A comprehensive quantification of global nitrous oxide sources and sinks.

Nitrous oxide (N2O), like carbon dioxide, is a long-lived greenhouse gas that accumulates in the atmosphere. Over the past 150 years, increasing atmospheric N2O concentrations have contributed to stratospheric ozone depletion1 and climate change2, with the current rate of increase estimated at 2 per cent per decade. Existing national inventories do not provide a full picture of N2O emissions, owing to their omission of natural sources and limitations in methodology for attributing anthropogenic sources. Here we present a global N2O inventory that incorporates both natural and anthropogenic sources and accounts for the interaction between nitrogen additions and the biochemical processes that control N2O emissions. We use bottom-up (inventory, statistical extrapolation of flux measurements, process-based land and ocean modelling) and top-down (atmospheric inversion) approaches to provide a comprehensive quantification of global N2O sources and sinks resulting from 21 natural and human sectors between 1980 and 2016. Global N2O emissions were 17.0 (minimum-maximum estimates: 12.2-23.5) teragrams of nitrogen per year (bottom-up) and 16.9 (15.9-17.7) teragrams of nitrogen per year (top-down) between 2007 and 2016. Global human-induced emissions, which are dominated by nitrogen additions to croplands, increased by 30% over the past four decades to 7.3 (4.2-11.4) teragrams of nitrogen per year. This increase was mainly responsible for the growth in the atmospheric burden. Our findings point to growing N2O emissions in emerging economies-particularly Brazil, China and India. Analysis of process-based model estimates reveals an emerging N2O-climate feedback resulting from interactions between nitrogen additions and climate change. The recent growth in N2O emissions exceeds some of the highest projected emission scenarios3,4, underscoring the urgency to mitigate N2O emissions.

Effects of aridification on soil total carbon pools in China's drylands.

Drylands are important carbon pools and are highly vulnerable to climate change, particularly in the context of increasing aridity. However, there has been limited research on the effects of aridification on soil total carbon including soil organic carbon and soil inorganic carbon, which hinders comprehensive understanding and projection of soil carbon dynamics in drylands. To determine the response of soil total carbon to aridification, and to understand how aridification drives soil total carbon variation along the aridity gradient through different ecosystem attributes, we measured soil organic carbon, inorganic carbon and total carbon across a ~4000 km aridity gradient in the drylands of northern China. Distribution patterns of organic carbon, inorganic carbon, and total carbon at different sites along the aridity gradient were analyzed. Results showed that soil organic carbon and inorganic carbon had a complementary relationship, that is, an increase in soil inorganic carbon positively compensated for the decrease in organic carbon in semiarid to hyperarid regions. Soil total carbon exhibited a nonlinear change with increasing aridity, and the effect of aridity on total carbon shifted from negative to positive at an aridity level of 0.71. In less arid regions, aridification leads to a decrease in total carbon, mainly through a decrease in organic carbon, whereas in more arid regions, aridification promotes an increase in inorganic carbon and thus an increase in total carbon. Our study highlights the importance of soil inorganic carbon to total carbon and the different effects of aridity on soil carbon pools in drylands. Soil total carbon needs to be considered when developing measures to conserve the terrestrial carbon sink.

Declined terrestrial ecosystem resilience.

Terrestrial ecosystem resilience is crucial for maintaining the structural and functional stability of ecosystems following disturbances. However, changes in resilience over the past few decades and the risk of future resilience loss under ongoing climate change are unclear. Here, we identified resilience trends using two remotely sensed vegetation indices, analyzed the relative importance of potential driving factors to resilience changes, and finally assessed the risk of future resilience loss based on the output data of eight models from CMIP6. The results revealed that more than 60% of the ecosystems experienced a conversion from an increased trend to a declined trend in resilience. Attribution analysis showed that the most important driving factors of declined resilience varied regionally. The declined trends in resilience were associated with increased precipitation variability in the tropics, decreased vegetation cover in arid region, increased temperature variability in temperate regions, and increased average temperature in cold regions. CMIP6 reveals that terrestrial ecosystems under SPP585 are expected to experience more intense declines in resilience than those under SSP126 and SSP245, particularly in cold regions. These results highlight the risk of continued degradation of ecosystem resilience in the future and the urgency of climate mitigation actions.

