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 meta-data 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 plant-mediated and/or fluxes from several sediment-water-air interfaces. 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 likely 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. Main text:Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), accounting for 16 to 25% of atmospheric warming to date 1,2 . Atmospheric methane nearly tripled since pre-industrial times with a steady rise between 1984 and 2000 (8.4 ± 0.6 ppb yr -1 ) 3 , little or no growth between 2000 and 2006 (0.5 ± 0.5 ppb yr -1 ) 3 , and a renewed growth to present day (2007 to 2020: 7.3 ± 0.6 ppb yr -1 ) 3-6 . Whether the renewed increase is caused by emissions from anthropogenic or natural sources, or by a decline in the oxidative capacity of the atmosphere, or a combination of all three factors remains unresolved [7][8][9] . Depending on the approach used, total Rivers (ice-corrected) 5.8 (1.8-21.0) 30.5 ± 17.1 This study Lakes (ice-cover, ice-melt corrected) < 0.001 km 2 21.2 (9.1-53.5) 54.5 ± 48.5 This study 0.001 -0.01 km 2 13.2 (5.6-33.1) 31.1 ± 23.7 This study 0.01 -0.1 km 2 4.4 (1.4-16.7) 22.4 ± 18.4 This study 0.1 -1 km 2 3.0 (1.1-8.0) 9.9 ± 7.0 This study > 1 km 2 14.0 (6.0-31.0) 33.0 ± 45.0 This study All lakes 55.8 (23.3-142.3) 150.9 ± 73.0 This study Reservoirs (ice-cover, ice-melt corrected) < 1 km 2 0.4 (0.1-1.3) 2.4 ± 4.7 This study > 1 km 2 14.7 (8.7-27.1) 22.0 ± 6.4 This study All reservoirs 15.1 (8.8-28.4) 24.3 ± 8.0 This study Freshwater wetlands 150.1 (138.3-164.6) 148.6 ± 15.2 Saunois et al. 11 (A) Freshwater aquaculture ponds 4.4 (0.4-7.9) 14.0 ± 18.8 This study Rice cultivation 29.9 (24.9-32.1) 29.8 ± 6.7 Saunois et al. 11 (B) Total inland waters 261.0 (197.5-396.2) 398.1 ± 79.4 This study Estuaries 0.23 (0.02-0.91) 0.90 ± 0.29 This study Coastal wetlands Saltmarshes 0.18 (0.02-0.89) 2.00 ± 1.51 This study Mangroves 0.21 (0.06-0.77) 1.46 ± 0.91 This study Seagrasses 0.13 (0.07-0.21) 0.18 ± 0.19 This study Tidal flats 0.17 (0.04...

Small artificial waterbodies are widespread and persistent emitters of methane and carbon dioxide

“…Since Holgerson and Raymond's (2016) synthesis was published, additional studies have found high GHG fluxes from vernal pools (Kifner et al, 2018), thaw ponds (Kuhn et al, 2018) and artificial ponds (Gorsky et al, 2019;Grinham et al, 2018;Martinez-Cruz et al, 2017;Ollivier et al, 2019a;Peacock et al, 2019;. A new synthesis further supports the inverse lake size-GHG flux relationship: 37% of total lentic CH 4 emissions (diffusive + ebullitive) came from waterbodies <0.001 km 2 in size (Rosentreter et al, 2021). These recent studies have also suggested that artificial ponds may have even higher GHG emissions per m 2 than natural ones.…”

Spatiotemporal Methane Emission From Global Reservoirs

“…vidual reservoir systems whereas Rosentreter et al (2021) and Harrison et al (2021) defined emission periods using air temperature for reservoirs in locations >0°C (>4°C for diffusion in Harrison et al (2021)) and ice-cover periods for those locations <0°C (Table 2). Specific season lengths are not reported for either of these studies.…”

Hidden Processes During Seasonal Isolation of a High-Altitude Watershed

“…Production of methane in fluvial systems is driven by prolonged anoxic conditions, which may be scarce in fastflowing headwater streams. Approximately one third of all global methane emissions are natural, yet emissions from natural systems are increasing as the climate is warming (Nisbet et al, 2014;Hamdan and Wickland, 2016;Rosentreter et al, 2021). Unconstrained sources of methane are responsible for >5 ppb yr −1 increases in the atmosphere, so the need to identify natural and anthropogenic factors contributing to this rise in emissions is urgent (Nisbet et al, 2019).…”