An official website of the United States government
Abstract Wetlands are the largest natural source of methane, yet bottom‐up models and top‐down models do not agree on global wetland methane emissions. In this study, we use TROPOMI methane data and inverse modeling to estimate the spatial and temporal distribution of global wetland methane emissions during the years 2019–2020 and compare inverse modeling results with an ensemble of 16 bottom‐up wetland models from the Global Carbon Project (GCP). We find that our inverse model increases wetland methane emissions near the equator (0–15) by 7% and decreases emissions in mid‐ and high‐latitude regions (31–90) by 26% compared to the mean of the GCP models. We also find that our inverse modeling estimate exhibits little seasonality in wetland methane emissions across most tropical wetland regions, even when emissions estimates within the prior include seasonality. This result is consistent with some bottom‐up models but not others. For mid‐ and high‐latitude wetland regions (e.g., the West Siberian lowland and Hudson Bay Great Lakes region), the seasonality of our inverse emissions estimate is consistent with most GCP models and suggests wetland methane emissions peak in July. Furthermore, we argue that an inundation map with accurate seasonality is a prerequisite for obtaining a bottom‐up methane emission estimate with appropriate seasonality. Overall, many of the bottom‐up models examined in this study agree with the magnitude and seasonality of the inverse model in major wetland regions, but there are nonetheless many opportunities to improve convergence between the bottom‐up and top‐down estimates.
more »
« less
Improved global wetland carbon isotopic signatures support post-2006 microbial methane emission increase
Abstract Atmospheric concentrations of methane, a powerful greenhouse gas, have strongly increased since 2007. Measurements of stable carbon isotopes of methane can constrain emissions if the isotopic compositions are known; however, isotopic compositions of methane emissions from wetlands are poorly constrained despite their importance. Here, we use a process-based biogeochemistry model to calculate the stable carbon isotopic composition of global wetland methane emissions. We estimate a mean global signature of −61.3 ± 0.7‰ and find that tropical wetland emissions are enriched by ~11‰ relative to boreal wetlands. Our model shows improved resolution of regional, latitudinal and global variations in isotopic composition of wetland emissions. Atmospheric simulation scenarios with the improved wetland isotopic composition suggest that increases in atmospheric methane since 2007 are attributable to rising microbial emissions. Our findings substantially reduce uncertainty in the stable carbon isotopic composition of methane emissions from wetlands and improve understanding of the global methane budget.
more »
« less
- PAR ID:
- 10420530
- Date Published:
- Journal Name:
- Communications Earth & Environment
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2662-4435
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Atmospheric methane levels were nearly steady between 1999 and 2006, but have been rising since then. Increases in wetland emissions, the largest natural global CH4 source, may be partly responsible. Tropical regions like Amazonia, host some of the largest wetlands on Earth, but there are few in-situ observations, which allow large-scale flux estimation. To improve estimates of its contribution to the global CH4 budget we measured 590 lower-troposphere methane concentrations vertical profiles at four Amazonian sites between 2010 and 2018. We estimate that Amazonia emits 46.2±10.3 TgCH4 y-1 (~8% of global emissions) with no emission trend. Non-fire sources (mainly wetlands) dominate emissions, with a smaller biomass burning contribution (~17%). We find a distinct east-west contrast with an emission peak in the northeast. Furthermore, while northwest-central Amazon emissions are nearly aseasonal, consistent with weak precipitation seasonality, southern emissions are strongly seasonal synchronously with equivalent water thickness seasonality.more » « less
-
Abstract While wetlands represent a small fraction (~5%–10%) of the world's land surface, it is estimated that one‐third of wetlands have been lost due to human activities. Wetland habitat loss decreases ecosystem benefits, including improved water quality and climate change mitigation. These microbially mediated functions are dependent on redox conditions, which are altered by soil hydrology and the presence of plants. We tested the overarching hypothesis that while microbial community composition would be resistant to change due to long‐term hydrologic history, key functions like greenhouse gas production would remain plastic and responsive to short‐term environmental shifts. Using a mesocosm design, we manipulated the duration of hydrologic conditions (i.e., stable dry, stable flooding, and alternating wet/dry) and the presence of plants to induce soil redox changes in wetland soils. We measured soil redox status, used targeted amplicon