Tg Yr Research Articles

Potential of optimizing irrigation and fertilization management for sustainable rice production in China

Flood irrigation and excessive nitrogen (N) fertilizers, which are often applied to paddy fields in China, may exacerbate irrigation water shortages, increase production costs, and cause environmental degradation. However, the effects of optimizing irrigation and fertilization management on N loss in paddy fields across China have not been evaluated comprehensively. In this study, we collected the data for synthetic N fertilizer application and irrigation water consumption in the paddy fields of China from 1979 to 2020 and 2004 to 2020, respectively. Synthetic N fertilizer use and irrigation water consumption in China increased by 145% and 54% during these periods, respectively. Based on the N fertilization rate and irrigation regimes, we constructed a N loss model and estimated the synthetic fertilizer-induced N loss from paddy fields. The N loss from paddy fields increased rapidly in 1979–1998, followed by significant fluctuations in 1998–2007, reaching a peak of 1.65 Tg in 2007 and decreasing steadily to 1.03 Tg in 2020. Notably, ammonia (NH3) volatilization was the dominant pathway of total N loss, followed by N runoff, N leaching, and nitrous oxide (N2O) emissions, contributing to 60.15%, 23.69%, 15.04% and 1.12% of the total loss on average, respectively. High N loss rates mainly occurred in the Yangtze river basin and Southeast coastal rice regions. When applying the optimal N management, water-saving irrigation regimes, and a combination of optimal N and water management, the N loss would decrease by 18.15%, 10.36%, and 26.29%, respectively. By the wide application of optimal N and water management, the N fertilizer use, irrigation water consumption, and N loss were likely to decrease by 0.55 Tg yr−1, 1.55 × 109 yuan yr−1, and 0.36 Tg N yr−1, respectively. Consequently, the net economic benefit rates became positive at the national scale, with the cost of N loss and water consumption reductions being 30.14% and 26.41%, respectively. Our study confirms that optimizing N application and irrigation management in paddy fields has noticeable potential to ensure the green agricultural sustainability and food security in China.

Identifying the main drivers of the spatiotemporal variations in wetland methane emissions during 2001–2020

Wetlands act as an important natural source of global methane (CH4). The emission rate of wetland CH4 is jointly affected by climate change, carbon dioxide (CO2) fertilization, and wetland distribution. In this study, we implemented a wetland CH4 emission module into the Yale Interactive Biosphere (YIBs) model to quantify the spatiotemporal variations of global wetland CH4 emissions in 2001–2020. Site-level validations showed that the YIBs model reasonably captures the seasonality and magnitude of CH4 emissions at 28 out of 33 sites with significantly positive correlations and low relative biases. On the global scale, the YIBs predicts an annual mean wetland CH4 emission of 147.5 Tg yr−1 in 2000–2017, very close to the estimate of 147.9 Tg yr−1 from the ensemble of 13 process-based models. Global wetland CH4 emissions showed a positive trend of 0.74 Tg yr−2 in the past 2 decades, leading to an increase of 7.4 Tg yr−1 (5.2%) in 2008–2017 than 2000–2009. Climate change and CO2 fertilization accounted for over 70% of global wetland CH4 emission changes. Among them, the impact of CO2 grew steadily and became the dominant factor after the year 2008. The most significant changes in wetland CH4 emissions were located in the tropical regions following the perturbations in temperature that drives the ecosystem productivity. We found limited changes in CH4 emissions over high latitudes because of the moderate variations in wetland area fraction. The rise of wetland CH4 emissions poses an emerging threat to the global warming and likely escalates the tropospheric air pollutants.

A Two‐Fold Increase of Phosphorus in Alpine Ice Over the Twentieth Century: Contributions From Dust, Primary Biogenic Emissions, Coal Burning, and Pig Iron Production

AbstractPhosphorus (P) is a key nutrient for many organisms but its global atmospheric budget is largely unconstrained. Estimates of major emissions sources such as fossil‐fuel combustion range from ∼0.02 to 1.1 Tg yr−1, and primary biogenic emissions range from 0.16 to 1.0 Tg yr−1. Here we used detailed measurements of phosphorus in Alpine ice cores extracted from the Col du Dôme (CDD) glacier located near the Mont Blanc summit and atmospheric model simulations to evaluate changes in western European emissions from pre‐industrial (PI) to modern times. The ice‐core records show that P concentrations during the PI were about 0.9 ng g−1, of which one third was of crustal origin and two thirds the result of primary biogenic emissions. Concentrations were higher throughout the 20th century, reaching 2.5 ng g−1 in the 1980s. Analysis of source tracers measured in the same ice, commodity productions statistics, and other information suggest that the increase in P throughout the 20th century was caused by enhanced emissions from natural and anthropogenic sources. Coal burning and steel industry represented the main anthropogenic sources during the first and second half of the century, respectively. After 1950, the increase in P was also caused by enhanced dust emissions, with increased biogenic emissions caused by recent changes of use‐land also contributing. These findings provide important constraints on the atmospheric P budget at the scale of western Europe during the recent centuries.

