CH4 Flux Data Research Articles

Upscaling Wetland Methane Emissions From the FLUXNET‐CH4 Eddy Covariance Network (UpCH4 v1.0): Model Development, Network Assessment, and Budget Comparison

AbstractWetlands are responsible for 20%–31% of global methane (CH4) emissions and account for a large source of uncertainty in the global CH4 budget. Data‐driven upscaling of CH4 fluxes from eddy covariance measurements can provide new and independent bottom‐up estimates of wetland CH4 emissions. Here, we develop a six‐predictor random forest upscaling model (UpCH4), trained on 119 site‐years of eddy covariance CH4 flux data from 43 freshwater wetland sites in the FLUXNET‐CH4 Community Product. Network patterns in site‐level annual means and mean seasonal cycles of CH4 fluxes were reproduced accurately in tundra, boreal, and temperate regions (Nash‐Sutcliffe Efficiency ∼0.52–0.63 and 0.53). UpCH4 estimated annual global wetland CH4 emissions of 146 ± 43 TgCH4 y−1 for 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 CH4 fluxes has the potential to produce realistic extra‐tropical wetland CH4 emissions 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).

Spatial-temporal variability of methane fluxes in lakes varying in latitude, area, and depth

Many previous studies have found spatial and seasonal variabilities in CH4 fluxes, which could significantly affect lake-wide CH4 budgets. However, the ways in which the spatial and seasonal patterns of CH4 fluxes vary among lakes on a global scale is largely unknown. We compiled literature on CH4 flux data from global lakes and analyzed the spatial and seasonal variabilities for lakes varying in latitude, maximum depth, and area. Spatially, we found a significant linear relationship between the ratio of littoral to profundal fluxes and lake morphology (more related to area than depth), while globally, half of the lakes would have within 5% error of CH4 emission estimation under single-zone sampling. Seasonally, mid-latitude lakes showed higher CH4 fluxes in the summer and autumn, indicating the influence of temperature and autumn overturn, and the latter being largely related to maximum depth. Globally, due to abundant shallow lakes in the mid-latitude zone, approximately 99% of lakes had higher fluxes in the summer, while 75% of lakes showed errors in CH4 emission estimation within 20% when only the summer flux was investigated. In the high-latitude lakes, CH4 evasion during the spring ice-off period was significantly correlated with lake maximum depth, while lake area was also important when analyzing the CH4 diffusive flux. Our study yields preliminary conclusions about spatial and seasonal patterns of CH4 flux in different lake types, which are fundamental to building an effective sampling strategy and to determining an accurate CH4 budget from global lakes.

A dataset of carbon and water flux observed in aquaculture ponds in Quanjiao, Anhui Province from 2016 to 2020

Aquaculture ponds are inland small water bodies subject to human activities. They are not only carbon sources, but important sources of regional water evaporation. Accurate observation of carbon and water fluxes in aquaculture ponds is the basis for quantifying carbon emission and evaporation contribution of inland water. Nanjing University of Information Science and Technology set up an atmospheric environment experiment site to observe the carbon and water flux over aquaculture water bodies in Guandu Village of Quanjiao County, Anhui Province. This dataset includes the CO2 flux data (measured by the multi-channel closed dynamic floating chamber method), CH4 flux data (measured by the eddy covariance method, multi-channel closed dynamic floating chamber method, and inverted funnel method), latent heat and sensible heat fluxes data (measured by the eddy covariance method), as well as key environmental factors data, such as air temperature, water temperature, and radiation. All data were processed by standardized procedures. Moreover, data quality controls were performed. This dataset can provide important data for accurately estimating carbon emission and evaporation contribution in regional and global inland small water bodies.

