Greenhouse Gas Budget Research Articles

A Greenhouse Gas Budget for Mexico During 2000–2019

AbstractApplication of the best available science to improve quantification of greenhouse gas (GHG) emissions at regional and national scales is key to climate action. Here, we present a two‐decade (2000–2019) GHG (CO2, CH4, and N2O) budget for Mexico derived from multiple products. Data from the National GHG Inventory, global observations, and the scientific literature were compared to identify knowledge gaps on GHG flux dynamics and discrepancies among estimates. Total mean annual GHG emissions were estimated at 695–910 TgCO2‐eq year−1 over these two decades, with 70% of the emissions attributable to CO2, 23% to CH4, and 5% to N2O (2% to other gases). When divided by sectors, we found agreement across emission estimates from various sources for fossil fuels, cattle, agriculture, and waste for all GHGs. However, considerable discrepancies were identified in the fluxes from terrestrial ecosystems. The disagreement was particularly large for the land CO2 sink, where net biome production estimations from the national inventory were double those from any other observational product. Extensive knowledge gaps exist, mainly related to aquatic systems (e.g., outgassing in rivers) and the lateral fluxes (e.g., wood trade). In addition, limited information is available on CH4 emissions from wetlands and soil CH4 consumption. We expect these results to guide future research to reduce estimation uncertainties and fill the information gaps across Mexico.

Seasonal patterns and key chemical predictors of dissolved greenhouse gases in small prairie pothole ponds

Shallow ponds can provide ideal conditions for production of greenhouse gases (GHGs) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), and thus are important to include in global and regional GHG budgets. The Canadian Prairie Pothole Region contains millions of shallow natural ponds, and we investigated GHG dynamics in 145 ponds across the region. Ponds were consistently supersaturated with CH4, often supersaturated with CO2 (57% occurrence), and often undersaturated with N2O (65% occurrence). Spring measurements showed higher N2O saturation ( p = 0.0037) than summer, while summer had higher CH4 ( p < 0.001) and CO2 ( p = 0.023) saturation than spring. Ponds exhibited large physicochemical variation, yet sulfate concentration and pH were strong predictors of dissolved CH4 and CO2, respectively. No predictor was identified for N2O. The link between sulfate and CH4 has important implications as dissolved CH4 in low sulfate (<178 mg L−1) systems was much more responsive to changes in temperature. This research fills an important knowledge gap about the GHG dynamics of prairie pothole ponds and the role of water chemistry for diffuse GHG release. Our work can also be used in ongoing efforts to describe ecosystem services (or disservices) assigned to ponds in this agriculture-dominated region.

Relationship between Greenhouse Gas Budget and Soil Carbon Storage Measured on Site in Zhalainuoer Grassland Mining Area

Grassland has great potential for carbon sequestration; however, the relationship between carbon storage (CS) and greenhouse gas (GHG) budget and their influencing factors in the natural restoration process in grassland mining areas are rarely studied. In this study, taking Zhalainuoer mining area in Inner Mongolia as an example, the subsidence soil for 1-, 2-, 5-, 10-, and 15-year and non-subsidence soil were selected as the research objects to explore the relationship between CS and the GHG budget and their influencing factors. The results show that there is a significant negative correlation between CS and the GHG budget. Soil organic carbon storage accounts for 99% of CS. CS is positively correlated with SOM and AP, and with the bacteria Entotheonellaeota. The GHG budget is mainly affected by CO2 emission, which is positively correlated with subsidence time, plant biomass, and coverage, negatively correlated with the bacteria Actinobacteriota and Deinococcota, and positively correlated with Cyanobacteria. In summary, soil plays a major role in storing carbon. Carbon sequestration is a physiological process produced by plants and organisms. Subsidence affects soil CS by changing soil properties and thus affecting its aboveground vegetation and soil microorganisms. This study investigates the changes in soil carbon storage following subsidence caused by mining activities. The findings contribute to our understanding of the impact of mining subsidence on soil CS and can inform the development of low-carbon remediation technologies.