Assimilation of POLDER observations to estimate aerosol emissions

Abstract. We apply a local ensemble transform Kalman smoother (LETKS) in combination with the global aerosol–climate model ECHAM–HAM to estimate aerosol emissions from POLDER-3/PARASOL (POLarization and Directionality of the Earth's Reflectances) observations for the year 2006. We assimilate aerosol optical depth at 550 mnm (AOD550), the Ångström exponent at 550 and 865 nm (AE550–865), and single-scattering albedo at 550 nm (SSA550) in order to improve modeled aerosol mass, size and absorption simultaneously. The new global aerosol emissions increase to 1419 Tg yr−1 (+28 %) for dust, 1850 Tg yr−1 (+75 %) for sea salt, 215 Tg yr−1 (+143 %) for organic aerosol and 13.3 Tg yr−1 (+75 %) for black carbon, while the sulfur dioxide emissions increase to 198 Tg yr−1 (+42 %) and the total deposition of sulfates to 293 Tg yr−1 (+39 %). Organic and black carbon emissions are much higher than their prior values from bottom-up inventories, with a stronger increase in biomass burning sources (+193 % and +90 %) than in anthropogenic sources (115 % and 70 %). The evaluation of the experiments with POLDER (assimilated) and AERONET as well as MODIS Dark Target (independent) observations shows a clear improvement compared with the ECHAM–HAM control run. Specifically based on AERONET, the global mean error in AOD550 improves from −0.094 to −0.006, while absorption aerosol optical depth at 550 nm (AAOD550) improves from −0.009 to −0.004 after the assimilation. A smaller improvement is also observed in the AE550–865 mean absolute error (from 0.428 to 0.393), with a considerably higher improvement over isolated island sites at the ocean. The new dust emissions are closer to the ensemble median of AEROCOM I, AEROCOM III and CMIP5 as well as some of the previous assimilation studies. The new sea salt emissions have become closer to the reported emissions from previous studies. Indications of a missing fraction of coarse dust and sea salt particles are discussed. The biomass burning changes (based on POLDER) can be used as alternative biomass burning scaling factors for the Global Fire Assimilation System (GFAS) inventory distinctively estimated for organic carbon (2.93) and black carbon (1.90) instead of the recommended scaling of 3.4 (Kaiser et al., 2012). The estimated emissions are highly sensitive to the relative humidity due to aerosol water uptake, especially in the case of sulfates. We found that ECHAM–HAM, like most of the global climate models (GCMs) that participated in AEROCOM and CMIP6, overestimated the relative humidity compared with ERA5 and as a result the water uptake by aerosols, assuming the kappa values are not underestimated. If we use the ERA5 relative humidity, sulfate emissions must be further increased, as modeled sulfate AOD is lowered. Specifically, over East Asia, the lower AOD can be attributed to the underestimated precipitation and the lack of simulated nitrates in the model.

Atmospheric data support a multi-decadal shift in the global methane budget towards natural tropical emissions