Potential methane production in oligohaline wetlands undergoing erosion and accretion in the Mississippi River Delta Plain, Louisiana, USA

Methane (CH4) is steadily increasing in the atmosphere from different sources including wetlands. However, there is limited landscape level CH4 flux data in deltaic coastal systems where the availability of freshwater is impacted by the combined effect of climate change and anthropogenic impacts. Here we determine potential CH4 fluxes in oligohaline wetlands and benthic sediments in the Mississippi River Delta Plain (MRDP), which is undergoing the highest rate of wetland loss and most extensive hydrological wetland restoration in North America. We evaluate potential CH4 fluxes in two contrasting deltaic systems, one undergoing sediment accretion as result of a freshwater and sediment diversions (Wax Lake Delta, WLD), and one experiencing net land loss (Barataria-Lake Cataouatche, BLC). Short- (<4 days) and long-term (36 days) incubations using soil and sediment intact cores and slurries were performed at different temperatures representing seasonal differences (10, 20, 30 °C). Our study revealed that all habitats were net sources of atmospheric CH4 in all seasons, and CH4 fluxes were generally the highest for the 20 °C incubation. The CH4 flux was higher in the marsh habitat of the recently formed delta system (WLD) with total carbon content of 5–24 mg C cm−3 compared to the marsh habitat in BLC, which had high soil carbon content of 67–213 mg C cm−3. This suggests that the quantity of soil organic matter might not be a determining factor in CH4 flux. Overall, benthic habitats were found to have the lowest CH4 fluxes indicating that projected future conversions of marshes to open water in this region will impact the total wetland CH4 emission, although the overall contribution of such conversions to the regional and global carbon budgets is still unknown. Further research is needed to expand the CH4 flux studies by simultaneously using several methods across different wetland habitats.

Global Climate Change Increases Terrestrial Soil CH4 Emissions

AbstractIncreased greenhouse gas emissions are causing unprecedented climate change, which is, in turn, altering emissions and removals (referring to the oxidation of atmospheric CH4 by methanotrophs within the soil) of the atmospheric CH4 in terrestrial ecosystems. In the global CH4 budget, wetlands are the dominant natural source and upland soils are the primary biological sink. However, it is unclear whether and how the soil CH4 exchanges across terrestrial ecosystems and the atmosphere will be affected by warming and changes in precipitation patterns. Here, we synthesize 762 observations of in situ soil CH4 flux data based on the chamber method from the past three decades related to temperature and precipitation changes across major terrestrial ecosystems worldwide. Our meta‐analysis reveals that warming (average warming of +2°C) promotes upland soil CH4 uptake and wetland soil CH4 emission. Decreased precipitation (ranging from −100% to −7% of local mean annual precipitation) stimulates upland soil CH4 uptake. Increased precipitation (ranging from +4% to +94% of local mean annual precipitation) accelerates the upland soil CH4 emission. By 2100, under the shared socioeconomic pathway with a high radiative forcing level (SSP585), CH4 emissions from global terrestrial ecosystems will increase by 22.8 ± 3.6 Tg CH4 yr−1 as an additional CH4 source, which may be mainly attributed to the increase in precipitation over 30°N latitudes. Our meta‐analysis strongly suggests that future climate change would weaken the natural buffering ability of terrestrial ecosystems on CH4 fluxes and thus contributes to a positive feedback spiral.

Evaluation of gap-filling methods for CH4 flux data based on eddy covariance method in the Lake Taihu, China