Significant inter-annual fluctuation in CO2 and CH4 diffusive fluxes from subtropical aquaculture ponds: Implications for climate change and carbon emission evaluations

Aquaculture ponds are potential hotspots for carbon cycling and emission of greenhouse gases (GHGs) like CO2 and CH4, but they are often poorly assessed in the global GHG budget. This study determined the temporal variations of CO2 and CH4 concentrations and diffusive fluxes and their environmental drivers in coastal aquaculture ponds in southeastern China over a five-year period (2017–2021). The findings indicated that CH4 flux from aquaculture ponds fluctuated markedly year-to-year, and CO2 flux varied between positive and negative between years. The coefficient of inter-annual variation of CO2 and CH4 diffusive fluxes was 168% and 127%, respectively, highlighting the importance of long-term observations to improve GHG assessment from aquaculture ponds. In addition to chlorophyll-a and dissolved oxygen as the common environmental drivers, CO2 was further regulated by total dissolved phosphorus and CH4 by dissolved organic carbon. Feed conversion ratio correlated positively with both CO2 and CH4 concentrations and fluxes, showing that unconsumed feeds fueled microbial GHG production. A linear regression based on binned (averaged) monthly CO2 diffusive flux data, calculated from CO2 concentrations, can be used to estimate CH4 diffusive flux with a fair degree of confidence (r2 = 0.66; p < 0.001). This algorithm provides a simple and practical way to assess the total carbon diffusive flux from aquaculture ponds. Overall, this study provides new insights into mitigating the carbon footprint of aquaculture production and assessing the impact of aquaculture ponds on the regional and global scales.

Smallholder Cropping Systems Contribute Limited Greenhouse Gas Fluxes in Upper Eastern Kenya

The contribution of smallholder farming systems to the National greenhouse gas (GHG) budget is missing in most developing countries, including Kenya. Data on the contribution of smallholder cropping systems to the GHG balance is essential for realising Sustainable Development Goal 13 on climate action, i.e., on nationally determined contributions (NDCs) and in compliance with the Paris Agreement. Do smallholder farming systems act as nature-based solutions for greenhouse gas emissions reduction? This study evaluated GHG emissions from cropping systems under on-farm smallholder farming conditions. We had five cropping systems on two smallholder farms: sole maize, maize-bean intercrop, coffee, banana, and agroforestry. Gas samples were collected using three static chambers per cropping system. The gas samples were analysed using gas chromatography (GC) fitted with a 63Ni-electron capture detector (ECD) for N2O and flame ionisation detector (FID) for CH4 and CO2 using N as carrier gas. Cumulative annual fluxes of (CH4, N2O, and CO2) varied significantly in farms one and two across the cropping systems. The cumulative soil GHG fluxes ranged from -1.34 kg CH4-C ha−1 yr−1 under agroforestry to -0.77 kg CH4-C ha−1 yr−1 under banana for CH4, 0.30 kg N2O-N ha−1 yr−1 to 1.23 kg N2O-N ha−1 yr−1 for N2O and 5949 kg CO2-C ha−1 yr−1 to 12954 kg CO2-C ha−1 yr−1 for CO2. The maize grain yields ranged from 0 to 3.38 Mg ha−1. The N2O yields scaled emissions ranged from 0.10 to 0.26 g kg−1 maize and 0.68 to 1.30 g kg−1 beans. Smallholder farmers in Upper Eastern Kenya contribute a limited amount of soil GHG emissions and thus could act as a nature-based solution for lowering agricultural emissions.

Mitigating Greenhouse Gas Emissions from Crop Production and Management Practices, and Livestock: A Review

Agriculture is the second most important greenhouse gas (GHG: methane (CH4) and nitrous oxide (N2O) emissions)-emitting sector after the energy sector. Agriculture is also recognized as the source and sink of GHGs. The share of agriculture to the global GHG emission records has been widely investigated, but the impact on our food production systems has been overlooked for decades until the recent climate crisis. Livestock production and feed, nitrogen-rich fertilizers and livestock manure application, crop residue burning, as well as water management in flood-prone cultivation areas are components of agriculture that produce and emit most GHGs. Although agriculture produces 72–89% less GHGs than other sectors, it is believed that reducing GHG emissions in agriculture would considerably lower its share of the global GHG emission records, which may lead to enormous benefits for the environment and food production systems. However, several diverging and controversial views questioning the actual role of plants in the current global GHG budget continue to nourish the debate globally. We must acknowledge that considering the beneficial roles of major GHGs to plants at a certain level of accumulation, implementing GHG mitigation measures from agriculture is indeed a complex task. This work provides a comprehensive review of agriculture-related GHG production and emission mechanisms, as well as GHG mitigation measures regarded as potential solutions available in the literature. This review also discusses in depth the significance and the dynamics of mitigation measures regarded as game changers with a high potential to enhance, in a sustainable manner, the resilience of agricultural systems. Some of the old but essential agricultural practices and livestock feed techniques are revived and discussed. Agricultural GHG mitigation approaches discussed in this work can serve as game changers in the attempt to reduce GHG emissions and alleviate the impact of climate change through sustainable agriculture and informed decision-making.