Abstract. We use the GEOS-Chem global 3-D model and two inverse methods (the maximum a posteriori and ensemble Kalman filter) to infer regional methane (CH4) emissions and the corresponding stable-carbon-isotope source signatures from 2004–2020 across the globe using in situ and satellite remote sensing data. We use the Siegel estimator to determine linear trends from the in situ data. Over our 17-year study period, we estimate a linear increase of 3.6 Tg yr−1 yr−1 in CH4 emissions from tropical continental regions, including North Africa, southern Africa, tropical South America, and tropical Asia. The second-largest increase in CH4 emissions over this period (1.6 Tg yr−1 yr−1) is from China. For boreal regions we estimate a negative emissions trend of −0.2 Tg yr−1 yr−1, and for northern and southern temperate regions we estimate trends of 0.03 Tg yr−1 yr−1 and 0.2 Tg yr−1 yr−1, respectively. These increases in CH4 emissions are accompanied by a progressively isotopically lighter atmospheric δ13C signature over the tropics, particularly since 2012, which is consistent with an increased biogenic emissions source and/or a decrease in a thermogenic/pyrogenic emissions source with a heavier isotopic signature. Previous studies have linked increased tropical biogenic emissions to increased rainfall. Over China, we find a weaker trend towards isotopically lighter δ13C sources, suggesting that heavier isotopic source signatures make a larger contribution to this region. Satellite remote sensing data provide additional evidence of emissions hotspots of CH4 that are consistent with the location and seasonal timing of wetland emissions. The collective evidence suggests that increases in tropical CH4 emissions are from biogenic sources, with a significant fraction from wetlands. To understand the influence of our results on changes in the hydroxyl radical (OH), we also report regional CH4 emissions estimates using an alternative scenario of a 0.5 % yr−1 decrease in OH since 2004, followed by a larger 1.5 % drop in 2020 during the first COVID-19 lockdown. We find that our main findings are broadly insensitive to those idealised year-to-year changes in OH, although the corresponding change in atmospheric CH4 in 2020 is inconsistent with independent global-scale constraints for the estimated annual-mean atmospheric growth rate.

Remotely sensed and surface measurement- derived mass-conserving inversion of daily NOx emissions and inferred combustion technologies in energy-rich northern China

Abstract. This work presents a new model-free inversion estimation framework (MFIEF) using daily TROPOspheric Monitoring Instrument (TROPOMI) NO2 columns and observed fluxes from the continuous emission monitoring system (CEMS) to quantify 3 years of daily scale emissions of NOx at 0.05∘×0.05∘ over Shanxi Province, a major world-wide energy-producing and energy-consuming region. The NOx emissions, day-to-day variability, and uncertainty on a climatological basis are computed to be 1.86, 1.03, and 1.05 Tg yr−1 respectively. The highest emissions are concentrated in the lower Fen River valley, which accounts for 25 % of the area, 53 % of the NOx emissions, and 72 % of CEMS sources. Two major forcing factors (10th to 90th percentile) are horizontal transport distance per day (63–508 km) and lifetime of NOx (7.1–18.1 h). Both of these values are consistent with NOx emissions to both the surface layer and the free troposphere. The third forcing factor, the ratio of NOx/NO2, on a pixel-to-pixel basis, is demonstrated to correlate with the combustion temperature and energy efficiency of large energy consuming sources. Specifically, thermal power plants, cement, and iron and steel companies have a relatively high NOx/NO2 ratio, while coking, industrial boilers, and aluminum oxide factories show a relatively lower ratio. Variance maximization is applied to daily TROPOMI NO2 columns, which facilitates identification of three orthogonal and statistically significant modes of variability, and successfully attributes them both spatially and temporally to (a) this work's computed emissions, (b) remotely sensed TROPOMI ultraviolet aerosol index (UVAI), and (c) computed transport based on TROPOMI NO2.

Nitrous oxide emissions dynamics in Indian agricultural landscapes: Revised emission rates and new insights

In the past five decades, nitrous oxide (N2O) emissions from Indian agricultural landscapes have significantly increased, making India the fifth-largest emitter of N2O globally (∼4.8%). The few studies that evaluated N2O at the India scale using a bottom-up approach were constrained to a brief period and employed country-level emission factors (EFs), despite the availability of long-term inventory data and spatially varying EFs. To fill this gap and comprehend the spatio-temporal N2O emissions dynamics at the district-level across India, this study created district- and crop-specific N2O EFs from 82 field plot studies with 861 observations across 13 locations. Through these improved EFs, district-level emissions were calculated from 1966 to 2017 using a modified IPCC Tier I-Tier II approach (specific EFs for synthetic fertilizer, animal manure, and crop residue return) and highlighted significant spatio-temporal drivers. Total N2O emissions in Indian agricultural soils increased from 73.7 ± 3.3 Gg in 1966–279.3 ± 15.8 Gg in 2017. The increase in total N2O emissions (3.9 Gg yr−1) from 1966 to 2017 is associated with the increase in crop production (4.1 Tg yr−1) driven by the increase in fertilization (0.3 Tg yr−1). N2O emissions from synthetic fertilizer, animal manure, crop residue return, soil organic matter, volatilization, leaching, and runoff increased from 1966 to 2017. On the national scale, N2O emissions from synthetic fertilization accounted for 57% in 2017, followed by animal manure (15%) and soil organic matter (9%). While the national emission totals are dominated by synthetic fertilizer application, district-level analyses revealed the possibility of the highest contributions from animal manure, crop residue return, and soil organic matter in 18% of the districts. The overall uncertainty from this modified method and refined EFs was 5.1%. This study is the first to capture long-term N2O dynamics at the district scale from Indian agricultural soils.