Eddy covariance method has become a key technique to measure CH4 flux continuously in lakes. A large number of CH4 flux data was missing due to variable reasons. In order to reconstruct a complete time series of CH4 flux, it is necessary to find an appropriate gap-filling method to insert the CH4 flux data gap. Based on the routine meteorological data and CH4 flux data measured at Bifenggang site in the eastern part of the Taihu eddy flux network during 2014 to 2017, we analyzed the control factors of CH4 flux at the half-hour scale and daily scale. With those data, we tested that whether nonlinear regression method and two machine learning methods, random forest algorithm and error back propagation algorithm, could fill the CH4 flux gap at the half-hour scale and daily scale. The results showed that CH4 flux at the half-hour scale was mainly influenced by sediment temperature, friction velocity, air temperature, relative humidity, latent heat flux and water temperature at 20 cm in the growing season, and was mainly affected by relative humidity, latent heat flux, wind speed, sensible heat flux and sediment temperature in non-growing season. The CH4 flux at the daily scale was mainly affected by latent heat flux and relative humidity. Random forest model was the best in CH4 flux data gap filling at both time scales. The random forest model with the input variables of day of year, solar elevation angle, sediment temperature, friction velocity, air temperature, water temperature at 20 cm, relative humidity, air pressure, and wind speed was more suitable for filling the CH4 flux data gap at the half-hour scale. The random forest model with the input variables of day of year, sediment temperature, friction velocity, air temperature, water temperature at 20 cm, relative humidity, air pressure, wind speed, and downward shortwave radiation was more suitable for filling CH4 flux data gap at the day scale. The interpolation models could fill the data gap better at daily scale than that at the half-hour scale.

Dynamic chamber as a more reliable technique for measuring methane emissions from aquatic ecosystems

Aquatic ecosystems are the largest natural source of atmospheric methane (“CH4”) worldwide. However, the current estimation of CH4 emissions from aquatic ecosystems still has extensive uncertainty due to large spatiotemporal variations in CH4 emissions as well as significant uncertainty in measurement methods. In this study, we initially investigated CH4 fluxes from a simulated eutrophic water body by using static chamber method (“SC”) during an incubation period of 36 days. Approximately 23 % of the total flux measurements were unsuccessful because they lacked a linear correlation between the accumulation of CH4 concentrations and enclosure time. CH4 fluxes could be achieved for most measurements. However, 5 min after enclosing, the initial CH4 concentrations measured in the chambers were too high (up to 507.4 ppm) to greatly suppress CH4 emissions from the diffusion process. Therefore, a dynamic chamber method (“DC”) was developed to overcome the shortcomings of the SC. To achieve the DC, air samples must be continuously collected at the inlet and outlet of the dynamic chamber at fixed flow rates. In contrast to the SC, effective CH4 flux data could be obtained by the DC for each measurement at different frequencies. The DC measured the diel and daily variations in CH4 fluxes and the displayed CH4 emissions from the simulated water were highly irregular. The displayed emissions had variations up to more than two orders of magnitude. These results implied that the SC measured few intermittent fluxes that were difficult to represent the actual CH4 emissions from eutrophic water. The DC developed in this study considers the temporal variations in CH4 emissions from aquatic ecosystems. Thus, the DC is expected to be applicable in the field flux measurements of CH4 as well as other greenhouse gases to reduce emissions uncertainties.

Evaluating alternative ebullition models for predicting peatland methane emission and its pathways via data–model fusion

Abstract. Understanding the dynamics of peatland methane (CH4) emissions and quantifying sources of uncertainty in estimating peatland CH4 emissions are critical for mitigating climate change. The relative contributions of CH4 emission pathways through ebullition, plant-mediated transport, and diffusion, together with their different transport rates and vulnerability to oxidation, determine the quantity of CH4 to be oxidized before leaving the soil. Notwithstanding their importance, the relative contributions of the emission pathways are highly uncertain. In particular, the ebullition process is more uncertain and can lead to large uncertainties in modeled CH4 emissions. To improve model simulations of CH4 emission and its pathways, we evaluated two model structures: (1) the ebullition bubble growth volume threshold approach (EBG) and (2) the modified ebullition concentration threshold approach (ECT) using CH4 flux and concentration data collected in a peatland in northern Minnesota, USA. When model parameters were constrained using observed CH4 fluxes, the CH4 emissions simulated by the EBG approach (RMSE = 0.53) had a better agreement with observations than the ECT approach (RMSE = 0.61). Further, the EBG approach simulated a smaller contribution from ebullition but more frequent ebullition events than the ECT approach. The EBG approach yielded greatly improved simulations of pore water CH4 concentrations, especially in the deep soil layers, compared to the ECT approach. When constraining the EBG model with both CH4 flux and concentration data in model–data fusion, uncertainty of the modeled CH4 concentration profiles was reduced by 78 % to 86 % in comparison to constraints based on CH4 flux data alone. The improved model capability was attributed to the well-constrained parameters regulating the CH4 production and emission pathways. Our results suggest that the EBG modeling approach better characterizes CH4 emission and underlying mechanisms. Moreover, to achieve the best model results both CH4 flux and concentration data are required to constrain model parameterization.

