Attributing the renewed growth of atmospheric methane

Determine how much of the atmospheric methane increase since 2007 comes from wetlands, fossil sources and agriculture versus a weakening sink, using public observations and reproducible inversions.

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Cite

@misc{cairn-methane-growth-attribution,
  title        = {Attributing the renewed growth of atmospheric methane},
  author       = {{Cairn Commons contributors}},
  howpublished = {\url{https://cairn-commons.com/problems/methane-growth-attribution}},
  year         = {2026},
  note         = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-09-28}
}

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Claims

Verified

Disputed

Refuted

On the literature board

The problem

Atmospheric methane resumed growing in 2007 after a near-plateau, with exceptionally fast growth around 2020–2021. The open question is attribution: how much of the rise is increased emissions (wetlands, fossil energy, livestock, waste) and how much is a slower loss through reaction with the hydroxyl radical — a split that matters because the two imply different mitigation levers.

Known status. NOAA's globally averaged marine surface record gives a global mean of about 1939 ppb (May 2026) with annual increases of 14.78 ppb (2020), 17.70 ppb (2021, the largest in the record), 13.01 (2022), 8.32 (2023), 7.91 (2024) and 5.14 ppb (2025); the data files are public. Peng et al. (Nature 2022) attributed the 2020 anomaly to 53 ± 10% weaker hydroxyl sink (driven by lower nitrogen oxide emissions during lockdowns) and 47 ± 16% higher natural emissions, with wetland emissions up 6.0 ± 2.3 Tg/yr. The Global Methane Budget (Saunois et al., Earth System Science Data 2025) gives 575 Tg/yr top-down for 2010–2019 against 669 Tg/yr bottom-up — a 16% gap that is itself an open problem — and 608 Tg/yr for 2020.

What counts as progress

Reproducible inverse-modelling or budget analyses on public data (NOAA and other surface networks, satellite column retrievals, isotopic records) with code, priors and transport-model settings released.
Reproducible analyses of the isotopic record testing whether a stated source mix is consistent with observed carbon-13 trends.
Analyses that quantify how much of the top-down/bottom-up gap is explained by a specific inventory or process assumption.
Documented negative results: an attribution that is not identifiable from the available observations, shown by a sensitivity or synthetic-data test.

How it is checked. A reviewer re-runs the analysis against the same public data versions, checks that prior assumptions and the sink treatment are explicit (since emissions and sink trade off), confirms that uncertainty is propagated, and that reported growth rates match the published data files.

Current state

The problem