Global Riverine Export of Dissolved Lignin Constrained by Hydrology, Geomorphology, and Land‐Cover

AbstractThe land‐to‐ocean export of terrestrial dissolved organic matter (tDOM) links the two largest organic carbon (OC) pools on Earth and is changing due to anthropogenic activity and climate change. Lignin phenols are often utilized as diagnostic markers for tDOM in aquatic systems, but information on the global dissolved lignin flux is still uncertain as linkages between future changes in export and global changes are poorly constrained. Thirty‐two new measurements from 15 Chinese rivers and 548 measurements collected from 97 global rivers were combined to analyze its distributions, fluxes, and potential controls. Dissolved lignin concentrations (in μg L−1) varied by more than two orders of magnitude (0.7–138 μg L−1) and were positively correlated with dissolved OC (DOC) concentrations globally, suggesting coupled export of dissolved lignin and DOC. The carbon‐normalized lignin yields (in mg g OC−1) varied by an order of magnitude (<1–30 mg (g OC)−1) and was significantly lower in the Arctic than in temperate and tropical rivers. Correlation analyses indicated that the dissolved lignin flux (FLignin) was mainly affected by water discharge and mean basin slope, while the area‐normalized yield of dissolved lignin as well as discharge‐weighted lignin concentration strongly and positively correlated with forest coverage. We, for the first time, estimated global FLignin ranges from 0.58 to 1.2 Tg yr−1 by using different approaches. Our synthesis reveals that future changes in land‐cover that is driven by climate change as well as other human activities would impose a significant influence on the land‐to‐ocean export of tDOM.

The Role of Midlatitude Cyclones in the Emission, Transport, Production, and Removal of Aerosols in the Northern Hemisphere

AbstractWe examine the distribution of aerosol optical depth (AOD) across 27,707 northern hemisphere (NH) midlatitude cyclones for 2005–2018 using retrievals from the Moderate Resolution Spectroradiometer (MODIS) sensor on the Aqua satellite. Cyclone‐centered composites show AOD enhancements of 20%–45% relative to background conditions in the warm conveyor belt (WCB) airstream. Fine mode AOD accounts for 68% of this enhancement annually. Relative to background conditions, coarse mode AOD is enhanced by more than a factor of two near the center of the composite cyclone, co‐located with high surface wind speeds. Within the WCB, MODIS AOD maximizes in spring, with a secondary maximum in summer. Cyclone‐centered composites of AOD from the Modern Era Retrospective analysis for Research and Applications, version 2 Global Modeling Initiative (M2GMI) simulation reproduce the magnitude and seasonality of the MODIS AOD composites and enhancements. M2GMI simulations show that the AOD enhancement in the WCB is dominated by sulfate (37%) and organic aerosol (25%), with dust and sea salt each accounting for 15%. MODIS and M2GMI AOD are 60% larger in North Pacific WCBs compared to North Atlantic WCBs and show a strong relationship with anthropogenic pollution. We infer that NH midlatitude cyclones account for 355 Tg yr−1 of sea salt aerosol emissions annually, or 60% of the 30–80°N total. We find that deposition within WCBs is responsible for up to 35% of the total aerosol deposition over the NH ocean basins. Furthermore, the cloudy environment of WCBs leads to efficient secondary sulfate production.

A high-resolution satellite-based map of global methane emissions reveals missing wetland, fossil fuel, and monsoon sources