Methane emissions only negligibly reduce the ecosystem service value of wetlands and rice paddies in the mature Ganges Delta.

Wetland provides a wide range of ecosystem services with immense value. However, methane (CH4) emissions adversely affect ecosystem services, and it requires fixation cost. The objective of the present study was to estimate CH4 emissions and ecosystem services value (ESV) and how much the fixation cost of CH4 reduces the ESV. Since rice cultivation is a very common practice here, the paddy fields were also incorporated in this study. CH4 flux and satellite data were employed for estimating the emissions with the help of two-factor (temperature and water availability) model. Global coefficients of ecosystem service value (ESV) that is defined as the monetary valuation of materialistic and non-materialistic services were adapted for estimating the ecosystem service of the CH4 emitting sources. Results show that during the boro season (pre-monsoon summer paddy cultivation season), average monthly emissions of paddy fields are equal to the wetlands which are 0.16 t/km2. During amon season (monsoon paddy cultivation season), this emissions is 0.7 t/km2 and 0.53 t/km2, respectively, from wetlands and paddy fields. Both wetlands and paddy fields emit a greater amount of CH4 during amon season than boro season. Behind this seasonal variation, water availability in terms of precipitation-evaporation ratio plays a more vital role than temperature. Total estimated ESV is 928.51 million US$, and CH4 fixation cost is 6.64 million US$ which is only 0.71% to total ESV. So, considering such huge net ESV, emphasis on wetland conservation and restoration are necessary.

BAWLD-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;: a comprehensive dataset of methane fluxes from boreal and arctic ecosystems

Abstract. Methane (CH4) emissions from the boreal and arctic region are globally significant and highly sensitive to climate change. There is currently a wide range in estimates of high-latitude annual CH4 fluxes, where estimates based on land cover inventories and empirical CH4 flux data or process models (bottom-up approaches) generally are greater than atmospheric inversions (top-down approaches). A limitation of bottom-up approaches has been the lack of harmonization between inventories of site-level CH4 flux data and the land cover classes present in high-latitude spatial datasets. Here we present a comprehensive dataset of small-scale, surface CH4 flux data from 540 terrestrial sites (wetland and non-wetland) and 1247 aquatic sites (lakes and ponds), compiled from 189 studies. The Boreal–Arctic Wetland and Lake Methane Dataset (BAWLD-CH4) was constructed in parallel with a compatible land cover dataset, sharing the same land cover classes to enable refined bottom-up assessments. BAWLD-CH4 includes information on site-level CH4 fluxes but also on study design (measurement method, timing, and frequency) and site characteristics (vegetation, climate, hydrology, soil, and sediment types, permafrost conditions, lake size and depth, and our determination of land cover class). The different land cover classes had distinct CH4 fluxes, resulting from definitions that were either based on or co-varied with key environmental controls. Fluxes of CH4 from terrestrial ecosystems were primarily influenced by water table position, soil temperature, and vegetation composition, while CH4 fluxes from aquatic ecosystems were primarily influenced by water temperature, lake size, and lake genesis. Models could explain more of the between-site variability in CH4 fluxes for terrestrial than aquatic ecosystems, likely due to both less precise assessments of lake CH4 fluxes and fewer consistently reported lake site characteristics. Analysis of BAWLD-CH4 identified both land cover classes and regions within the boreal and arctic domain, where future studies should be focused, alongside methodological approaches. Overall, BAWLD-CH4 provides a comprehensive dataset of CH4 emissions from high-latitude ecosystems that are useful for identifying research opportunities, for comparison against new field data, and model parameterization or validation. BAWLD-CH4 can be downloaded from https://doi.org/10.18739/A2DN3ZX1R (Kuhn et al., 2021).