Terrestrial biodiversity threatened by increasing global aridity velocity under high-level warming.

Global aridification is projected to intensify. Yet, our knowledge of its potential impacts on species ranges remains limited. Here, we investigate global aridity velocity and its overlap with three sectors (natural protected areas, agricultural areas, and urban areas) and terrestrial biodiversity in historical (1979 through 2016) and future periods (2050 through 2099), with and without considering vegetation physiological response to rising CO2 Both agricultural and urban areas showed a mean drying velocity in history, although the concurrent global aridity velocity was on average +0.05/+0.20 km/yr-1 (no CO2 effects/with CO2 effects; "+" denoting wetting). Moreover, in drylands, the shifts of vegetation greenness isolines were found to be significantly coupled with the tracks of aridity velocity. In the future, the aridity velocity in natural protected areas is projected to change from wetting to drying across RCP (representative concentration pathway) 2.6, RCP6.0, and RCP8.5 scenarios. When accounting for spatial distribution of terrestrial taxa (including plants, mammals, birds, and amphibians), the global aridity velocity would be -0.15/-0.02 km/yr-1 ("-" denoting drying; historical), -0.12/-0.15 km/yr-1 (RCP2.6), -0.36/-0.10 km/yr-1 (RCP6.0), and -0.75/-0.29 km/yr-1 (RCP8.5), with amphibians particularly negatively impacted. Under all scenarios, aridity velocity shows much higher multidirectionality than temperature velocity, which is mainly poleward. These results suggest that aridification risks may significantly influence the distribution of terrestrial species besides warming impacts and further impact the effectiveness of current protected areas in future, especially under RCP8.5, which best matches historical CO2 emissions [C. R. Schwalm et al., Proc. Natl. Acad. Sci. U.S.A. 117, 19656-19657 (2020)].

Imbalance in lake variability but not embodying driving factors on the Qinghai-Tibetan Plateau calls on heterogeneous lake management.

Comprehensive regional remote analysis tends to neglect lakes in exorheic basins on the Qinghai-Tibetan Plateau (QTP), and a concurrent lack of discussions on whether there exist imbalanced explanations for the driving forces of both internal and external lakes is also present. We integrate multisourced lake datasets, high-resolution information, and available altimetry datasets to establish multiple mathematical models to meta-simulate lake volume changes, extend current lake variation datasets, and quantify the imbalance of variations and factors driving the water mass budget. The results showed that the primary cause of lake variations in QTP is net precipitation (57.75 ± 31.46%), followed by glacier runoff (33.53 ± 31.42%), and permafrost (8.34 ± 7.87%). Even though glacier runoff is currently considered as a weak factor of lake variation, heterogeneous results call for remaining attention in glacier-induced lake basins. Imbalance embodying in lake variability but not in contributions of driving factors, which calls for special lake management ways in different watersheds.

Soil moisture determines the recovery time of ecosystems from drought.

Recovery time, the time it takes for ecosystems to return to normal states after experiencing droughts, is critical for assessing the response of ecosystems to droughts; however, the spatial dominant factors determining recovery time are poorly understood. We identify the global patterns of terrestrial ecosystem recovery time based on remote sensed vegetation indices, analyse the affecting factors of recovery time using random forest regression model, and determine the spatial distribution of the dominant factors of recovery time based on partial correlation. The results show that the global average recovery time is approximately 3.3 months, and that the longest recovery time occurs in mid-latitude drylands. Analysis of affecting factors of recovery time suggests that the most important environmental factor affecting recovery time is soil moisture during the recovery period, followed by temperature and vapour pressure deficit (VPD). Recovery time shortens with increasing soil moisture and prolongs with increasing VPD; however, the response of recovery time to temperature is nonmonotonic, with colder or hotter temperatures leading to longer recovery time. Soil moisture dominates the drought recovery time over 58.4% of the assessed land area, mostly in the mid-latitudes. The concern is that soil moisture is projected to decline in more than 65% regions in the future, which will lengthen the drought recovery time and exacerbate drought impacts on terrestrial ecosystems, especially in southwestern United States, the Mediterranean region and southern Africa. Our research provides methodological insights for quantifying recovery time and spatially identifies dominant factors of recovery time, improving our understanding of ecosystem response to drought.

