Global ocean methane emissions dominated by shallow coastal waters

Global Change BiologyVolume 26, Issue 9 p. 5342-5342 CORRIGENDUMFree Access Corrigendum This article corrects the following: A synthesis of methane emissions from shallow vegetated coastal ecosystems Alia N. Al-Haj, Robinson W. Fulweiler, Volume 26Issue 5Global Change Biology pages: 2988-3005 First Published online: March 16, 2020 First published: 22 July 2020 https://doi.org/10.1111/gcb.15192Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL In the paper by Al-Haj and Fulweiler (2020), global warming potential (GWP) and sustained flux global warming potential (SGWP) were calculated incorrectly. GWP should be calculated using gas emissions in gas mass units (Neubauer & Megonigal, 2015). We thank Judith Rosentreter for bringing the calculation error to our attention and Damien Maher and Patrick Megonigal for consultation on the correction. The updated version of Table 1 includes global flux rate, GWP or SGWP calculated via the following equations for the mean CH4 flux rate of each ecosystem: where r is the mean CH4 flux rate in µmol m−2 day−1 for each ecosystem and A is the area of the ecosystem in km2. where GFR is the mean global CH4 flux rate for each ecosystem and we use the 100 year global warming potential multiplier (32 or 45, respectively) from Neubauer and Megonigal (2015). TABLE 1. Global CH4 emission estimates from vegetated coastal ecosystems (VCEs) and their impact on global carbon cycling. Median CH4 fluxes, global CH4 flux estimates (Supporting Text 3 in Data S1), increase in global marine methane budget (Supporting Text 4 in Data S1), global warming potential (above) and sustained flux global warming potential (above) from VCEs. Values with the same lower case letter are not significantly different Mangrove Salt marsh Seagrass CH4 flux rate (µmol CH4 m−2 day−1) Mean ± SE 4,556.96 ± 1,102.06 3,534.90 ± 1,331.21 108.24 ± 19.72 Median 279.17a 224.44a 64.80b Range −67.33 to 72,867.83 −92.60 to 94,129.68 1.25–401.50 Aerial extent (km2) 137,760–152,361 55,000 788,000–1,646,788 Global CH4 flux rate (Tmol CH4 year−1) Mean ± SE 0.23 ± 0.06 0.25 ± 0.06 0.071 ± 0.027 0.031 ± 0.006 0.065 ± 0.012 Increase in global marine CH4 budget (%) 40.2–44.5 12.5 5.4–11.4 Global C burial (Tg C year−1)* Mean ± SE 31.1 ± 5.4 34.4 ± 5.9 11.99 ± 1.32 108.74 ± 29.90 227.26 ± 62.57 Global warming potential (Tg CO2eq year−1) Mean 117.61–130.08 36.42 15.98–33.39 Sustained flux global warming potential (Tg CO2eq year−1) Mean 165.39–182.92 51.22 22.47–46.96 Global area source McLeod et al. (2011) Mcowen et al. (2017) Jayathilake and Costello (2018), Davidson and Finlayson (2019) * Global C burial rates from McLeod et al. (2011). Subsequently, section 6: Global warming potential is also incorrect. Using the correct calculations for GWP and SGWP, mangroves and salt marshes release ~1–1.5 g of CO2 equivalent CH4 for every gram of CO2-eq C stored when considering the mean CH4 flux rate and seagrasses remain CO2 sinks releasing 1 g of CO2 equivalent CH4 for every 17 g of CO2-eq C stored (Table 1). These results indicate that mangroves and salt marsh ecosystems may be net C storers in some systems and may have net C loss in others. Code detailing step-by-step calculations can be found at https://github.com/aliaalhaj/Al-HajFulweiler2020_MethanefromVCEs_Code. REFERENCE Al-Haj, A. N., & Fulweiler, R. W. (2020). A synthesis of methane emissions from shallow vegetated coastal ecosystems. Global Change Biology, 26, 2988– 3005. https://doi.org/10.1111/gcb.15046 Citing Literature Volume26, Issue9September 2020Pages 5342-5342 ReferencesRelatedInformation

Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions to the total atmospheric methane budget. However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage, which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage, we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories. Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

Abstract. Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning. The data presented here can be downloaded from https://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from the Global Carbon Project.