Ebullition Controls on CH4 Emissions in an Urban, Eutrophic River: A Potential Time-Scale Bias in Determining the Aquatic CH4 Flux.

Rivers and streams contribute significant quantities of methane (CH4) to the atmosphere. However, there is a lack of CH4 flux and ebullitive (bubble) emission data from urban rivers, which might lead to large underestimations of global aquatic CH4 emissions. Here, we conducted high-frequency surveys using the boundary layer model (BLM) supplemented with floating chambers (FCs) and bubble traps to investigate the seasonal and diurnal variability in CH4 emissions in a eutrophic urban river and to evaluate whether the contribution of bubbles is important. We found that ebullition contributed nearly 99% of CH4 emissions and varied on hourly to seasonal time scales, ranging from 0.83 to 230 mmol m-2 d-1, although diffusive emissions and CH4 concentrations in bubbles did not exhibit temporal variability. Ebullitive CH4 emissions presented high temperature sensitivity (r = 0.6 and p < 0.01) in this urban river, and eutrophication might have triggered this high temperature sensitivity. The ebullitive CH4 flux is more likely to be underestimated at low temperatures because capturing the bubble flux is more difficult, given the low frequency of ebullition events. This study suggests that future ebullition measurements on longer time scales are needed to accurately quantify the CH4 budgets of eutrophic urban rivers.

Methanotrophy in geothermal soils, an overlooked process: The example of Nisyros island (Greece)

A multidisciplinary field campaign was carried out at Nisyros Island (Greece). Hydrothermal gases were sampled and analysed, and CH4 and CO2 fluxes from the soils were measured with the accumulation chamber method. The sampling area (Lakki plain) covers an area of about 0.08 km2, and includes the main fumarolic areas of Kaminakia, Stefanos, Ramos, Lofos and Phlegeton. Flux values measured at 130 sites range from −3.4 to 1420 mg m−2 d−1 for CH4 and from 0.1 to 383 g m−2 d−1 for CO2. The fumarolic areas show very different CH4 degassing patterns, Kaminakia showing the highest CH4 output values (about 0.8 t a−1 from an area of about 30,000 m2) and Phlegeton the lowest (about 0.01 t a−1 from an area of about 2500 m2). The total output from the entire geothermal system of Nisyros should not exceed 2 t a−1. Previous indirect estimates of the CH4 output at Nisyros, based on soil CO2 output and CH4/CO2 ratios in fumarolic gases, were more than one order of magnitude higher. The present work further underscores the utmost importance of direct CH4 flux data because indirect methods totally disregard methanotrophic activity within the soil. Ten soil samples were collected for CH4 consumption experiments and for metagenomic analysis. Seven of the soil samples showed small but significant CH4 consumption (up to 39.7 ng g−1 h−1) and were positive for the methanotrophs-specific gene (pmoA) confirming microbial CH4 oxidation in the soil, notwithstanding the harsh environmental conditions (high temperature and H2S concentrations and low pH).

A Method for Estimating Annual Cumulative Soil/Ecosystem Respiration and CH4 Flux from Sporadic Data Collected Using the Chamber Method