Abstract. We interpret space-borne observations from the TROPOspheric Monitoring Instrument (TROPOMI) in a multi-inversion framework to characterize the 2018–2019 global methane budget. Evaluation of the inverse solutions indicates that simultaneous source + sink optimization using methane observations alone remains an ill-posed problem – even with the dense TROPOMI sampling coverage. Employing remote carbon monoxide (CO) and hydroxyl radical (OH) observations with independent methane measurements to distinguish between candidate solutions, we infer from TROPOMI a global methane source of 587 (586–589) Tg yr−1 and sink of 571 Tg yr−1 for our analysis period. We apply a new downscaling method to map the derived monthly emissions to 0.1∘ × 0.1∘ resolution, using the results to uncover key gaps in the prior methane budget. The TROPOMI data point to an underestimate of tropical wetland emissions (a posteriori increase of +13 % [6 %–25 %] or 20 [7–25] Tg yr−1), with adjustments following regional hydrology. Some simple wetland parameterizations represent these patterns as accurately as more sophisticated process-based models. Emissions from fossil fuel activities are strongly underestimated over the Middle East (+5 [2–6] Tg yr−1 a posteriori increase) and over Venezuela. The TROPOMI observations also reveal many fossil fuel emission hotspots missing from the prior inventory, including over Mexico, Oman, Yemen, Turkmenistan, Iran, Iraq, Libya, and Algeria. Agricultural methane sources are underestimated in India, Brazil, the California Central Valley, and Asia. Overall, anthropogenic sources worldwide are increased by +19 [11–31] Tg yr−1 over the prior estimate. More than 45 % of this adjustment occurs over India and Southeast Asia during the summer monsoon (+8.5 [3.1–10.7] Tg in July–October), likely due to rainfall-enhanced emissions from rice, manure, and landfills/sewers, which increase during this season along with the natural wetland source.

Trends in anthropogenic ammonia emissions in China since 1980: A review of approaches and estimations

Ammonia (NH3) emissions from intensive anthropogenic activities is an important component in the global nitrogen cycle that has triggered large negative impacts on air quality and ecosystems worldwide. An accurate spatially explicit high resolution NH3 emission inventory is essential for modeling atmospheric aerosol pollution and nitrogen deposition. However, existing NH3 emission inventories in China are still subject to several uncertainties. In this review we firstly summarize the widely used methods for the estimate of NH3 emissions and discuss their advantages and major limitations. Secondly, we present aggregated data from ten NH3 emission inventories to assess the trends in total anthropogenic NH3 emissions in China over the period 1980–2019. Almost emission estimates reported that NH3 emissions in China have doubled in the last four decades. We find a substantial differences in annual total NH3 emissions, spatial distributions and seasonal variations among selected datasets. In 2012, the median emission (Tg yr−1) and associated minimum-maximum ranges are 12.4 (8.5_17.2) for total emission, 9.9 (8.1_13.8) for agriculture, 0.3 (0.2_1.0) for industry, 0.4 (0.2_1.1) for residential and 0.1 (0.1_0.3) for transport and other emission of 1.5 (0.3_2.6). In general, peak emissions occur in summer but in different months, the higher NH3 emission intensities are concentrated in the NCP area, and in eastern and south-central China but distinct regional discrepancy among selected datasets. Finally, we made an analysis of the reasons and levels of difference in NH3 emission estimates with recommendations for improvement of China’s NH3 emission inventory.

Isoprene and monoterpene simulations using the chemistry–climate model EMAC (v2.55) with interactive vegetation from LPJ-GUESS (v4.0)

Abstract. Earth system models (ESMs) integrate previously separate models of the ocean, atmosphere and vegetation into one comprehensive modelling system enabling the investigation of interactions between different components of the Earth system. Global isoprene and monoterpene emissions from terrestrial vegetation, which represent the most important source of volatile organic compounds (VOCs) in the Earth system, need to be included in global and regional chemical transport models given their major chemical impacts on the atmosphere. Due to the feedback of vegetation activity involving interactions with weather and climate, a coupled modelling system between vegetation and atmospheric chemistry is recommended to address the fate of biogenic volatile organic compounds (BVOCs). In this work, further development in linking LPJ-GUESS, a global dynamic vegetation model, to the atmospheric-chemistry-enabled atmosphere–ocean general circulation model EMAC is presented. New parameterisations are included to calculate the foliar density and leaf area density (LAD) distribution from LPJ-GUESS information. The new vegetation parameters are combined with existing LPJ-GUESS output (i.e. leaf area index and cover fractions) and used in empirically based BVOC modules in EMAC. Estimates of terrestrial BVOC emissions from EMAC's submodels ONEMIS and MEGAN are evaluated using (1) prescribed climatological vegetation boundary conditions at the land–atmosphere interface and (2) dynamic vegetation states calculated in LPJ-GUESS (replacing the “offline” vegetation inputs). LPJ-GUESS-driven global emission estimates for isoprene and monoterpenes from the submodel ONEMIS were 546 and 102 Tg yr−1, respectively. MEGAN determines 657 and 55 Tg of isoprene and monoterpene emissions annually. The new vegetation-sensitive BVOC fluxes in EMAC are in good agreement with emissions from the semi-process-based module in LPJ-GUESS. The new coupled system is used to evaluate the temperature and vegetation sensitivity of BVOC fluxes in doubling CO2 scenarios. This work provides evidence that the new coupled model yields suitable estimates for global BVOC emissions that are responsive to vegetation dynamics. It is concluded that the proposed model set-up is useful for studying land–biosphere–atmosphere interactions in the Earth system.