Hot spots and hot moments of greenhouse gas emissions in agricultural peatlands

AbstractDrained agricultural peatlands occupy only 1% of agricultural land but are estimated to be responsible for approximately one third of global cropland greenhouse gas emissions. However, recent studies show that greenhouse gases fluxes from agricultural peatlands can vary by orders of magnitude over time. The relationship between these hot moments (individual fluxes with disproportionate impact on annual budgets) of greenhouse gas emissions and individual chamber locations (i.e. hot spots with disproportionate observations of hot moments) is poorly understood, but may help elucidate patterns and drivers of high greenhouse gas emissions from agricultural peatland soils. We used continuous chamber-based flux measurements across three land uses (corn, alfalfa, and pasture) to quantify the spatiotemporal patterns of soil greenhouse gas emissions from temperate agricultural peatlands in the Sacramento-San Joaquin Delta of California. We found that the location of hot spots of emissions varied over time and were not consistent across annual timescales. Hot moments of nitrous oxide (N2O) and carbon dioxide (CO2) fluxes were more evenly distributed across space than methane (CH4). In the corn system, hot moments of CH4 flux were often isolated to a single location but locations were not consistent across years. Spatiotemporal variability in soil moisture, soil oxygen, and temperature helped explain patterns in N2O fluxes in the annual corn agroecosystem but were less informative for perennial alfalfa N2O fluxes or CH4 fluxes across ecosystems, potentially due to insufficient spatiotemporal resolution of the associated drivers. Overall, our results do not support the concept of persistent hot spots of soil CO2, CH4, and N2O emissions in these drained agricultural peatlands. Hot moments of high flux events generally varied in space and time and thus required high sample densities. Our results highlight the importance of constraining hot moments and their controls to better quantify ecosystem greenhouse gas budgets.

Biogeography of microbial communities in high-latitude ecosystems: Contrasting drivers for methanogens, methanotrophs and global prokaryotes.

Methane-cycling is becoming more important in high-latitude ecosystems as global warming makes permafrost organic carbon increasingly available. We explored 387 samples from three high-latitudes regions (Siberia, Alaska and Patagonia) focusing on mineral/organic soils (wetlands, peatlands, forest), lake/pond sediment and water. Physicochemical, climatic and geographic variables were integrated with 16S rDNA amplicon sequences to determine the structure of the overall microbial communities and of specific methanogenic and methanotrophic guilds. Physicochemistry (especially pH) explained the largest proportion of variation in guild composition, confirming species sorting (i.e., environmental filtering) as a key mechanism in microbial assembly. Geographic distance impacted more strongly beta diversity for (i) methanogens and methanotrophs than the overall prokaryotes and, (ii) the sediment habitat, suggesting that dispersal limitation contributed to shape the communities of methane-cycling microorganisms. Bioindicator taxa characterising different ecological niches (i.e., specific combinations of geographic, climatic and physicochemical variables) were identified, highlighting the importance of Methanoregula as generalist methanogens. Methylocystis and Methylocapsa were key methanotrophs in low pH niches while Methylobacter and Methylomonadaceae in neutral environments. This work gives insight into the present and projected distribution of methane-cycling microbes at high latitudes under climate change predictions, which is crucial for constraining their impact on greenhouse gas budgets.

The greenhouse gas budget for China's terrestrial ecosystems.

The first greenhouse gas (GHG) budget accounting over China shows that China's land ecosystems is close to GHG neutral, in contrast to the net GHG source of global land ecosystems.

Maximizing Grains While Minimizing Yield-Scaled Greenhouse Gas Emissions for Wheat Production in China

Researchers have previously described the response of crop productivity and greenhouse gas (GHG) emissions to fertilizer nitrogen (N) additions, but they have not determined how to maximize yields while minimizing GHG emissions. We conducted an experiment at 2293 sites with four N levels to simulate both grain yield and yield-scaled GHG emissions in response to the N addition. The yield-scaled GHG emissions decreased by 16% as the N rate increased from treatments without the N addition to the minimum yield-scaled GHG emissions, which was comparable to the values associated with the maximum grain yields. The sites with both high soil productivity and high crop productivity had the highest yield and lowest yield-scaled GHG emissions, with 43% higher yield and 38% lower yield-scaled GHG emissions than sites with low soil and low crop productivity. These findings are expected to enhance evaluations of wheat production and GHG emissions in China, and thereby contribute to addressing disparities in the global food and GHG budget.