Measurements of greenhouse gas fluxes over many ecosystems have been made as part of the attempt to quantify global carbon and nitrogen cycles. In particular, annual flux observations are of great value for regional flux assessments, as well as model development and optimization. The chamber method is a popular approach for soil/ecosystem respiration and CH4 flux observations of terrestrial ecosystems. However, in situ flux chamber measurements are usually made with non-continuous sampling. To date, efficient methods for the application of such sporadic data to upscale temporally and obtain annual cumulative fluxes have not yet been determined. To address this issue, we tested the adequacy of non-continuous sampling using multi-source data aggregation. We collected 330 site-years monthly soil/ecosystem respiration and 154 site-years monthly CH4 flux data in China, all obtained using the chamber method. The data were randomly divided into a training group and verification group. Fluxes of all possible sampling months of a year, i.e., 4094 different month combinations were used to obtain the annual cumulative flux. The results showed a good linear relationship between the monthly flux and the annual cumulative flux. The flux obtained during the warm season from May to October generally played a more important role in annual flux estimations, as compared to other months. An independent verification analysis showed that the monthly flux of 1 to 4 months explained up to 67%, 89%, 94%, and 97% of the variability of the annual cumulative soil/ecosystem respiration and 92%, 99%, 99%, and 99% of the variability of the annual cumulative CH4 flux. This study supports the use of chamber-observed sporadic flux data, which remains the most commonly-used method for annual flux estimating. The flux estimation method used in this study can be used as a guide for designing sampling programs with the intention of estimating the annual cumulative flux.

Monthly gridded data product of northern wetland methane emissions based on upscaling eddy covariance observations

Abstract. Natural wetlands constitute the largest and most uncertain source of methane (CH4) to the atmosphere and a large fraction of them are found in the northern latitudes. These emissions are typically estimated using process (“bottom-up”) or inversion (“top-down”) models. However, estimates from these two types of models are not independent of each other since the top-down estimates usually rely on the a priori estimation of these emissions obtained with process models. Hence, independent spatially explicit validation data are needed. Here we utilize a random forest (RF) machine-learning technique to upscale CH4 eddy covariance flux measurements from 25 sites to estimate CH4 wetland emissions from the northern latitudes (north of 45∘ N). Eddy covariance data from 2005 to 2016 are used for model development. The model is then used to predict emissions during 2013 and 2014. The predictive performance of the RF model is evaluated using a leave-one-site-out cross-validation scheme. The performance (Nash–Sutcliffe model efficiency =0.47) is comparable to previous studies upscaling net ecosystem exchange of carbon dioxide and studies comparing process model output against site-level CH4 emission data. The global distribution of wetlands is one major source of uncertainty for upscaling CH4. Thus, three wetland distribution maps are utilized in the upscaling. Depending on the wetland distribution map, the annual emissions for the northern wetlands yield 32 (22.3–41.2, 95 % confidence interval calculated from a RF model ensemble), 31 (21.4–39.9) or 38 (25.9–49.5) Tg(CH4) yr−1. To further evaluate the uncertainties of the upscaled CH4 flux data products we also compared them against output from two process models (LPX-Bern and WetCHARTs), and methodological issues related to CH4 flux upscaling are discussed. The monthly upscaled CH4 flux data products are available at https://doi.org/10.5281/zenodo.2560163 (Peltola et al., 2019).

Uncertainty in United States coastal wetland greenhouse gas inventorying

Coastal wetlands store carbon dioxide (CO2) and emit CO2 and methane (CH4) making them an important part of greenhouse gas (GHG) inventorying. In the contiguous United States (CONUS), a coastal wetland inventory was recently calculated by combining maps of wetland type and change with soil, biomass, and CH4 flux data from a literature review. We assess uncertainty in this developing carbon monitoring system to quantify confidence in the inventory process itself and to prioritize future research. We provide a value-added analysis by defining types and scales of uncertainty for assumptions, burial and emissions datasets, and wetland maps, simulating 10 000 iterations of a simplified version of the inventory, and performing a sensitivity analysis. Coastal wetlands were likely a source of net-CO2-equivalent (CO2e) emissions from 2006–2011. Although stable estuarine wetlands were likely a CO2e sink, this effect was counteracted by catastrophic soil losses in the Gulf Coast, and CH4 emissions from tidal freshwater wetlands. The direction and magnitude of total CONUS CO2e flux were most sensitive to uncertainty in emissions and burial data, and assumptions about how to calculate the inventory. Critical data uncertainties included CH4 emissions for stable freshwater wetlands and carbon burial rates for all coastal wetlands. Critical assumptions included the average depth of soil affected by erosion events, the method used to convert CH4 fluxes to CO2e, and the fraction of carbon lost to the atmosphere following an erosion event. The inventory was relatively insensitive to mapping uncertainties. Future versions could be improved by collecting additional data, especially the depth affected by loss events, and by better mapping salinity and inundation gradients relevant to key GHG fluxes. Social Media : US coastal wetlands were a recent and uncertain source of greenhouse gasses because of CH4 and erosion.

