 2014 and 2020. Things really spiked after that, with 2020, 2021, and 2022 showing record annual growth of 15.2, 17.9, and 13.1 parts per billion in each year, before falling below 8 ppb (but still rising overall) between July, 2023 and July, 2024.
By mid-summer this year, atmospheric methane stood at 1921.33 ppb, according to the Global Monitoring Laboratory at the U.S. National Oceanic and Atmospheric Administration (NOAA).
The methane spike galvanized atmospheric researchers around the world, including the University of Colorado, Germany’s Max Planck Institute for Biogeochemistry, New Zealand’s National Institute of Water and Atmospheric Research (NIWA), and Tohoku University in Japan (TU).
“When we saw how drastic the change was in 2020-2022, we reached out to our friends at Max Planck, NIWA, and TU and they were seeing it, too. We wanted to get this news out there,” lead author Sylvia Michel told The Energy Mix.
Michel is a senior research assistant at the Institute of Arctic and Alpine Research, University of Colorado, Boulder, and a doctoral candidate at CU Boulder’s Department of Atmospheric and Oceanic Sciences.
Microbe Metabolism Responsible
Michel’s team, with colleagues from the NOAA Global Monitoring Lab, found that the sudden spike in methane emissions in 2020-2022 owed largely to methane-producing microbes that thrive in wet environments like wetlands, but also human-made rice paddies, landfill sites, and reservoirs. That is, the methane surge did not originate with leaky fossil fuel infrastructure, as previously claimed.
“Some prior studies have suggested that human activities, especially fossil fuels, were the primary source of methane growth in recent years,” PNAS article co-author Xin Lan told CU Boulder Today. “These studies failed to look at the isotope profile of methane, which could lead to a different conclusion and an incomplete picture of global methane emissions.”
Lan is a carbon cycle scientist with the Cooperative Institute for Research in Environmental Sciences of University of Colorado Boulder and NOAA Global Monitoring Laboratory.
The new research redresses that data gap. Using isotope ratio mass spectrometry, Michel and her team were able to distinguish between methane molecules generated as a waste product of microbe metabolism and “fugitive” ones escaping from leaky fossil infrastructure, on the basis of how many isotopes of “heavy” carbon, known as C13, they carry.
The “carbon signature” of methane produced by microbes contains fewer C13 isotopes and is correspondingly “lighter” than the carbon signature of methane produced by fossil fuels.
Putting the relative lightness of microbial methane alongside findings that even as levels of atmospheric methane surged, the overall carbon signature of that methane was growing lighter, the researchers concluded that “only large increases in microbial emissions could explain both the rising methane and its changing weight,” explains the Washington Post.
A Warmer, Wetter World
Just what caused microbial methane production to skyrocket between 2020-2022 is a complex question.
“Methane production by microbes increases with increasing temperature, so you would expect emissions to increase from all microbial sources exposed to ambient temperature as climate warms, if that were the only variable affecting methane production,” methane expert and retired NOAA scientist Ed Dlugokencky told the Mix.
But microbial activity in natural environments like wetlands also increases with precipitation, Dlugokencky said. Thus, the 2020-2022 spike in atmospheric methane could also correspond to the La Niña phase of the El Niño-Southern Oscillation cycle “when more tropical rain than average typically falls on land rather than the ocean.”
Satellite data over some of the world’s tropical wetlands during the period of study strongly supports this hypothesis, he added.
And there are signs that the return of El Niño in 2023 may have reduced microbial activity in wetlands.
“The 2023 CH4 (methane) growth rate is lower than 2020-2022, which could be related to the El Niño weather pattern that caused dry conditions over the global wetland area and thus reduced wetland CH4 emissions,” co-author Lan told The Mix.
However, Lan was quick to add that El Niño’s sweltering heat helped take carbon dioxide emissions to record levels last year.
A dangerous ‘climate feedback’?
The new findings point to an unsettling possibility. If much of the spike in microbial methane comes from wetlands, a dangerous feedback loop could already be under way—namely, that climate-induced warming (and periodic waterlogging) of these natural ecosystems is causing resident microbes to pump out more methane, which will cause further warming, further enabling higher CH4 production in the microbes.
However, microbial methane is also produced in human-generated sources like rice paddies, landfills, and reservoirs, and so it is possible that the spike in methane is largely due to there simply being more habitat for microbes to thrive in.
In theory, that latter result might be less alarming. Whereas microbial emissions in tropical wetlands will be very hard to control, human-made habitats like landfills “could be better managed,” research lead Michel told The Mix. “But that is hard, too, especially in developing countries.”
Determining the relative contributions of different kinds of microbial methane will require further study, she added. “Our data don’t tell us exactly what kind of microbial emissions they were, and our model is too simple to say where the emissions were coming from.”
And getting it right matters, she said. “We need to know where the methane is coming from in order to know how to minimize its release to the atmosphere.”
Urgent Need for Action
Stanford University climate scientist Rob Jackson—recently featured in this Scientific American interview about his new book, Into the Clear Blue Sky: The Path to Restoring Our Atmosphere—agrees.
“Methane contributes a third of current warming and is the only greenhouse gas for which reducing emissions today can slow climate change over the next decade or two,” Jackson, who also chairs the Global Carbon Project, told The Mix.
“Because of methane’s relatively short lifetime, if we could eliminate all methane emissions from human activities, including agriculture, waste, and fossil fuels—a big if—methane’s concentration would return quickly to preindustrial levels.”
“Doing so would save us 0.5°C of warming. Nothing else gives us this much power to slow global warming over the next few decades,” Jackson said.
