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Half of global methane emissions come from highly variable aquatic ecosystem sources

Abstract

Atmospheric methane is a potent greenhouse gas that plays a major role in controlling the Earth’s climate. The causes of the renewed increase of methane concentration since 2007 are uncertain given the multiple sources and complex biogeochemistry. Here, we present a metadata analysis of methane fluxes from all major natural, impacted and human-made aquatic ecosystems. Our revised bottom-up global aquatic methane emissions combine diffusive, ebullitive and/or plant-mediated fluxes from 15 aquatic ecosystems. We emphasize the high variability of methane fluxes within and between aquatic ecosystems and a positively skewed distribution of empirical data, making global estimates sensitive to statistical assumptions and sampling design. We find aquatic ecosystems contribute (median) 41% or (mean) 53% of total global methane emissions from anthropogenic and natural sources. We show that methane emissions increase from natural to impacted aquatic ecosystems and from coastal to freshwater ecosystems. We argue that aquatic emissions will probably increase due to urbanization, eutrophication and positive climate feedbacks and suggest changes in land-use management as potential mitigation strategies to reduce aquatic methane emissions.

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Fig. 1: Inland water and coastal ocean areal methane fluxes.
Fig. 2: Global aquatic methane emissions compared with other global methane sources and sinks.
Fig. 3: Global aquatic methane emissions from headwater streams to the open ocean.

Data availability

The datasets that support the findings of this study are available in the Figshare repository: https://doi.org/10.6084/m9.figshare.13611296. Source data are provided with this paper.

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Acknowledgements

J.A.R. and B.D.E. were supported by ARC Grants DP160100248 and LP150100519. A.V.B. is a research director at the Fonds National de la Recherche Scientifique (FNRS). C.S. was supported by The Second Tibetan Plateau Scientific Expedition and Research programme grant 2019QZKK0304. J.M. received funding from NASA grant NNX17AK49G. B.P. acknowledges support from the NASA Terrestrial Ecology Program and the Gordon and Betty Moore Foundation (GBMF5439). D.O. was supported by funding from the Campus Alberta Innovates Program (CAIP). Thanks to M. F. Billett, K. McKenzie and M. Wallin for providing additional information for the streams and rivers dataset. Thanks to A. Grinham, L. Gómez-Gener, T. DelSontro, K. Kuhn and K. Delwich for providing ancillary data to the lake and reservoir dataset. We thank P. del Giorgio and Y. Prairie for providing feedback on earlier versions of this work. We thank J.-J. Chen for translating several Chinese papers. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government.

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Contributions

J.A.R. did the synthesis for mangroves, salt marshes, seagrasses and tidal flats and produced all figures in the main manuscript; A.V.B. did the synthesis for estuaries and continental shelves; A.V.B. and J.A.R. did the synthesis for aquaculture ponds; C.S. compiled the data for streams, rivers, lakes and reservoirs with help from S.L.; B.R.D. and M.A.H. updated the compiled data for lakes and reservoirs and analysed the data with input from J.M.; C.S., S.L., G.H.A. and P.A.R. analysed the data for streams and rivers; G.H.A. determined zonal estimates of river surface area and stream and lake ice corrections; B.D.E. and J.A.R. conceived the project; J.A.R. drafted the first manuscript, and all authors reviewed and edited the manuscript and approved the final version.

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Correspondence to Judith A. Rosentreter.

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Peer review information Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: Clare Davis; Rebecca Neely.

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Extended data

Extended Data Fig. 1 Areal methane fluxes from continental shelf regions.

Boxplot showing median, lower (Q1), upper (Q3) quartiles and 1.5 times the length of the interquartile range of diffusive methane fluxes in areas with natural gas seeps, estuarine plumes, upwelling areas and the remaining (other) continental shelves. Differences are statistically significant (Kruskal-Wallis test, p<0.0001).

Extended Data Fig. 2 Areal methane fluxes from aquatic ecosystems over latitudes.

Natural log (ln) transformed methane fluxes over latitudes of all aquatic ecosystems compiled in this study, and individual plots for rivers and streams, lakes, reservoirs, estuaries, coastal wetlands (mangroves, salt marshes, seagrasses), and continental shelves.

Extended Data Fig. 3 Areal methane fluxes from natural and impacted estuaries, mangroves, and salt marshes.

Boxplots showing median, lower (Q1), upper (Q3) quartiles and 1.5 times the length of the interquartile range of methane fluxes from impacted and more natural (low disturbed) estuaries, mangroves and salt marshes. Several sites that could not be classified as ‘impacted’ or ‘natural’ were excluded from this plot.

Extended Data Fig. 4 Areal methane fluxes from eutrophic, mesotrophic and oligotrophic lakes and reservoirs.

Boxplots showing median, lower (Q1), upper (Q3) quartiles and 1.5 times the length of the interquartile range of total (diffusive and ebullitive) methane fluxes from eutrophic, mesotrophic and oligotrophic lakes and reservoirs.

Extended Data Fig. 5 Areal methane fluxes from coastal wetlands and relationships of methane fluxes versus temperature and salinity.

a) Boxplots showing median, lower (Q1), upper (Q3) quartiles and 1.5 times the length of the interquartile range of methane fluxes from salt marshes, mangroves and seagrasses. b) Linear relationships of coastal wetland methane fluxes and temperature (r2 = 0.04, p = 0.07) and salinity (r2 = 0.02, p = 0.1). Salt marsh extreme methane flux values (n = 2) are not shown.

Extended Data Table 1 Areal methane fluxes from inland and coastal ocean ecosystems

Supplementary information

Supplementary Information

Supplementary Fig. 1 and Tables 1–5.

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Statistical Source Data.

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Rosentreter, J.A., Borges, A.V., Deemer, B.R. et al. Half of global methane emissions come from highly variable aquatic ecosystem sources. Nat. Geosci. 14, 225–230 (2021). https://doi.org/10.1038/s41561-021-00715-2

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