Widespread Frequent Methane Emissions From the Oil and Gas Industry in the Permian Basin.

Emissions of methane (CH4) in the Permian basin (USA) have been derived for 2019 and 2020 from satellite observations of the Tropospheric Monitoring Instrument (TROPOMI) using the divergence method, in combination with a data driven method to estimate the background column densities. The resulting CH4 emission data, which have been verified using model data with known emissions, have a spatial resolution of approximately 10km. The CH4 emissions show moderate spatial correlation with the locations of oil and gas production and drilling activities in the Permian basin, as well as with emissions of nitrogen oxides (NOx). Analysis of the emission maps and time series indicates that a significant fraction of methane emissions in the Permian basin is from frequent widespread emissions sources, rather than from a few infrequent very large unplanned releases, which is important considering possible CH4 emission mitigation strategies. In addition to providing spatially resolved emissions, the divergence method also provides the total emissions of the Permian basin and its main sub-basins. The total CH4 emission of the Permian is estimated as 3.0±0.7Tgyr-1 for 2019, which agrees with other independent estimates based on TROPOMI data. For the Delaware sub-basin, it is estimated as 1.4±0.3Tgyr-1 for 2019, and for the Midland sub-basin 1.2±0.3Tgyr-1. In 2020 the emissions are 9% lower compared to 2019 in the entire Permian basin, and respectively 19% and 27% for the Delaware and Midland sub-basins.

Flaring efficiencies and NOx emission ratios measured for offshore oil and gas facilities in the North Sea

Abstract. Gas flaring is a substantial global source of carbon emissions to atmosphere and is targeted as a route to mitigating the oil and gas sector carbon footprint due to the waste of resources involved. However, quantifying carbon emissions from flaring is resource-intensive, and no studies have yet assessed flaring emissions for offshore regions. In this work, we present carbon dioxide (CO2), methane (CH4), ethane (C2H6), and NOx (nitrogen oxide) data from 58 emission plumes identified as gas flaring, measured during aircraft campaigns over the North Sea (UK and Norway) in 2018 and 2019. Median combustion efficiency, the efficiency with which carbon in the flared gas is converted to CO2 in the emission plume, was 98.4 % when accounting for C2H6 or 98.7 % when only accounting for CH4. Higher combustion efficiencies were measured in the Norwegian sector of the North Sea compared with the UK sector. Destruction removal efficiencies (DREs), the efficiency with which an individual species is combusted, were 98.5 % for CH4 and 97.9 % for C2H6. Median NOx emission ratios were measured to be 0.003 ppm ppm−1 CO2 and 0.26 ppm ppm−1 CH4, and the median C2H6:CH4 ratio was measured to be 0.11 ppm ppm−1. The highest NOx emission ratios were observed from floating production storage and offloading (FPSO) vessels, although this could potentially be due to the presence of alternative NOx sources on board, such as diesel generators. The measurements in this work were used to estimate total emissions from the North Sea from gas flaring of 1.4 Tg yr−1 CO2, 6.3 Gg yr−1 CH4, 1.7 Gg yr−1 C2H6 and 3.9 Gg yr−1 NOx.

Reconciling the bottom-up and top-down estimates of the methane chemical sink using multiple observations