Compound droughts slow down the greening of the Earth.

Vegetation response to soil and atmospheric drought has raised extensively controversy, however, the relative contributions of soil drought, atmospheric drought, and their compound droughts on global vegetation growth remain unclear. Combining the changes in soil moisture (SM), vapor pressure deficit (VPD), and vegetation growth (normalized difference vegetation index [NDVI]) during 1982-2015, here we evaluated the trends of these three drought types and quantified their impacts on global NDVI. We found that global VPD has increased 0.22 ± 0.05 kPa·decade-1 during 1982-2015, and this trend was doubled after 1996 (0.32 ± 0.16 kPa·decade-1 ) than before 1996 (0.16 ± 0.15 kPa·decade-1 ). Regions with large increase in VPD trend generally accompanied with decreasing trend in SM, leading to a widespread increasing trend in compound droughts across 37.62% land areas. We further found compound droughts dominated the vegetation browning since late 1990s, contributing to a declined NDVI of 64.56%. Earth system models agree with the dominant role of compound droughts on vegetation growth, but their negative magnitudes are considerably underestimated, with half of the observed results (34.48%). Our results provided the evidence of compound droughts-induced global vegetation browning, highlighting the importance of correctly simulating the ecosystem-scale response to the under-appreciated exposure to compound droughts as it will increase with climate change.

Socioeconomic development alters the effects of 'green' and 'grain' on evapotranspiration in China's loess plateau after the grain for green programme.

Revegetation has been conducted extensively to restore degraded ecosystems, thereby accelerating water consumption and affecting water availability for other human demands. Examining evapotranspiration (ET) can guide regional management to promote revegetation sustainability and address the contradiction in water demand. We characterised ET variation on China's Loess Plateau from 2003 to 2013, after the 'Grain for Green' revegetation programme implementation. Annual ET significantly increased, with an average trend of 4.87 mm yr-2; the highest increasing trends were in the southern part of the plateau. Combining zero-order correlation and partial correlation, we found that climate and crop production were the key factors influencing ET, while revegetation also had significant effects. We also explored how multiple influencing factors affected ET through partial least-squares path modelling. Revegetation and socioeconomic development were found to impose indirect effects on ET by promoting rural household income and altering agricultural production. The specified linkages and regulating pathways among revegetation and human needs including socioeconomic development and agricultural production should be considered in solving the conflicts between the ecosystem and human water use in water-limited regions.

The community perception of human-water connections is indirectly influenced by the landscape context: A case study in the lower reaches of the Yellow river.

Humans and water are closely connected in large river basins and form social-ecological systems (SESs). However, cross-scale effect in SESs make it difficult to identify the key forces driving human-water connections at the community scale when ignoring the landscape context. Focusing on the incongruous human-water relationships in the lower reaches of the Yellow River, we built local resident perception-based networks linking the agricultural subsystem, environmental subsystem, and cultural subsystem by distributing farmer household questionnaires and extracted 13 indicators from 7 kinds of network metrics to indicate human-water connections. We applied analysis of variance (ANOVA), random forest (RF) and multilevel linear model (MLM) methods to identify the driving forces of perception-based human-water connections among 20 factors at both the community and landscape scales. The results showed that the perception-based network indicators were mainly directly influenced by community-level driving factors, especially the accessibility of information, such as the frequency of going out, the frequency of accessing the Yellow River channel, and the information source for the national policy on the Yellow River. The influences of community-level driving factors on network indicators were affected by landscape-level driving factors, e.g., the nighttime light, population density, gross domestic product and proportion of artificial land, thus indicating indirect influences from the landscape context. These analyses and findings can enrich the methods by which social, ecological and hydrological elements are structurally linked in sociohydrologic research and highlight the cross-scale effect of the landscape context on human-water systems at the community level.