<p>The air-sea gas flux is proportional to the difference of partial pressure between the sea-water and the overlying atmosphere multiplied by gas transfer velocity <em>k</em>, a measure of the effectiveness of the gas exchange. Because wind is the source of turbulence making the gas exchange more effective, <em>k</em> is usually parameterized by wind speed. Unfortunately, measured values of gas transfer velocity at a given wind speed have a large spread in values. Surfactants have been long suspected as the main reason of this variability but few measurements of gas exchange and surfactants have been performed at open sea simultaneously and therefore their results were inconclusive. Only recently, it has been shown that surfactants may decrease the CO<sub>2</sub> air-sea exchange by up to 50%. However the labour intensive methods used for surfactant study make it impossible to collect enough data to map the surfactant coverage or even create a gas transfer velocity parameterization involving a measure of surfactant activity. This is why we propose to use optical fluorescence as a proxy of surfactant activity.</p><p> </p><p>Previous research done by our group showed that fluorescence parameters allow estimation the surfactant enrichment of the surface microlayer, as well as types and origin of fluorescent organic matter involved. We plan to measure, from a research ship, all the variables needed for calculation of gas transfer velocity <em>k</em> (namely CO<sub>2</sub> partial pressure both in water and in air as well as vertical flux of this trace gas) and to use mathematical optimization methods to look for a parameterization involving wind speed and one of the fluorescence parameters which will minimize the residual <em>k</em> variability. Although our research will still involve water sampling and laboratory fluorescence measurements, the knowledge of which absorption and fluorescence emission bands are the best proxy for surfactant activity may allow to create remote sensing products (fluorescence lidars) allowing continuous measurements of surfactant activity at least from the ship board, if not from aircraft and satellites. The improved parameterization of the CO<sub>2</sub> gas transfer velocity will allow better constraining of basin-wide and global air-sea fluxes, an important component of global carbon budget.</p><p> </p><p>If an improved gas transfer velocity parametrization based on surfactant fluorescence spectrum in concert with a turbulence proxy (wind) were to be found, a tantalizing possibility arises of a remote sensing estimation of <em>k</em>. Namely a UV lidar can both excite and measure the fluorescence band identified as proxy of the surfactant effect on the gas transfer velocity. Depending on the wavelength bands needed to be utilized, the effect could be measured from a moving ship (already an improvements on methods needing sampling), an aircraft or possibly even a satellite. We intend to pursue this idea in cruises to both the Baltic and the North Atlantic, possibly in cooperation with other air-sea interaction groups (this presentation is in part an invitation to cooperation).</p>

Abstract. The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase, making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (∼ biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003–2012 decade, global methane emissions are estimated by top-down inversions at 558 Tg CH4 yr−1, range 540–568. About 60 % of global emissions are anthropogenic (range 50–65 %). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher than all other RCP scenarios. Bottom-up approaches suggest larger global emissions (736 Tg CH4 yr−1, range 596–884) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the top-down budget, it is likely that some of the individual emissions reported by the bottom-up approaches are overestimated, leading to too large global emissions. Latitudinal data from top-down emissions indicate a predominance of tropical emissions (∼ 64 % of the global budget, < 30° N) as compared to mid (∼ 32 %, 30–60° N) and high northern latitudes (∼ 4 %, 60–90° N). Top-down inversions consistently infer lower emissions in China (∼ 58 Tg CH4 yr−1, range 51–72, −14 %) and higher emissions in Africa (86 Tg CH4 yr−1, range 73–108, +19 %) than bottom-up values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for top-down inversions than for bottom-up inventories and models. The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute 30–40 % on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center (http://doi.org/10.3334/CDIAC/GLOBAL_METHANE_BUDGET_2016_V1.1) and the Global Carbon Project.