After-use of cutover peatland from the perspective of landowners: Future effects on the national greenhouse gas budget in Finland

Recently, increased attention has been drawn to the greenhouse gas emissions of the Land use, Land use Change and Forestry (LULUCF) sector. In the Finnish LULUCF sector, the soil-originated emissions from the after-use of cutover peatland have become a more relevant question. This is due to the rapid increase in the number of former peat-harvesting sites, caused by the decline in the combustion of energy peat due to its high CO2 emissions. We conducted a nationally representative survey to study which after-use options of cutover peatlands are preferred by landowners. Based on the preferred after-use options, we estimated their impact on the national greenhouse gas emissions in the Finnish regions in 2035, and what factors influence the landowners’ choice of after-use options. We found that the four most popular after-use options for landowners were afforestation (71%), agriculture (24%), production of wind and solar power (22%) and rewetting (18%) when the respondent chose the three most attractive options. However, there were clear regional differences in the preferred after-use options, which might lead, if realized, to different soil-originated emissions at the regional level. The preferred after-use options could produce emissions of 0.35 million tonnes of carbon dioxide equivalent a year (Mt CO2 eq y−1) by 2035 in Finland. Most of these emissions (30.1%) would come from South Ostrobothnia in western Finland, mainly due to the indicated high interest in agricultural after-use. Most landowners prefer financially profitable after-use options, such as forestry, agriculture, as well as wind and solar energy production in cutover peatlands. However, environmentally oriented landowners favour many kinds of actions to support biodiversity, carbon sequestration, and water protection in cutover peatlands. It is crucial that economic productivity and the greenhouse gas sinks of cutover peatlands be considered simultaneously to meet the climate targets of the LULUCF sector in Finland.

Spatially-explicit environmental assessment of bioethanol from miscanthus and switchgrass in France

Bioethanol is promoted as a means of tackling climate change, diversifying energy sources and securing energy supply. However, there also concerns that their wider deployment could lead to unintended environmental consequences. Life cycle assessment (LCA) is a widely used methodology to assess the environmental performance of biofuels. However, its outcomes strongly depend on the inventory data and modeling assumptions. Agronomic variables such as crop yields, nitrogen fertilizer rates or field emissions of nitrous oxide are very sensitive inputs, as are soil carbon dynamics in response to land use changes (LUC) entailed by the deployment of energy crops. Models simulating agroecosystem processes and the economics of agricultural farms are promising tools to predict such variables and improve the reliability of LCA.Here, we combined the agro-ecosystem model CERES-EGC, the farm economic model AROPAj and the LCA approach to investigate the effect of local drivers on the environmental impacts of bioethanol from miscanthus and switchgrass over France.Overall, lignocellulosic bioethanol achieved GHG abatement targets in the 74 %–94 % range compared to gasoline, and complied with the 50 % minimum imposed by European regulations. Miscanthus-based ethanol achieved up to twice lower environmental impacts than switchgrass due to 50 % higher biomass yields overall. Low fertilizer N input rates (in the 0-30 kg N ha-1 yr-1 range) proved the most efficient strategy to optimize energy return. Significant inter-regional variability occurred, especially in terms of soil C sequestration rates, which weighed in substantially on GHG budgets. Some regions were more efficient than others as a result, which advocates a site-specific approach and a potential prioritization when planning biorefineries, taking into account local production and environmental performance potentials. Compared to previous studies, ours provided high-resolution data in terms of crop yields, nitrous oxide emissions and soil C dynamics, factoring in LUC effects at local to regional scales.

Inclusive greenhouse gas budget assessment in turfs: From turf production to disposal of grass clippings

