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Anthropogenic activities have substantially enhanced the loadings of reactive nitrogen (Nr) in the Earth system since pre-industrial times1,2, contributing to widespread eutrophication and air pollution3-6. Increased Nr can also influence global climate through a variety of effects on atmospheric and land processes but the cumulative net climate effect is yet to be unravelled. Here we show that anthropogenic Nr causes a net negative direct radiative forcing of -0.34 [-0.20, -0.50] W m-2 in the year 2019 relative to the year 1850. This net cooling effect is the result of increased aerosol loading, reduced methane lifetime and increased terrestrial carbon sequestration associated with increases in anthropogenic Nr, which are not offset by the warming effects of enhanced atmospheric nitrous oxide and ozone. Future predictions using three representative scenarios show that this cooling effect may be weakened primarily as a result of reduced aerosol loading and increased lifetime of methane, whereas in particular N2O-induced warming will probably continue to increase under all scenarios. Our results indicate that future reductions in anthropogenic Nr to achieve environmental protection goals need to be accompanied by enhanced efforts to reduce anthropogenic greenhouse gas emissions to achieve climate change mitigation in line with the Paris Agreement.

Process-based land surface models are important tools for estimating global wetland methane (CH4) emissions and projecting their behavior across space and time. So far there are no performance assessments of model responses to drivers at multiple time scales. In this study, we apply wavelet analysis to identify the dominant time scales contributing to model uncertainty in the frequency domain. We evaluate seven wetland models at 23 eddy covariance tower sites. Our study first characterizes site-level patterns of freshwater wetland CH4 fluxes (FCH4) at different time scales. A Monte Carlo approach was developed to incorporate flux observation error to avoid misidentification of the time scales that dominate model error. Our results suggest that (a) significant model-observation disagreements are mainly at multi-day time scales (32 days) for the boreal and Arctic tundra wetland sites but have significant bias in variability at seasonal time scales for temperate and tropical/subtropical sites; (c) model errors exhibit increasing power spectrum as time scale increases, indicating that biases at time scales

Forests play a major role in the global carbon cycle. Previous studies on the capacity of forests to sequester atmospheric CO2 have mostly focused on carbon uptake, but the roles of carbon turnover time and its spatiotemporal changes remain poorly understood. Here, we used long-term inventory data (1955 to 2018) from 695 mature forest plots to quantify temporal trends in living vegetation carbon turnover time across tropical, temperate, and cold climate zones, and compared plot data to 8 Earth system models (ESMs). Long-term plots consistently showed decreases in living vegetation carbon turnover time, likely driven by increased tree mortality across all major climate zones. Changes in living vegetation carbon turnover time were negatively correlated with CO2 enrichment in both forest plot data and ESM simulations. However, plot-based correlations between living vegetation carbon turnover time and climate drivers such as precipitation and temperature diverged from those of ESM simulations. Our analyses suggest that forest carbon sinks are likely to be constrained by a decrease in living vegetation carbon turnover time, and accurate projections of forest carbon sink dynamics will require an improved representation of tree mortality processes and their sensitivity to climate in ESMs.

Terrestrial ecosystems carbon and water cycles are tightly coupled through photosynthesis and evapotranspiration processes. The ratios of carbon stored to carbon uptake and water loss to carbon gain are key ecophysiological indicators essential to assess the magnitude and response of the terrestrial plant to the changing climate. Here, we use estimates from 10 terrestrial ecosystem models to quantify the impacts of climate, atmospheric CO2 concentration, and nitrogen (N) deposition on water use efficiency (WUE), and carbon use efficiency (CUE). We find that across models, WUE increases over the 20th Century particularly due to CO2 fertilization and N deposition and compares favorably to experimental studies. Also, the results show a decrease in WUE with climate for the last 3 decades, in contrasts with up-scaled flux observations that demonstrate a constant WUE. Modeled WUE responds minimally to climate with modeled CUE exhibiting no clear trend across space and time. The divergence between simulated and observationally-constrained WUE and CUE is driven by modeled NPP and autotrophic respiration, nitrogen cycle, carbon allocation, and soil moisture dynamics in current ecosystem models. We suggest that carbon-modeling community needs to reexamine stomatal conductance schemes and the soil-vegetation interactions for more robust modeling of carbon and water cycles.

Clarifying how increased atmospheric CO2 concentration (eCO2) contributes to accelerated land carbon sequestration remains important since this process is the largest negative feedback in the coupled carbon–climate system. Here, we constrain the sensitivity of the terrestrial carbon sink to eCO2 over the temperate Northern Hemisphere for the past five decades, using 12 terrestrial ecosystem models and data from seven CO2 enrichment experiments. This constraint uses the heuristic finding that the northern temperate carbon sink sensitivity to eCO2 is linearly related to the site-scale sensitivity across the models. The emerging data-constrained eCO2 sensitivity is 0.64 ± 0.28 PgC yr−1 per hundred ppm of eCO2. Extrapolating worldwide, this northern temperate sensitivity projects the global terrestrial carbon sink to increase by 3.5 ± 1.9 PgC yr−1 for an increase in CO2 of 100 ppm. This value suggests that CO2 fertilization alone explains most of the observed increase in global land carbon sink since the 1960s. More CO2 enrichment experiments, particularly in boreal, arctic and tropical ecosystems, are required to explain further the responsible processes.