Identification of critical ecological areas using the ecosystem multifunctionality-stability-integrity framework: A case study in the Yellow River basin, China.

Critical ecological areas (CEAs), as important regions for biodiversity and ecosystem functions, are crucial for ecological conservation and environmental management at regional and global scales. However, the methodology and framework of CEA identification have not been well established. In this study, a comprehensive CEA identification method was developed based on the ecosystem multifunctionality-stability-integrity framework by using K-means clustering, critical slowing down theory and possible connectivity. Taking the Yellow River basin (YRB) as a case study, our results showed that ecosystem multifunctionality gradually decreased from the southeast to northwest. A decrease in ecosystem stability was observed since 2017 and was mainly due to the increased impacts of human activities and urbanization within the 10-20 km distance threshold from the ecosystem. Based on the proposed framework, 15.13% of the YRB was identified as CEAs with reliable estimates, and most areas were distributed in the Three-River Headwaters, Qinling and Taihang Mountains. Moreover, urbanization and precipitation were found to be the dominant environmental factors affecting the CEA distribution in the YRB. Our results indicated that the proposed framework could provide a comprehensive approach for CEA identification and useful implications for ecological conservation and environmental management.

Response to concerns about the African fire trends controlled by precipitation over recent decades.

Croplands expanded in Africa over recent decades, even though the increasing trends are spatially heterogeneous.

Determining critical thresholds of ecological restoration based on ecosystem service index: A case study in the Pingjiang catchment in southern China.

Considering that the degradation of ecological systems is an urgent environmental challenge, promoting multiple ecosystem services (ES) through ecological restoration has recently become more critical. However, the complicated interactions among multiple ES are not fully considered in ecological restoration planning and management, which prevents simultaneous improvements to environmental and human welfare. In this study, the spatio-temporal variations of multiple ES and their interactions were investigated in the Pingjiang catchment, which used to suffer severe soil erosion and has been the target of the ecological restoration projects. The results showed that five individual ES were heterogeneously distributed, and each individual ES and their overall benefits have increased with the implementation of ecological restoration (except for water yield). However, significantly negative correlations existed in over half of ten ES pairs, and the trade-offs among the five individual ES also increased. Through redundancy analysis, the forest proportion (FP) was identified as the major socio-ecological factor that determines multiple ES patterns; therefore, determining the appropriate FP for restoration areas is important for regulating the supply of ES. The constraint effects of FP on each ES and their overall benefits and trade-offs were examined, and inconsistent thresholds were detected for some relationships. Thus, a comprehensive index (ES index) that incorporates the overall trade-offs was proposed to reflect the complicated interactions among multiple ES and the preferences of different stakeholder groups. The constraint effect of FP on the ES index was explored, and the threshold values detected in the hump-shaped curve of the constraint lines provided references for determining the appropriate FP. This study established an integrated land use management framework by proposing a comprehensive ES index and determining its critical thresholds through the constraint line method. The results provide insights for the better planning and targeting of ecological restoration and land use management projects worldwide.

Identifying ecological security patterns based on the supply, demand and sensitivity of ecosystem service: A case study in the Yellow River Basin, China.

Ecological security is the basis for ecosystems to provide various ecosystem services (ESs) to humans. Identifying ecological security patterns (ESPs) is an effective approach to determine the priority conservation areas and ensure regional ecological security. However, most previous studies on ESPs were based mainly on the supply of ESs, while the demand and sensitivity of ESs were not fully considered. In this study, a comprehensive ESP identification framework was developed by integrating the supply, demand and sensitivity of ESs with the fuzzy multicriteria decision-making and circuit theory. Taking the Yellow River Basin (YRB) as a case study, our results show that the ecological sources (139,633 km2 or 17.3%) of the YRB were located mainly in the transition area between the Qinghai-Tibet Plateau and Loess Plateau, and in the Qinling Mountains and eastern plains; these areas reliably exhibited high conservation efficiency and low decision-making risk and tradeoff levels. However, the northern and western YRB had few ecological sources due to mismatches among the supply, demand and sensitivity of ESs. Based on circuit theory, ecological corridors (36,905 m and 76,878 km2) effectively connected the western, southern and eastern parts of the YRB. These ecological sources and corridors were both dominated by grassland, forest and cropland. However, ten pinch points, primarily covered by cropland, were also recognized in the eastern YRB and should be considered as priority areas for ecological conservation. Moreover, our results indicate that this comprehensive ESP identification framework could provide useful guidance to decision-makers for maintaining ESs and ecological conservation.

