Carbon
isotopes can easily reveal methane sources. Methane is generated in two ways: decomposition
of organic matter by microbes (biogenic methane) and methane generated by
hydrocarbons under high pressure and temperature (thermogenic methane). Most
natural gas produced around the world is thermogenic in origin.
A new PNAS
paper shows pretty conclusively that microbial methane is the source of the
sharp rise in atmospheric methane from 2020-2022. The paper title uses the term
“suggests” but isotopes don’t lie so I would call the evidence pretty
conclusive.
Wetlands, landfills, and cattle are three of the major sources of biogenic methane and the likely sources for the sharp 2020-2022 increase. Landfills and cattle are anthropogenic sources while wetlands are predominantly natural sources. however, there are significant anthropogenic wetlands such as reservoirs and dams.
One straightforward
conclusion offered by the authors of the paper is that global warming is
increasing the metabolism of the microorganisms that break down the organic
matter into methane. The researchers measured the13C:12C ratio of CH4
(expressed as δ13CCH4). The staff at Phys.org writes:
“By examining the types of carbon atoms, or isotopes,
that the methane sample contains, Michel, Li and the team can identify its
source. For example, methane from fossil fuels has more carbon-13 isotope than
methane in the air, and methane from microbial sources contains even less
carbon-13. The lab has been measuring isotopes of methane since 1998.”
“Scientists have observed a rapid increase in
atmospheric methane levels since 2007, following a period of stabilization in
the early 21st century. In 2020, NOAA reported the highest growth rate of
methane since it began collecting data in 1983, and that record was shattered
again in 2021.”
“At the same time, Michel noticed a surprising
decrease in the carbon-13 isotope over the past 17 years. She and the team set
out to understand what was driving it.”
“They found that between 2020 and 2022, the drastic
increase in atmospheric methane was driven almost entirely by microbial
sources. Since 2007, scientists have observed microbes playing a significant
role in methane emissions, but their contribution has surged to over 90%
starting in 2020.”
According to
the paper:
“These results are consistent with previous inverse
modeling studies (8, 11, 12) that suggested approximately 85% of CH4 growth
during 2007–2020 was due to increased microbial emissions.”
Some data for
global methane emissions from the IEA is shown below. Some things to note are
that the oil sector is responsible for 63% of emissions from oil and gas while
the natural gas sector is responsible for 37%, This is due to the continued prevalence
of flaring and venting natural gas at oil wells. Methane emitted from coal
mining is about 30% higher than methane emitted from natural gas. Thus, the
narrative that natural gas production is responsible for most methane emissions
is not accurate. Natural gas emits less than two-thirds more methane than bioenergy
production. If we add biomass burning to that it would double bioenergy’s contribution
to just one third less than natural gas’s contribution. Contributions from
agriculture and waste are more than double the contributions from all fossil
fuels.
Some prior
studies suggested that the increase was being driven by fossil fuel emissions,
but this research shows (pretty firmly IMO) that this is not the case. While
the study showed that microbial methane is responsible for the recent
increases, it does not show whether these are coming from natural or
anthropogenic sources. However, a September paper in Environmental Research
Letters suggests that it is natural sources that are driving most of it.
The lead author of the paper, Rob Jackson of
Stanford University, is also the chair of the Global Carbon Project. I have criticized
his work in the past erroneously suggesting that stray methane in some water
wells in the region producing gas from the Marcellus Shale in Northeastern Pennsylvania
came from the Marcellus reservoir. This was proven false by carbon isotope
analysis as the gas was identified as coming from minor gas reservoirs just
below freshwater aquifers. I also criticized his recent work in targeting
natural gas stoves as dangerous since his experiments showed emissions of NOx
and other components in small, confined, non-ventilated conditions that did not
reflect real-world conditions. Based on these and some of his other statements
about fracking and methane issues, and the ability of his research to draw
media attention, I have regarded him as an activist biased scientist and tend
to take his conclusions with a grain of salt. However, in this paper, that does
not seem to be the case.
Importantly,
the paper attributes a portion of the emissions from wetlands as anthropogenic
since human-made reservoirs and dams contribute significantly to wetland emissions.
Inundated organic matter decomposes via anaerobic bacteria, which in time
releases methane.
“Methane concentrations have risen faster over the
past five-year period than in any period since record-keeping began.
Understanding where and why this is happening is a central goal of the Global
Methane Budget. At least two-thirds of global methane emissions are now
attributable to anthropogenic sources, an outcome that cannot continue if we
are to maintain a habitable climate.”
Another
conclusion of the paper which supports the PNAS paper’s conclusions is where
the increases from 2019-2023 have derived, mainly tropical regions.
“Tropical regions contribute the most to recent
emission increases (>7 Tg from 2019 to 2023), particularly in the Congo and,
to a lesser extent, parts of southeast Asia and southern Brazil (figure 4).
Another area of increase from 2019 to 2023 is observed near Beijing, China
(figure 4). As a full ensemble of inversions becomes available with more data
through 2023, additional estimates will provide more comprehensive coverage of
emission increases.”
The authors
also note that identifying and quantifying emissions with new satellites such
as those from MethaneSAT and Carbon Mapper will lead to better mitigation,
especially in the fossil fuel sector. The sector has been busy identifying
leaks, quantifying leak rates, and repairing leaks. This is mostly done in advanced
industrial economies like the U.S. and E.U. Leaks from less advanced countries
like Russia, Turkmenistan, China, and Iraq, all big emitters, are less likely to
be addressed.
References:
Microbes
in environment drove methane emissions more than fossil fuels between 2020 and
2022, analysis finds. Science X Staff. Phys.org. October 21, 2024. Microbes
in environment drove methane emissions more than fossil fuels between 2020 and
2022, analysis finds (msn.com)
Rapid
shift in methane carbon isotopes suggests microbial emissions drove record high
atmospheric methane growth in 2020–2022. Sylvia Englund Michel, Xin Lan, John
Miller, and Jianghanyang Li. Edited by Mark Thiemens, University of California
San Diego. Proceedings of the National Academy of Science (PNAS). October 21, 2024. 121 (44) e2411212121. Rapid shift in methane
carbon isotopes suggests microbial emissions drove record high atmospheric
methane growth in 2020–2022 | PNAS
Understanding
methane emissions. Global Methane Tracker 2024. International Energy Agency. Understanding
methane emissions – Global Methane Tracker 2024 – Analysis - IEA
Methane
emissions are rising faster than ever. Stanford Report. September 9, 2024. The
world hasn’t hit the brakes on methane emissions | Stanford Report
Human
activities now fuel two-thirds of global methane emissions. R B Jackson, M
Saunois, A Martinez, J G Canadell, X Yu1, M Li, B Poulter, P A Raymond, P
Regnier, P Ciais. Published 10 September 2024. Environmental Research Letters,
Volume 19, Number 10. Human
activities now fuel two-thirds of global methane emissions - IOPscience
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