Urban ecosystems are complex systems with anthropogenic features that generate considerable C[O.sub.2] emissions, which contributes to global climate change. Quantitative estimates of the carbon footprint of urban ecosystems are crucial for developing low-carbon development policies to mitigate climate change. Herein, we reviewed more than 195 urban carbon footprint and carbon footprint related studies, collated the recent progress in carbon footprint calculation methods and research applications of the urban ecosystem carbon footprint, analyzed the research applications of the carbon footprint of different cities, and focused on the need to study the urban ecosystem carbon footprint from a holistic perspective. Specifically, we aimed to: (i) compare the strengths and weaknesses of five existing carbon footprint calculation methods [life cycle assessment, input-output analysis, hybrid life cycle assessment, carbon footprint calculator, and Intergovernmental Panel on Climate Change (IPCC)]; (ii) analyze the status of current research on the carbon footprint of different urban subregions based on different features; and (iii) highlight new methods and areas of research on the carbon footprint of future urban ecosystems. Not all carbon footprint accounting methods are applicable to the carbon footprint determination of urban ecosystems; although the IPCC method is more widely used than the others, the hybrid life cycle assessment method is more accurate. With the emergence of new science and technology, quantitative methods to calculate the carbon footprint of urban ecosystems have evolved, becoming more accurate. Further development of new technologies, such as big data and artificial intelligence, to assess the carbon footprint of urban ecosystems is anticipated to help address the emerging challenges in urban ecosystem research effectively to achieve carbon neutrality and urban sustainability under global change. Key words: accounting systems, climate change, carbon sequestration, C[O.sub.2] emissions assessment, hybrid life cycle assessment. Les ecosystemes urbains sont des systemes complexes presentant des caracteristiques anthropiques qui generent des emissions considerables de C[O.sub.2] contribuant aux changements climatiques mondiaux. Les estimations quantitatives de l'empreinte carbone des ecosystemes urbains sont cruciales pour l'elaboration de politiques de developpement a faible emission de carbone visant a attenuer les changements climatiques. Dans cet article, les auteurs ont passe en revue plus de 195 etudes sur l'empreinte carbone urbaine et reliees a l'empreinte carbone urbaine, collige les progres recents en matiere de methodes de calcul de l'empreinte carbone et les applications de la recherche sur l'empreinte carbone des ecosystemes urbains, analyse les applications de recherche de l'empreinte carbone de differentes villes et mis l'accent sur la necessite d'etudier l'empreinte carbone des ecosystemes urbains d'un point de vue holistique. Plus precisement, ils ont cherche a: (i) comparer les forces et les faiblesses de cinq methodes existantes de calcul de l'empreinte carbone [evaluation du cycle de vie, analyse entree-sortie, evaluation hybride du cycle de vie, calculateur de l'empreinte carbone et celle du Groupe d'experts intergouvernemental sur revolution du climat (GIEC)]; (ii)analyser l'etat de la recherche actuelle sur l'empreinte carbone de differentes sous-regions urbaines en fonction de differentes caracteristiques; et (iii) mettre en lumiere de nou-velles methodes et de nouveaux domaines de recherche sur l'empreinte carbone des futurs ecosystemes urbains. Toutes les methodes de comptabilisation de l'empreinte carbone ne sont pas applicables a la determination de l'empreinte carbone des ecosystemes urbains; bien que la methode du GIEC soit plus largement utilisee que les autres, la methode hybride d'evaluation du cycle de vie est plus precise. Avec les avancees de la science et l'emergence de nouvelles technologies, les methodes quantitatives de calcul de l'empreinte carbone des ecosystemes urbains ont evolue et sont devenues plus precises. Le developpement ulterieur de nouvelles technologies, telles que celles qui utilisent les donnees massives et l'intelligence artificielle, pour evaluer l'empreinte carbone des ecosystemes urbains devrait aider a relever efficacement les nouveaux defisde la recherche sur les ecosystemes urbains afind'atteindre la neutralite carbone et la durabilite urbaine dans le contexte des changements climatiques mondiaux. [Traduit par la Redaction] Mots-cles: systemes de comptabilite, changements climatiques, sequestration du carbone, evaluation des emissions de CO2, analysehybrideducycledevie., Introduction Approximately half of the Earth's terrestrial surface is currently affected by human activities that are accelerating C[O.sub.2] emissions and increasingly becoming detrimental for several ecosystems (Lambin et al. 2003). [...]