To better understand the current global CH4 budget, biogenic, fossil fuel, and biomass burning CH4 fluxes for the period 1995–2013 were inversely estimated from the observed mole fraction data of atmospheric CH4 using a three‐dimensional chemical transport model. Then, forward simulations of carbon and hydrogen isotope ratios of atmospheric CH4 (δ13C‐CH4 and δD‐CH4) were conducted using the inversion fluxes to evaluate the source proportion of the global total CH4 emission. Model‐simulated spatiotemporal variations of atmospheric CH4 reproduce the observational results well; however, the simulated δ13C‐CH4 and δD‐CH4 values significantly underestimate their observed values as a whole. This implies that the proportion of biogenic CH4 sources in the global CH4 emission, deduced by inverse modeling, is overestimated, although the proportion is fairly comparable with the medians of recent multiple CH4 inverse modeling. To reduce the disagreement between the observed and calculated isotope ratios, the CH4 fluxes of individual source categories were adjusted using our atmospheric δ13C‐CH4 and δD‐CH4 data observed at Arctic and Antarctic surface stations. The resultant global average biogenic, fossil fuel, and biomass burning CH4 fluxes over 2003–2012 are 346 ± 11, 162 ± 2, and 50 ± 2 TgCH4 year−1, respectively. It is also strongly suggested that the leveling‐off of atmospheric CH4 in the early 2000s and the renewed growth after 2006/2007 are, respectively, explainable by the decrease in biogenic and biomass burning CH4 emissions for 2000–2006 and the increase in biogenic CH4 emissions after that period. These emission changes mainly originate in the tropics.

Global atmospheric methane (CH4) emissions have risen significantly, tripling in atmospheric concentrations since preindustrial times. Wetlands, as the largest natural source of CH4 emissions, contribute significantly to the global CH4 budget. However, quantifying wetland CH4 emissions remains highly uncertain due to the complex interplay of hydrological and biogeochemical processes. In this study, we develop a random forest (RF) and SHapley Additive exPlanations (SHAP) framework to identify the main predictors of CH4 emissions across different climate zones and on a global scale. We used monthly global environmental variables and CH4 flux emissions from FLUXNET-CH4 dataset, incorporating 39 wetland sites over the globe. These sites are classified into tropical, temperate, and boreal regions by latitude. Key variables considered in the analysis included mineral-associated organic carbon, soil organic carbon, soil moisture, and canopy height. Our findings reveal that air temperature and latent heat are the most important predictors of CH4 at both global and regional scale. Regionally, tropical wetlands are primarily influenced by canopy height, water table level and soil organic carbon while soil temperature emerges as the dominant driver in temperate and boreal wetlands. Furthermore, we analyze the similarities and differences in CH4 predictors across climate zones to improve our understanding of regional and global wetlands CH4 dynamics. Understanding the main predictors of CH4 emissions across wetland regions is essential for improving CH4 budget accuracy on both regional and global scales.

Land and freshwater ecosystems play a significant role in affecting the global methane budget. With future warming, the increase of methane emissions could create large positive feedbacks to the global climate system.  We have used observation data of methane fluxes from diverse land and freshwater ecosystems to calibrate and evaluate extant land and freshwater biogeochemistry models of the Terrestrial Ecosystem Model (TEM) and the Arctic Lake Biogeochemistry Model (ALBM) to quantify the global methane emissions for the past few decades and the 21st century in a temporally and spatially explicit manner. Land ecosystems could emit methane from wetlands while uplands could uptake atmospheric methane. TEM simulates that global wetlands emissions are 212 ± 62 and 212 ± 32 Tg CH4 yr−1 due to uncertain parameters and wetland type distribution, respectively, during 2000–2012. After combining the global upland methane consumption of −34 to −46 Tg CH4 yr−1, we estimate that the global net land methane emissions are 149–176 Tg CH4 yr−1 due to uncertain wetland distribution and meteorological input. During 1950–2016, both wetland emissions and upland consumption increased during El Niño events and decreased during La Niña events. For freshwater ecosystems, we find that current emissions are 24.0 ± 8.4 Tg CH4 yr−1 from lakes larger than 0.1 km2. Future projections under the RCP8.5 scenario suggest a 58–86% growth in emissions from lakes.  Warming enhanced methane oxidation in lake water can be an effective sink to reduce the net release from global lakes. Additionally, these studies identify the key biogeochemical and physical processes of controlling methane production, consumption, and transport in various hotspot emission regions.  We also highlight the need for more in situ methane flux data, more accurate wetland and lake type and their area distribution dynamics information to better constrain the quantification uncertainty of global biogenic methane emissions across the landscape.