and shotgun metagenomic sequencing to characterize microbial communities, and measured greenhouse gas production to assess microbial function. The 8‐week hydrologic treatment shifted community composition but did not override the stronger effects of long‐term hydrologic history. Methane and carbon dioxide fluxes were altered by short‐term hydrologic treatment, with methane production favored in the wet treatment and carbon dioxide production favored in the dry treatment. Plant presence versus absence manipulation had little impact on soil microbiome composition or soil greenhouse gas production. The results highlight the resistance of microbial community structure shaped by historical hydrologic regimes, and emphasize that hydrologic conditions exert a stronger influence than plant presence on microbial composition and function. Predicting the outcomes of wetland disturbance and restoration requires an enhanced understanding of community stability and functional plasticity. Our results suggest that wetland hydrologic restoration can establish a stable microbial community that is resistant to environmental shifts, but microbial functions such as greenhouse gas emissions remain responsive to hydrologic disturbances, including flooding and drought.more » « less
-
In the global context, wetlands are small, both in overall area, and in typical single-site extent. However, wetlands are responsible for a very large fraction of the global methane flux and for a large fraction of terrestrially sequestered CO2. Wetlands are intrinsically heterogenous, with temporal variation of water elevation and seasonal and long-term vegetation dynamics, as well as spatially, through microtopographic variation of water elevation that drives differences in vegetation within any wetland sites. This heterogeneity in hydrological dynamics and vegetation structure within wetlands leads to large differences in biogeochemical processing of carbon and extreme within-site variability of CO2 uptake and methane emission rates. My group have been working in coastal wetlands in Louisiana, and a lake coastal wetland, at an estuary of Lake Erie in Ohio. The Ohio wetland, Old Woman Creek (OWC), has one of the highest methane fluxes ever observed with a longterm EC system, but not atypical for wetlands in agricultural watersheds. We have been combining point-scale measurements of methane and CO2 concentrations and fluxes, with site-level eddy covariance flux measurements to identify and quantify the differences between patches within wetlands. We show different patterns of regulation of methane transport through plants. We developed a remote-sensing based approach to classify the area and location of different vegetation patches and follow their decadal changes. We identify long-term and short-term effects of hydrology of greenhouse gas dynamics. Our results from the field demonstrate that any sampling of methane fluxes in a wetland must account for the areas of the ecological and hydrological patch types that each of the samples within the wetland represent. We incorporated our insight from the field into the E3SM-ELM Earth-system land-surface model, for which we developed the ELM-Wet version. ELM-Wet resolves wetland at the of eco-hydrological sub-patch scale within the wetland landunit, to improve the global predictability of methane fluxes and carbon sequestrations from wetlands.more » « less
-
Abstract. Methane (CH4) flux estimates from high-latitude North American wetlands remain highly uncertain in magnitude, seasonality, and spatial distribution. In this study, we evaluate a decade (2007–2017) of CH4 flux estimates by comparing 16 process-based models with atmospheric CH4 observations collected from in situ towers. We compare the Global Carbon Project (GCP) process-based models with a model inter-comparison from a decade earlier called The Wetland and Wetland CH4 Intercomparison of Models Project (WETCHIMP). Our analysis reveals that the GCP models have a much smaller inter-model uncertainty and have an average magnitude that is a factor of 1.5 smaller across Canada and Alaska. However, current GCP models likely overestimate wetland fluxes by a factor of two or more across Canada and Alaska based on tower-based atmospheric CH4 observations. The differences in flux magnitudes among GCP models are more likely driven by uncertainties in the amount of soil carbon or spatial extent of inundation than in temperature relationships, such as Q10 factors. The GCP models do not agree on the timing and amplitude of the seasonal cycle, and we find that models with a seasonal peak in July and August show the best agreement with atmospheric observations. Models that exhibit the best fit to atmospheric observation also have a similar spatial distribution; these models concentrate fluxes near Canada's Hudson Bay Lowlands. Current, state-of-the-art process-based models are much more consistent with atmospheric observations than models from a decade ago, but our analysis shows that there are still numerous opportunities for improvement.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1038/s43247-022-00488-5