Abstract. The methane chemical sink estimated by atmospheric chemistry models (bottom-up method) is significantly larger than estimates based on methyl chloroform (MCF) inversions (top-down method). The difference is partly attributable to large uncertainties in hydroxyl radical (OH) concentrations simulated by the atmospheric chemistry models used to derive the bottom-up estimates. In this study, we propose a new approach based on OH precursor observations and a chemical box model. This approach contributes to improving the 3D distributions of tropospheric OH radicals obtained from atmospheric chemistry models and reconciling bottom-up and top-down estimates of the chemical loss of atmospheric methane. By constraining simulated OH precursors with observations, the global mean tropospheric column-averaged air-mass-weighted OH concentration ([OH]trop-M) is ∼10×105 molec. cm−3 (which is 2×105 molec. cm−3 lower than the original model-simulated global [OH]trop-M) and agrees with that obtained by the top-down method based on MCF inversions. With OH constrained by precursor observations, the methane chemical loss is 471–508 Tg yr−1, averaged from 2000 to 2009. The new adjusted estimate is in the range of the latest top-down estimate of the Global Carbon Project (GCP) (459–516 Tg yr−1), contrary to the bottom-up estimates that use the original model-simulated OH fields (577–612 Tg yr−1). The overestimation of global [OH]trop-M and methane chemical loss simulated by the atmospheric chemistry models is caused primarily by the models' underestimation of carbon monoxide and total ozone column, and overestimation of nitrogen dioxide. Our results highlight that constraining the model-simulated OH fields with available OH precursor observations can help improve bottom-up estimates of the global methane sink.

Aerobic methane emissions from the shoots of Scots pine

Plants are recognized sources of methane (CH4) but plant-mediated CH4 emissions have mostly been studied on herbaceous species, although also trees are known to emit CH4. Emissions from tree canopies likely mostly derive from an aerobic, abiotic process. Aerobic CH4 production from trees has not been thoroughly studied, leaving uncertainties to the global source strength estimates, which vary from 0 to 240 Tg yr-1. Even less is known about how aerobic emissions affect the CH4 cycles in boreal forest ecosystems, as the environmental and physiological drivers are not fully understood. In this study, shoot-level CH4 fluxes of boreal conifer trees were measured outdoors and in the greenhouse, to investigate the environmental and physiological drivers and regulators of shoot-level CH4 fluxes. Most of the measurements were done from saplings of Pinus sylvestris L. (Scots pine), one of the most important tree species of the boreal region of the Eurasian continent, by using chamber enclosure methods with spectral, online greenhouse gas analysers. The measurements were conducted either manually, or with an automated measurement system, developed to overcome issues related to manual measurement techniques. The shoots of Scots pine showed small but significant emissions of CH4 in all experimental setups, the emissions were driven by light, enhanced by elevated temperature, and occurred independently from drought and photosynthesis. Solar radiation was a more significant driver of these CH4 emissions than artificial light with UV-A. These results show that Scots pine canopies have the potential to produce CH4 in a similar process that has been described before for the foliage of herbaceous plants, but these emissions are smaller than the initial estimates of the aerobic CH4 source from vegetation. The boreal forest canopies are sources of CH4 and have the capacity to decrease the CH4 sink strength of boreal upland forests by ~ 5 %.

Landscape Change Affects Soil Organic Carbon Mineralization and Greenhouse Gas Production in Coastal Wetlands

AbstractPlant invasion and aquaculture activities have drastically modified the landscape of coastal wetlands in many countries, but their impacts on soil organic carbon (SOC) mineralization and greenhouse gas production remain poorly understood. We measured SOC mineralization rate and soil CO2 and CH4 production rates in three habitat types from 21 coastal sites across the tropical and subtropical zones in China: native mudflats (MFs), Spartina alterniflora marshes (SAs), and aquaculture ponds (APs). Landscape change from MFs to SAs or APs increased total and labile fraction of SOC, as well as carbon mineralization rate and greenhouse gas production, but there were no discernible differences in SOC source‐sink dynamics between SAs and APs. SOC mineralization rate was highest in SAs (20.4 μg g−1 d−1), followed by APs (16.9 μg g−1 d−1) and MFs (11.9 μg g−1 d−1), with CO2 as the dominant by‐product. Bioavailable SOC was less than 2% and was turned over within 60 days in all three habitat types. Proliferation of S. alterniflora marshes and expansion of AP construction had resulted in a net increase in soil CO2‐eq production of 0.4–4.3 Tg yr−1 in the last three decades. Future studies will benefit from better census and monitoring of coastal habitats in China, complementary in situ measurements of greenhouse gas emissions, and more sampling in the southern provinces to improve spatial resolution.

