An official website of the United States government
Abstract Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. Salinity is a useful proxy for tidal marsh CH4emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes, where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by comparing emissions from four strata differing in hydrology and plant community composition. Mean CH4chamber-collected emissions measured as mg CH4m−2 h−1ranked asS. alterniflora(1.2 ± 0.3) ≫ High-elevationJ. roemerianus(0.4 ± 0.06) > Low-elevationJ. roemerianus(0.3 ± 0.07) = S. patens(0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater depletion in theS. alterniflorastratum (61 ± 4%) than in theS. patensstratum (1 ± 9%) with theJ. roemerianusstrata falling in between. We attribute the high CH4emissions in theS. alterniflorastratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH4emissions in theS. patensstratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH4emissions from theJ. roemerianusstrata were likely due to plant traits that favor CH4oxidation over CH4production. Hydrology and plant community composition have significant potential as proxies to estimate CH4emissions at the site scale.
more »
« less
Global methane emissions from rivers and streams
Abstract Methane (CH4) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH4emissions from freshwater ecosystems1,2, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH4emissions remain highly uncertain3,4. Here we report a spatially explicit global estimate of CH4emissions from running waters, accounting for 27.9 (16.7–39.7) Tg CH4 per year and roughly equal in magnitude to those of other freshwater systems5,6. Riverine CH4emissions are not strongly temperature dependent, with low average activation energy (EM = 0.14 eV) compared with that of lakes and wetlands (EM = 0.96 eV)1. By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land–water connections in regulating CH4supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land.
more »
« less
- Award ID(s):
- 2025982
- PAR ID:
- 10492685
- Publisher / Repository:
- Nature
- Date Published:
- Journal Name:
- Nature
- Volume:
- 621
- Issue:
- 7979
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 530 to 535
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Wetlands are responsible for 20%–31% of global methane (CH4) emissions and account for a large source of uncertainty in the global CH4budget. Data‐driven upscaling of CH4fluxes from eddy covariance measurements can provide new and independent bottom‐up estimates of wetland CH4emissions. Here, we develop a six‐predictor random forest upscaling model (UpCH4), trained on 119 site‐years of eddy covariance CH4flux data from 43 freshwater wetland sites in the FLUXNET‐CH4 Community Product. Network patterns in site‐level annual means and mean seasonal cycles of CH4fluxes were reproduced accurately in tundra, boreal, and temperate regions (Nash‐Sutcliffe Efficiency ∼0.52–0.63 and 0.53). UpCH4 estimated annual global wetland CH4emissions of 146 ± 43 TgCH4 y−1for 2001–2018 which agrees closely with current bottom‐up land surface models (102–181 TgCH4 y−1) and overlaps with top‐down atmospheric inversion models (155–200 TgCH4 y−1). However, UpCH4 diverged from both types of models in the spatial pattern and seasonal dynamics of tropical wetland emissions. We conclude that upscaling of eddy covariance CH4fluxes has the potential to produce realistic extra‐tropical wetland CH4emissions estimates which will improve with more flux data. To reduce uncertainty in upscaled estimates, researchers could prioritize new wetland flux sites along humid‐to‐arid tropical climate gradients, from major rainforest basins (Congo, Amazon, and SE Asia), into monsoon (Bangladesh and India) and savannah regions (African Sahel) and be paired with improved knowledge of wetland extent seasonal dynamics in these regions. The monthly wetland methane products gridded at 0.25° from UpCH4 are available via ORNL DAAC (https://doi.org/10.3334/ORNLDAAC/2253).more » « less
-
Abstract Methane (CH4) is a potent greenhouse gas with a warming potential 84 times that of carbon dioxide (CO2) over a 20‐year period. Atmospheric CH4concentrations have been rising since the nineteenth century but the cause of large increases post‐2007 is disputed. Tropical wetlands are thought to account for ∼20% of global CH4emissions, but African tropical wetlands are understudied and their contribution is uncertain. In this work, we use the first airborne measurements of CH4sampled over three wetland areas in Zambia to derive emission fluxes. Three independent approaches to flux quantification from airborne measurements were used: Airborne mass balance, airborne eddy‐covariance, and an atmospheric inversion. Measured emissions (ranging from 5 to 28 mg m−2 hr−1) were found to be an order of magnitude greater than those simulated by land surface models (ranging from 0.6 to 3.9 mg m−2hr−1), suggesting much greater emissions from tropical wetlands than currently accounted for. The prevalence of such underestimated CH4sources may necessitate additional reductions in anthropogenic greenhouse gas emissions to keep global warming below a threshold of 2°C above preindustrial levels.more » « less
-
Abstract Small freshwater reservoirs are ubiquitous and likely play an important role in global greenhouse gas (GHG) budgets relative to their limited water surface area. However, constraining annual GHG fluxes in small freshwater reservoirs is challenging given their footprint area and spatially and temporally variable emissions. To quantify the GHG budget of a small (0.1 km2) reservoir, we deployed an Eddy covariance (EC) system in a small reservoir located in southwestern Virginia, USA over 2 years to measure carbon dioxide (CO2) and methane (CH4) fluxes near‐continuously. Fluxes were coupled with in situ sensors measuring multiple environmental parameters. Over both years, we found the reservoir to be a large source of CO2(633–731 g CO2‐C m−2 yr−1) and CH4(1.02–1.29 g CH4‐C m−2 yr−1) to the atmosphere, with substantial sub‐daily, daily, weekly, and seasonal timescales of variability. For example, fluxes were substantially greater during the summer thermally stratified season as compared to the winter. In addition, we observed significantly greater GHG fluxes during winter intermittent ice‐on conditions as compared to continuous ice‐on conditions, suggesting GHG emissions from lakes and reservoirs may increase with predicted decreases in winter ice‐cover. Finally, we identified several key environmental variables that may be driving reservoir GHG fluxes at multiple timescales, including, surface water temperature and thermocline depth followed by fluorescent dissolved organic matter. Overall, our novel year‐round EC data from a small reservoir indicate that these freshwater ecosystems likely contribute a substantial amount of CO2and CH4to global GHG budgets, relative to their surface area.more » « less
-
Abstract Inland waters play a major role in global greenhouse gas (GHG) budgets. The smallest of these systems (i.e., ponds) have a particularly large—but poorly constrained—emissions footprint at the global scale. Much of this uncertainty is due to a poor understanding of temporal variability in emissions. Here, we conducted high‐resolution temporal sampling to quantify GHG exchange between four temperate constructed ponds and the atmosphere on an annual basis. We show these ponds are a net source of GHGs to the atmosphere (564.4 g CO2‐eq m−2 yr−1), driven by highly temporally variable diffusive methane (CH4) emissions. Diffusive CH4release to the atmosphere was twice as high during periods when the ponds had a stratified water column than when it was mixed. Ebullitive CH4release was also higher during stratification. Building ponds to favor mixed conditions thus presents an opportunity to minimize the global GHG footprint of future pond construction.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-023-06344-6