Globally, greenhouse gas (GHG) reduction is a serious concern. To evaluate whether turfs serve as a GHG sink or source, GHG budget assessments for life cycle are required. However, previous studies have only focused on the use of turfs. To bridge these gaps in literature, this study investigated GHG (CO2, N2O, and CH4) emissions from the disposal of grass clippings and soil GHG fluxes in turfs. Additionally, GHG budgets in the turf production phase were assessed. Finally, inclusive GHG budgets from turf production to disposal of grass clippings for four turf uses (soccer stadium, golf course, office, and urban park) were assessed. Grass clippings were disposed in three forms (incineration, leaving as-is, and biochar). We found that GHG emissions from incineration and leaving 1 t-fresh weight (FW) of grass clippings were 0.711 and 0.207 t-CO2e, respectively. Contrastingly, the GHG emissions from the biochar yield from 1 t-FW of grass clippings were −0.200 t-CO2e. Further, annual soil GHG fluxes in newly established Zoysia and Kentucky bluegrass turfs were calculated at 0.067 and 0.040 tCO2e･ha-1･yr-1, respectively. As the turf grass in production fields sequester large amounts of CO2, GHG budgets in turf production phase were estimated at approximately −20 t-CO2e･ha-1･yr-1. Inclusive GHG budget assessment from turf production to disposal of grass clippings showed that turfs only in the urban parks served as a GHG sink and this ability was comparable to CO2 sequestration in forests. To enhance the ability of GHG sinks and to promote changes from a GHG source to GHG sink, our study revealed the importance of reduction of GHG emissions from energy and resource uses (especially fertilizers and gasoline) for turf management.

High Intra‐Seasonal Variability in Greenhouse Gas Emissions From Temperate Constructed Ponds

AbstractInland waters play a major role in global greenhouse gas (GHG) budgets. The smallest of these systems (i.e., ponds) have a particularly large—but poorly constrained—emissions footprint at the global scale. Much of this uncertainty is due to a poor understanding of temporal variability in emissions. Here, we conducted high‐resolution temporal sampling to quantify GHG exchange between four temperate constructed ponds and the atmosphere on an annual basis. We show these ponds are a net source of GHGs to the atmosphere (564.4 g CO2‐eq m−2 yr−1), driven by highly temporally variable diffusive methane (CH4) emissions. Diffusive CH4 release to the atmosphere was twice as high during periods when the ponds had a stratified water column than when it was mixed. Ebullitive CH4 release was also higher during stratification. Building ponds to favor mixed conditions thus presents an opportunity to minimize the global GHG footprint of future pond construction.

Urban Sewage Canal sediment in Kolkata Metropolis (India) is a potent producer of greenhouse gases

The large network of sewage canals criss-crossing Kolkata Metropolis carry considerable sewage load of about 4.5 million people. In an incubation study, substantial carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) production were observed in sediment samples collected from the canal bed at five sites, located at city outskirt, urban slum area, industrial area, residential area and agricultural area. The maximum weekly total production of CO2 and CH4 were 201.6, 137.4 mg kg−1 sediment, respectively, while N2O production was 37.1 μg kg−1 sediment. This translated into maximum annual production of 28609.5, 19498.6 and 5.3 t y−1 CO2, CH4 and N2O, respectively, from the sediment pool lying within 0.30 m depth in entire canal network. The CO2-equivalent (100-year time horizon) production in the sediment of canal network was estimated to be 0.56 Tg y−1. The estimated productions underline the chances of substantial emissions of CO2, CH4 and N2O from such large urban sewage canal networks. Sewage canals as GHG source remains under-represented in greenhouse gas (GHG) inventories due to lack of understanding and data on GHG production in canal sediments and their potential subsequent emissions. This study highlights that sewage canals deserve serious consideration in the preparation of GHG budgets in regions hosting large network of sewage canals.

Anthropogenic activities significantly increase annual greenhouse gas (GHG) fluxes from temperate headwater streams in Germany

Abstract. Anthropogenic activities increase the contributions of inland waters to global greenhouse gas (GHG; CO2, CH4, and N2O) budgets, yet the mechanisms driving these increases are still not well constrained. In this study, we quantified year-long GHG concentrations, fluxes, and water physico-chemical variables from 28 sites contrasted by land use across five headwater catchments in Germany. Based on linear mixed-effects models, we showed that land use was more significant than seasonality in controlling the intra-annual variability of the GHGs. Streams in agriculture-dominated catchments or with wastewater inflows had up to 10 times higher daily CO2, CH4, and N2O emissions and were also more temporally variable (CV &gt; 55 %) than forested streams. Our findings also suggested that nutrient, labile carbon, and dissolved GHG inputs from the agricultural and settlement areas may have supported these hotspots and hot-moments of fluvial GHG emissions. Overall, the annual emission from anthropogenic-influenced streams in CO2 equivalents was up to 20 times higher (∼ 71 kg CO2 m−2 yr−1) than from natural streams (∼ 3 kg CO2 m−2 yr−1), with CO2 accounting for up to 81 % of these annual emissions, while N2O and CH4 accounted for up to 18 % and 7 %, respectively. The positive influence of anthropogenic activities on fluvial GHG emissions also resulted in a breakdown of the expected declining trends of fluvial GHG emissions with stream size. Therefore, future studies should focus on anthropogenically perturbed streams, as their GHG emissions are much more variable in space and time and can potentially introduce the largest uncertainties to fluvial GHG estimates.