High Spatial and Temporal Resolution Methane Emissions Inventory from Terrestrial Ecosystems in China, 2010–2020

Methane (CH4) is not only an important greenhouse gas next to carbon dioxide (CO2), but also an important chemically active gas. Under the background of climate warming, the measurement of CH4 emissions from terrestrial ecosystems in China is not only very important for exploring the impact of climate change on the ecological environment, but also of great significance for the in-depth study of ecosystem carbon cycling. In this study, we used the Emission-Factor Approach to estimate CH4 emissions from terrestrial ecosystems in China from 2010–2020 and explored the spatial distribution characteristics of CH4 emissions. The estimated CH4 emission inventory of terrestrial ecosystems with 0.05 spatial resolution on monthly time scale is in good agreement with the results of the latest emission inventory. It is estimated that CH4 emissions from terrestrial ecosystems in China are 19.955 Tg yr−1, including 18.61% (3.713 Tg yr−1) from vegetation, 21.47% (4.285 Tg yr−1) from wetlands and 59.92% (11.957 Tg yr−1) from paddy fields, with the largest contribution from paddy fields. The regions with high CH4 emissions from terrestrial ecosystems in China are mainly located in the central, eastern and southeastern regions of China, and show a decreasing trend from southeast to northwest. The CH4 emission from terrestrial ecosystems in China has obvious seasonal variation characteristics, with the lowest emission in January (0.248 Tg month−1) and the highest emission in August (3.602 Tg month−1). The emissions are high in summer and autumn and low in spring and winter. CH4 emissions from terrestrial ecosystems in China showed an overall upward trend from 2010–2020.

Paddy rice methane emissions across Monsoon Asia

Although rice cultivation is one of the most important agricultural sources of methane (CH4) and contributes ∼8% of total global anthropogenic emissions, large discrepancies remain among estimates of global CH4 emissions from rice cultivation (ranging from 18 to 115 Tg CH4 yr−1) due to a lack of observational constraints. The spatial distribution of paddy-rice emissions has been assessed at regional-to-global scales by bottom-up inventories and land surface models over coarse spatial resolution (e.g., > 0.5°) or spatial units (e.g., agro-ecological zones). However, high-resolution CH4 flux estimates capable of capturing the effects of local climate and management practices on emissions, as well as replicating in situ data, remain challenging to produce because of the scarcity of high-resolution maps of paddy-rice and insufficient understanding of CH4 predictors. Here, we combine paddy-rice methane-flux data from 23 global eddy covariance sites and MODIS remote sensing data with machine learning to 1) evaluate data-driven model performance and variable importance for predicting rice CH4 fluxes; and 2) produce gridded up-scaling estimates of rice CH4 emissions at 5000-m resolution across Monsoon Asia, where ∼87% of global rice area is cultivated and ∼ 90% of global rice production occurs. Our random-forest model achieved Nash-Sutcliffe Efficiency values of 0.59 and 0.69 for 8-day CH4 fluxes and site mean CH4 fluxes respectively, with land surface temperature, biomass and water-availability-related indices as the most important predictors. We estimate the average annual (winter fallow season excluded) paddy rice CH4 emissions throughout Monsoon Asia to be 20.6 ± 1.1 Tg yr−1 for 2001–2015, which is at the lower range of previous inventory-based estimates (20–32 CH4 Tg yr−1). Our estimates also suggest that CH4 emissions from paddy rice in this region have been declining from 2007 through 2015 following declines in both paddy-rice growing area and emission rates per unit area, suggesting that CH4 emissions from paddy rice in Monsoon Asia have likely not contributed to the renewed growth of atmospheric CH4 in recent years.

The Shortwave Radiative Flux Response to an Injection of Sea Salt Aerosols in the Gulf of Mexico

AbstractMarine cloud brightening (MCB) has been proposed as a potential means of geoengineering the climate, temporarily providing cooling to offset some of the effects of climate change. Marine sky brightening (MSB), involving the direct scattering of sunlight from sea salt injection into the marine boundary layer, has been proposed as an additional geoengineering method that could work in areas that are not regularly cloudy. Here, we use a regional atmospheric model to simulate MCB and MSB over the Gulf of Mexico and nearby land, a highly populated and economically important region that is not characterized by persistent marine stratocumulus cloud cover. Injection of sea salt in the Aitken mode from a region in the central Gulf of Mexico equivalent to 10.8 Tg yr−1 produces an upwards 8.4 W m−2 radiative flux change across the region at the top of the atmosphere, largely due to cloud property changes. Comparatively, a similar mass injection in the accumulation mode produces a 3.1 W m−2 radiative flux change driven primarily by direct scattering. Injection of even larger particles produces a much smaller radiative flux change. Shortwave flux changes due to clouds are largely driven by an increase in cloud droplet number concentration and an increase in cloud liquid water path (each contributing about 45% to the flux change), with a much lower contribution from cloud fraction changes (10%).