Accelerated increase in vegetation carbon sequestration in China after 2010: A turning point resulting from climate and human interaction.

China has increased its vegetation coverage and enhanced its terrestrial carbon sink through ecological restoration since the end of the 20th century. However, the temporal variation in vegetation carbon sequestration remains unclear, and the relative effects of climate change and ecological restoration efforts are under debate. By integrating remote sensing and machine learning with a modelling approach, we explored the biological and physical pathways by which both climate change and human activities (e.g., ecological restoration, cropland expansion, and urbanization) have altered Chinese terrestrial ecosystem structures and functions, including vegetation cover, surface heat fluxes, water flux, and vegetation carbon sequestration (defined by gross and net primary production, GPP and NPP). Our study indicated that during 2001-2018, GPP in China increased significantly at a rate of 49.1-53.1 TgC/yr2 , and the climatic and anthropogenic contributions to GPP gains were comparable (48%-56% and 44%-52%, respectively). Spatially, afforestation was the dominant mechanism behind forest cover expansions in the farming-pastoral ecotone in northern China, on the Loess Plateau and in the southwest karst region, whereas climate change promoted vegetation cover in most parts of southeastern China. At the same time, the increasing trend in NPP (22.4-24.9 TgC/yr2 ) during 2001-2018 was highly attributed to human activities (71%-81%), particularly in southern, eastern, and northeastern China. Both GPP and NPP showed accelerated increases after 2010 because the anthropogenic NPP gains during 2001-2010 were generally offset by the climate-induced NPP losses in southern China. However, after 2010, the climatic influence reversed, thus highlighting the vegetation carbon sequestration that occurs with ecological restoration.

Food-Energy-Water Crises in the United States and China: Commonalities and Asynchronous Experiences Support Integration of Global Efforts.

Food, energy, and water (FEW) systems have been recognized as an issue of critical global importance. Understanding the mechanisms that govern the FEW nexus is essential to develop solutions and avoid humanitarian crises of displacement, famine, and disease. The U.S. and China are the world's leading economies. Although the two nations are shaped by fundamentally different political and economic systems, they share FEW trajectories in several complementary ways. These realities place the U.S. and China in unique positions to engage in problem definition, dialogue, actions, and transdisciplinary convergence of research to achieve productive solutions addressing FEW challenges. By comparing the nexus and functions of the FEW systems in the two nations, this perspective aims to facilitate collaborative innovations that reduce disciplinary silos, mitigate FEW challenges, and enhance environmental sustainability. The review of experiences and challenges facing the U.S. and China also offers valuable insights for other nations seeking to achieve sustainable development goals.

Greater increases in China's dryland ecosystem vulnerability in drier conditions than in wetter conditions.

Dryland ecosystems are experiencing dramatic climate change, either drier or wetter. However, the differences in response amplitudes of dryland ecosystems to drier and wetter climates have not been frequently discussed, especially when using composite indicators at large scales. This study explores the changing patterns of ecosystem vulnerability in China's drylands by comprehensively considering exposure, sensitivity and resilience indicators using leaf area index (LAI) datasets and meteorological data within two periods from 1982 to 1999 (P1) and from 2000 to 2016 (P2). The results show that nearly 57% of China's drylands have experienced drier conditions in 2000-2016 based on the average aridity index (AI) values compared with the conditions in 1982-1999. Compared with the conditions in 1982-1999, ecosystem vulnerability has increased in 78% of dryland, and ecosystem resilience has decreased in 46% of the area in 2000-2016. The amplitudes of vulnerability increase are higher in drier conditions than in wetter conditions. Ecosystem resilience has obviously increased in wetter conditions but has decreased in drier conditions, especially in farming-pastoral ecotones with an obvious land use change. Consequently, vegetation-climate composite indicators provide a holistic pattern of China's dryland ecosystem response to climate change, and the decreased ecosystem resilience in drier conditions in northeast China should be a warning signal under the national vegetation greening background. This research highlights that the impact of drying on ecosystem resilience leads the response of ecosystems to drier environment.