Toward understanding the contribution of waterbodies to the methane emissions of a permafrost landscape on a regional scale-A case study from the Mackenzie Delta, Canada.

Waterbodies in the arctic permafrost zone are considered a major source of the greenhouse gas methane (CH4 ) in addition to CH4 emissions from arctic wetlands. However, the spatio-temporal variability of CH4 fluxes from waterbodies complicates spatial extrapolation of CH4 measurements from single waterbodies. Therefore, their contribution to the CH4 budget of the arctic permafrost zone is not yet well understood. Using the example of two study areas of 1,000km² each in the Mackenzie Delta, Canada, we approach this issue (i) by analyzing correlations on the landscape scale between numerous waterbodies and CH4 fluxes and (ii) by analyzing the influence of the spatial resolution of CH4 flux data on the detected relationships. A CH4 flux map with a resolution of 100m was derived from two aircraft eddy-covariance campaigns in the summers of 2012 and 2013. We combined the CH4 flux map with high spatial resolution (2.5m) waterbody maps from the Permafrost Region Pond and Lake Database and classified the waterbody depth based on Sentinel-1 SAR backscatter data. Subsequently, we reduced the resolution of the CH4 flux map to analyze if different spatial resolutions of CH4 flux data affected the detectability of relationships between waterbody coverage, number, depth, or size and the CH4 flux. We did not find consistent correlations between waterbody characteristics and the CH4 flux in the two study areas across the different resolutions. Our results indicate that waterbodies in permafrost landscapes, even if they seem to be emission hot spots on an individual basis or contain zones of above average emissions, do currently not necessarily translate into significant CH4 emission hot spots on a regional scale, but their role might change in a warmer climate.

Assessing diel variation of CH4 flux from rice paddies through temperature patterns

The diel variation in methane (CH4) flux from irrigated rice was characterized during the dry and wet cropping seasons in 2013 and 2014 using the eddy covariance (EC) technique. The EC technique has the advantage of obtaining measurements of fluxes at an extremely high temporal resolution (10Hz), meaning it records 36,000 measurements per hour. The EC measurements can very well capture the temporal variations of the diel (both diurnal and nocturnal) fluxes of CH4 and the environmental factors (temperature, surface energy flux, and gross ecosystem photosynthesis) at 30-min intervals. The information generated by this technique is important to enhance our mechanistic understanding of the different factors affecting the landscape scale diel CH4 flux. Distinct diel patterns of CH4 flux were observed when the data were partitioned into different cropping periods (pre-planting, growth, and fallow). The temporal variations of the diel CH4 flux during the dry seasons were more pronounced than during the wet seasons because the latter had so much climatic disturbance from heavy monsoon rains and occasional typhoons. Pearson correlation analysis and Granger causality test were used to confirm if the environmental factors evaluated were not only correlated with but also Granger-causing the diel CH4 flux. Soil temperature at 2.5 cm depth (Ts 2.5 cm) can be used as simple proxy for predicting diel variations of CH4 fluxes in rice paddies using simple linear regression during both the dry and wet seasons. This simple site-specific temperature response function can be used for gap-filling CH4 flux data for improving the estimates of CH4 source strength from irrigated rice production.