The massive quantities of phytoplankton in the North Atlantic and Antarctic oceans producing dimethylsulfoniopropionate (DMSP) as an osmoprotectant, much of which is degraded by marine bacteria to dimethylsulfide (DMS), ensures an important role for both compounds in the global sulfur cycle. The closest to a comprehensive review on this topic is a book of symposium proceedings edited by Kiene et al. (75); the more recent developments related specifically to DMSP degradation by microbial communities are found elsewhere (68). This article is more comprehensive, as it includes some of the earlier literature in describing the sources of DMSP, its release and linkage to the marine (primarily microbial) food web and subsequent degradation via cleavage to DMS and acrylic acid or demethylation and demethiolation to methanethiol. DMS production from DMSP has long been associated with marine algae according to the following reaction (20, 22): (1) DMSP is a tertiary sulfonium compound produced in high concentration by certain species of marine algae and plant halophytes for the regulation of their internal osmotic environment (1, 41, 47, 120), although its role in plants remains unclear. This alga-associated, i.e., particulate DMSP (DMSPp), when released into the marine environment as dissolved DMSP (DMSPd), can serve as a link between primary production and the microbial population, as it is readily degraded by chemoheterotrophic bacteria (59). DMSP turnover usually exceeds DMS production in natural waters (60) because DMSP is also demethylated to 3-methiolpropionate, which can be further demethylated to 3-mercaptopropionate or demethiolated, releasing methanethiol (72, 118). These reactions will be discussed in more detail below. The biogeochemical significance of DMSP cleavage was first suggested in 1972, when DMS was found to be universally present in seawater and emitted at a significant rate to the atmosphere (87). It was proposed that DMS, rather than H2S from coastal waters and mud flats, was the missing gaseous sulfur compound needed to enable the steady-state flow of sulfur between marine and terrestrial environments, making DMS emissions a key step in the global sulfur cycle (87). Atmospheric H2S, which arises primarily from dissimilatory sulfate reduction in organic matter-rich environments, could never be measured in sufficient quantity to be the vehicle for transferring large quantities of sulfur from sea to air to land. The total annual flux of biogenic DMS released to the atmosphere ranges from 28 to 45 Tg of S year−1, at least 10-fold higher than from all other sources (Table ​(Table1).1). Recent, more comprehensive calculations of global annual DMS flux from the oceans gave values that ranged from 13 to 37 Tg of S year−1 (57). This sea-to-air flux represents about 50% of the global biogenic sulfur flux to the atmosphere (3). However, anthropogenic sulfur emissions dominate the sulfur flux, representing 80 to 90% of the input to the global sulfur cycle (12, 23, 88). TABLE 1. Estimates of natural emissions of organosulfur compoundsa The magnitude of the marine DMS emissions is all the more remarkable considering that over half of the DMSP released is demethylated (68) and that a significant fraction of the DMS is oxidized by bacteria in the water column before it can be released to the atmosphere (13, 64). While most of the biogenic sulfur emissions (primarily DMS) come from the oceans, those coming from salt marshes and coastal wetlands are many times higher on a unit area basis (112). DMS flux per unit area from these marine wetlands is also much higher than from any known terrestrial soil (2). The biogeochemical cycling of DMSP and its biological degradation products are shown in Fig. ​Fig.11. FIG. 1. Scheme representing the mechanisms of DMSP and DMS cycling in the marine water column and atmosphere. DMSO, dimethyl sulfoxide; CCN, cloud-condensing nuclei; MMPA, 3-methiolpropionate; β-HP, β-hydroxypropionate; 3-MPA, 3-mercaptopropionate; ...

An understanding of potential factors controlling methane emissions from natural wetlands is important to accurately project future atmospheric methane concentrations. Here, we examine the relative contributions of climatic and environmental factors, such as precipitation, temperature, atmospheric CO2 concentration, nitrogen deposition, wetland inundation extent, and land-use and land-cover change, on changes in wetland methane emissions from preindustrial to present day (i.e., 1850–2005). We apply a mechanistic methane biogeochemical model integrated in the Community Land Model version 4.5 (CLM4.5), the land component of the Community Earth System Model. The methane model explicitly simulates methane production, oxidation, ebullition, transport through aerenchyma of plants, and aqueous and gaseous diffusion. We conduct a suite of model simulations from 1850 to 2005, with all changes in environmental factors included, and sensitivity studies isolating each factor. Globally, we estimate that preindustrial methane emissions were higher by 10% than present-day emissions from natural wetlands, with emissions changes from preindustrial to the present of +15%, −41%, and −11% for the high latitudes, temperate regions, and tropics, respectively. The most important change is due to the estimated change in wetland extent, due to the conversion of wetland areas to drylands by humans. This effect alone leads to higher preindustrial global methane fluxes by 33% relative to the present, with the largest change in temperate regions (+80%). These increases were partially offset by lower preindustrial emissions due to lower CO2 levels (10%), shifts in precipitation (7%), lower nitrogen deposition (3%), and changes in land-use and land-cover (2%). Cooler temperatures in the preindustrial regions resulted in our simulations in an increase in global methane emissions of 6% relative to present day. Much of the sensitivity to these perturbations is mediated in the model by changes in methane substrate production and the areal extent of wetlands. The detrended interannual variability of high-latitude methane emissions is explained by the variation in substrate production and wetland inundation extent, whereas the tropical emission variability is explained by both of those variables and precipitation.