Impact of thunderstorm activities on tropospheric NOx adversely affecting the climate change over Bagmati province of Nepal

Lightning is a complex electrical discharge that occurs in the atmosphere. Huge currents associated with the lightning discharge raises the ambient temperature resulting in the change in atmospheric chemistry. The extreme temperatures within lightning channels break apart molecular nitrogen (N2) and oxygen (O2) to produce nitrogen oxides (NOx). NOx act as indirect greenhouse gases by producing the tropospheric Ozone. Likewise, NOx gases also affect the global greenhouse gas budget through the change in concentration of hydroxyl radical (OH) and Methane. This study analyzes the association of lightning stroke density with the tropospheric NOx over the Bagmati province (central region) of Nepal, during pre-monsoon and post-monsoon for 3 years (2018 to 2020). The tropospheric NOx was examined by utilizing the data from Ozone Monitoring Instrument (OMI), whereas the lightning stroke data was obtained from VAISALA’s Global Lightning Detection (GLD-360) Network. The lightning stroke density for each season were plotted against the corresponding average value of NOx to obtain the correlation coefficient over the period of study. Strong positive correlations between lightning and NOx production during per-monsoon periods were obtained for all the three years of study period, whereas, comparatively weak correlations are obtained for the post-monsoon seasons. Nevertheless, the NOx production due to lightning is found to be strong, during the pre-monsoon period, a relatively dry season over the Bagmati province. This clearly indicates that lightning is a big source of tropospheric NOx,that in turn produces greenhouse gases and hence contributes to the climatic changes over the central region on Nepal.

Analysis of Greenhouse Gas Emissions Characteristics and Emissions Reduction Measures of Animal Husbandry in Inner Mongolia

Global warming has had a profound impact on human life, with animal husbandry being a significant contributor to greenhouse gas emissions and playing a crucial role in the global greenhouse gas budget. Inner Mongolia is a major contributor to these emissions, making it vital to study the link between greenhouse gas emissions and animal husbandry in this region for the purpose of reducing emissions. In this study, the emissions of greenhouse gases (CH4, N2O, and CO2) from livestock and poultry breeding from 2010 to 2020 and the emissions of each city from 2020 were estimated, the emissions characteristics were analysed, and the low carbon emissions reduction technical measures were proposed. The results show that (1) the overall greenhouse gas emissions from 2010 to 2020 in Inner Mongolia showed a fluctuating trend; the main emissions sources were gastrointestinal fermentation and faecal management. The annual average CH4 emissions were 994,400 ta−1, and the annual average N2O emissions were 35,100 ta−1. (2) In 2020, the total emissions of each league city were 38.05 million t equivalent of CO2, and the emissions gradually decreased from east to west, with a significant emissions reduction potential. Based on these findings, this study also proposed technical measures for reducing carbon emissions, offering theoretical support to drive the industrial transformation and upgrading of the livestock industry, and promoting green economic development in Inner Mongolia as part of its carbon peaking and neutrality goals.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition.

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Green frontrunner or indebted culprit? Assessing Denmark’s climate targets in light of fair contributions under the Paris Agreement

This paper contributes to academic and policy debates about climate leadership by illustrating an approach to examining national emission reduction targets focusing on Denmark. Widely recognized as a climate leader, Denmark is cherished for both its historical track record and its current climate targets. With a target of 70% emissions reduction by 2030 compared to 1990 stipulated in national law, central actors in Danish policymaking claim that domestic climate policy is aligned with the Paris temperature goals and present Denmark as a ‘green frontrunner.’ We examine the pledges and targets enshrined in the Danish Climate Act in reference to a 1.5 °C global greenhouse gas budget using five different approaches to burden sharing. For all five approaches, we find that the Danish climate target is inadequate given the 1.5 °C goal. Moreover, when only looking at equity approaches for distributive climate justice globally, the Danish target appears drastically insufficient. Denmark is, in this sense, not a green frontrunner but rather an indebted culprit, challenging the dominant narrative in Danish climate policy. Our results thus call into question the premise of the claim of Danish climate leadership, which works to legitimize existing policy and obscure the many dimensions of climate change.