Strong geologic methane emissions from discontinuous terrestrial permafrost in the Mackenzie Delta, Canada

Arctic permafrost caps vast amounts of old, geologic methane (CH4) in subsurface reservoirs. Thawing permafrost opens pathways for this CH4 to migrate to the surface. However, the occurrence of geologic emissions and their contribution to the CH4 budget in addition to recent, biogenic CH4 is uncertain. Here we present a high-resolution (100 m × 100 m) regional (10,000 km²) CH4 flux map of the Mackenzie Delta, Canada, based on airborne CH4 flux data from July 2012 and 2013. We identify strong, likely geologic emissions solely where the permafrost is discontinuous. These peaks are 13 times larger than typical biogenic emissions. Whereas microbial CH4 production largely depends on recent air and soil temperature, geologic CH4 was produced over millions of years and can be released year-round provided open pathways exist. Therefore, even though they only occur on about 1% of the area, geologic hotspots contribute 17% to the annual CH4 emission estimate of our study area. We suggest that this share may increase if ongoing permafrost thaw opens new pathways. We conclude that, due to permafrost thaw, hydrocarbon-rich areas, prevalent in the Arctic, may see increased emission of geologic CH4 in the future, in addition to enhanced microbial CH4 production.

Microseepage of methane to the atmosphere from the Dawanqi oil‐gas field, Tarim Basin, China

AbstractThe microseepage of natural gas from subsurface hydrocarbon reservoirs is a widespread process in petroleum basins. On a global scale, microseepage represents an important natural source of atmospheric methane (CH4). To date, microseepage CH4 flux data have been obtained from ~20 petroleum systems in North America, Europe, and Asia. While the seasonal variations of gas flux due to soil methanotrophic activity are known, the role of geological factors in controlling gas fluxes has been poorly investigated. Here we present new microseepage data from the Dawanqi oil‐gas field located within the Tarim Basin (China), a petroleum system characterized by intense faulting and shallow (&lt;700 m) reservoirs. We measured CH4 fluxes from the ground at 51 sites along three transects by using a closed‐chamber connected to a portable gas sensor using off‐axis integrated cavity output spectroscopy. Our results indicate that the highest CH4 fluxes occur over faults and/or shallow reservoirs, especially those that were not developed and that have higher fluid pressures. Microseeping CH4 is thermogenic, like that occurring within the Dawanqi reservoirs, as demonstrated by 13C enrichment (δ13C from −46.3‰ to −30.7‰) in the chamber. Mean and range microseepage values (17 mg m−2d−1; from −1.4 to 330 mg m−2d−1) are similar to those reported for other petroleum fields with active tectonics. Our results confirm that dry soil over petroleum fields can be a net source of atmospheric CH4 and its flux is primarily controlled by faulting, and reservoir depth and pressure. These factors shall be considered in global bottom‐up seepage emission estimates.

Hydrothermal methane fluxes from the soil at Lakki plain (Nisyros Island, Greece)

Methane and CO2 flux measurements from the soils were made with the accumulation chamber method in Lakki plain covering an area of about 0.06 km2 including the main fumarolic areas of Kaminakia, Stefanos and Phlegeton. Flux values measured at 77 sites range from –3.4 to 1420 mg m-2 d-1 for CH4 and from 0.1 to 383 g m-2 d-1 for CO2. The three fumarolic areas show very different methane degassing patterns, Kaminakia showing the highest flux values. Methane output can be estimated in about 0.01 t a-1 from an area of about 2500 m2 at Phlegeton, about 0.1 t a-1 from an area of about 20,000 m2 at Stefanos and about 0.25 t a-1 from an area of about 30,000 m2 at Kaminakia. The total output from the entire geothermal system of Nisyros should not exceed 1 t a-1. Previous estimates of the CH4 output at Nisyros, based on soil CO2 output and CH4/CO2 ratios in fumarolic gases, were more than one order of magnitude higher. The present work further underscores the utmost importance of direct CH4 flux data.