Atmospheric methane levels (the second largest contributor to climate change) have more than doubled over the last 200 years, though with highly variable trends over time. The relative contribution of different sources and sinks to the global CH4 budget remains uncertain despite ongoing efforts to improve the estimates based on various approaches, and particularly the causes for an accelerated increase in recent years remain unclear. Therefore, understanding and quantifying methane sources at global to regional scales is essential to reduce uncertainties in the global methane budget and its feedback with the climate system.Within the Arctic region, wetlands and lakes constitute a major natural source of methane. With temperatures rising at rates at least twice the global average over the last decades, Arctic permafrost is increasingly thawing. Associated disturbance processes hold the potential to increase methane emissions, and as a consequence result in a positive feedback to climate change. However, until now neither observations nor model estimates could provide clear evidence of such a trend in emissions. As a consequence, current and possible future contributions of Arctic ecosystems to the accelerated increase in the global atmospheric methane levels remain highly uncertain.To help reduce methane emission uncertainties in the high northern latitudes, we estimated global CH4 fluxes to the atmosphere using the Jena CarboScope Global Inversion System, with a strong focus of our analysis on the Arctic region. We used wetland flux from JSBACH model as prior and assimilated atmospheric observations from regional networks available over the last years for the region above 60°N latitude (a total of 23 towers) to quantify the methane emissions over this region between 2010 to 2020. We found a clear seasonal pattern with emission peaks during July and August. As a sensitivity test to evaluate the improvement to constrain the Arctic methane fluxes with the assimilation of the regional data, we also conducted an inversion using just the global background surface stations (a total of 30 global stations). We found higher mean annual methane flux to the atmosphere when assimilating the regional data, with the largest difference between May to August. These estimates were finally evaluated against an ensemble of inverse model estimates from Global Methane Project available for the period between 2010 to 2017.

Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO2 from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO2 fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the 3He and SF6 dual tracer technique in Florida Bay near Bob Allen Keys (25.02663° N, 80.68137° W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO2 in freshwater at 20 °C, k(600), was 4.8 ± 1.8 cm h-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO2 fluxes in this area. The deviation in k(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (k(600)=0.125u102), which might be applicable to other seagrass ecosystems and wind fetch limited environments.

Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO2 from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO2 fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the 3He and SF6 dual tracer technique in Florida Bay near Bob Allen Keys (25.02663° N, 80.68137° W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO2 in freshwater at 20 °C, k(600), was 4.8 ± 1.8 cm h-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO2 fluxes in this area. The deviation in k(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (k(600)=0.125u102), which might be applicable to other seagrass ecosystems and wind fetch limited environments.

<strong class="journal-contentHeaderColor">Abstract.</strong> Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO₂ from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO₂ fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the ³He and SF₆ dual tracer technique in Florida Bay near Bob Allen Keys (25.02663&deg; N, 80.68137&deg; W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO₂ in freshwater at 20 &deg;C, <em>k</em>(600), was 4.8 &plusmn; 1.8 cm h^-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO₂ fluxes in this area. The deviation in <em>k</em>(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (<em>k</em>(600)=0.125<em>u</em>₁₀²), which might be applicable to other seagrass ecosystems and wind fetch limited environments.

<strong class="journal-contentHeaderColor">Abstract.</strong> Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO₂ from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO₂ fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the ³He and SF₆ dual tracer technique in Florida Bay near Bob Allen Keys (25.02663&deg; N, 80.68137&deg; W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO₂ in freshwater at 20 &deg;C, <em>k</em>(600), was 4.8 &plusmn; 1.8 cm h^-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO₂ fluxes in this area. The deviation in <em>k</em>(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (<em>k</em>(600)=0.125<em>u</em>₁₀²), which might be applicable to other seagrass ecosystems and wind fetch limited environments.