Objective indicators contribute more than subjective beliefs to resident willingness to pay for ecosystem services on the Tibetan Plateau.

Effective ecosystem management on the Tibetan Plateau will contribute to regional environmental sustainability, and these efforts need broad public support, especially that of residents, over the long run. Although residents' subjective perceptions often directly influence practices, the interactive effects of subjective and objective indicators at the individual level often interfere with resident participation in ecosystem management. With the objective of decoupling the effects of multiple variables on resident participation in environmental sustainability, we launched a questionnaire survey on the topic of willingness to pay (WTP) on the Tibetan Plateau, and explored the effects of single variables and pairwise variables on WTP via dummy regression and proposed specific management suggestions. The results showed that objective indicators were the key drivers of WTP. First, it not only had strong direct effects on WTP (2770.32 CNY/year) but also interacted with subjective beliefs (3805.92 CNY/year); second, it had indirect effects on participation attitudes (R = 0.79) through subjective beliefs (R = 0.38). Put differently, the challenge of achieving sustainable management in the TP is how to enhance and satisfy the sociodemographic and socio-economic attributes of indigenous residents.

Nonlinear dynamics of fires in Africa over recent decades controlled by precipitation.

Dynamics of fires in Africa are of critical importance for understanding changes in ecosystem properties and effects on the global carbon cycle. Given increasing fire risk from projected warming on the one hand and a documented human-driven decline in fires on the other, it is still unknown how the complex interplay between climate and human factors affects recent changes of fires in Africa. Moreover, the impact of recent strong El Niño events on fire dynamics is not yet known. By applying an ensemble empirical mode decomposition method to satellite-derived fire burned area, we investigated the spatio-temporal evolution of fires in Africa over 2001-2016 and identified the potential dominant drivers. Our results show an overall decline of fire rates, which is continuous over the time period and mainly caused by cropland expansion in northern sub-Saharan Africa. However, we also find that years of high precipitation have caused an initial increase in fire rates in southern Africa, which reversed to a decline in later years. This decline is caused by a high frequency of dry years leading to very low fuel loads, suggesting that recent drought causes a general reduction of burned areas, in particular in xeric savannas. In some mesic regions (10°-15°S), solar radiation and increased temperature caused increase in fires. These findings show that climate change overrules the impact of human expansion on fire rates at the continental scale in Africa, reducing the fire risk.

The Regional Impact of Ecological Restoration in the Arid Steppe on Dust Reduction over the Metropolitan Area in Northeastern China.

A massive ecological restoration program has been implemented in northern China with the aim of protecting the Beijing-Tianjin-Hebei metropolitan area of eastern China from dust events. However, some current studies have cast doubt on the efficacy of such ecological restoration projects, partly due to the constraint of available water in northern China, leading to poor survival rates of planted trees in semiarid regions (15%). In this study, using a logical framework combining statistical analysis, partial least-squares path model analysis, and a regional climate model (RegCM) simulation with multisource dust indicators, we found that there was a reduction of dust in northern China that was synchronous with the increase in vegetation growth after ecological restoration. In contrast to previous reports of a decrease in wind speed due to ecological restoration, this study found that the increase in vegetation had an insignificant impact on local wind speed (p = 0.30). Instead, ecological restoration mainly reduced the sand emission in steppe area by improving the soil conditions of the underlying surface, and hence contributed 15% of the reduction of dust events in the Beijing-Tianjin-Hebei metropolitan area through dust transmission (p = 0.002). The effect of ecological restoration in the northern steppe on dust reduction over the northeastern metropolitan area of China should not be overstated.