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River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget.

Coastal ecosystems release or absorb carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), but the net effects of these ecosystems on the radiative balance remain unknown. We compiled a dataset of observations from 738 sites from studies published between 1975 and 2020 to quantify CO2, CH4 and N2O fluxes in estuaries and coastal vegetation in ten global regions. We show that the CO2-equivalent (CO(2)e) uptake by coastal vegetation is decreased by 23-27% due to estuarine CO(2)e outgassing, resulting in a global median net sink of 391 or 444 TgCO(2)e yr(-1) using the 20- or 100-year global warming potentials, respectively. Globally, total coastal CH4 and N2O emissions decrease the coastal CO2 sink by 9-20%. Southeast Asia, North America and Africa are critical regional hotspots of GHG sinks. Understanding these hotspots can guide our efforts to strengthen coastal CO2 uptake while effectively reducing CH4 and N2O emissions. The authors show that estuarine and coastal vegetation are collectively a greenhouse gas (GHG) sink for the atmosphere, but methane and nitrous oxide emissions counteract the carbon dioxide uptake. Critical coastal GHG sink hotspots are identified in Southeast Asia, North America and Africa.

Inland waters are important sources of the greenhouse gasses (GHGs) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) to the atmosphere. While a growing number of global estimates of inland water GHG emissions exists, the integration of inland waters into regional GHG budgets is often hampered by the lack of adequate geo-spatial datasets. Moreover, existing estimates diverge substantially, in part due to persisting uncertainties related to the size and distribution of effective inland water surface areas. In the framework of the 2nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP-2) initiative, we synthesize existing estimates of GHG emissions from streams, rivers, lakes and reservoirs, and homogenize them with regard to underlying global maps of inland water surface areas and the effects of seasonal ice cover. We then produce estimates of inland water GHG emissions for 10 extensive land regions that are used for the regional land budgets of RECCAP2. According to our synthesis, global inland waters emit 5.6 (3.5-9.1) Pg CO2 yr-1, 101 (83-135) Tg CH4 yr-1 and 326 (254-592) Gg N2O yr-1. South American rivers contribute about one third of global inland water CO2 emissions. North-American and Russian lakes contribute together one third of global inland water CH4 emissions. Finally, North America alone contributes one fourth of global inland water N2O emissions.The global inland water emissions sum up to a global warming potential (GWP) of an equivalent emission of 13.6 (10.0-20.3) and 8.3 (5.8-12.7) Pg CO2 yr-1 at a 20- and 100-year horizon, respectively. At 100-year horizon, the contribution of CO2 dominates the GWP of global inland water GHG emissions, with rivers being the largest emitters. At the 20-year horizon, on the contrary, lakes and rivers are equally important emitters, and the contributions of CH4 to the GWP of inland water GHG emissions even exceed those of CO2. Contributions of N2O to the GWP appear to be less significant at both time horizons. Normalized to the area of the RECCAP-2 land regions, South America and South East Asia show the highest inland water emission rates in terms of GWP, dominated by riverine CO2 emissions.

This contribution presents a new view of the global carbon cycle which accounts for the land-to-ocean transport of carbon through inland waters, estuaries, tidal wetlands and continental shelf waters—the ‘land-to-ocean aquatic continuum’ (LOAC). We highlight how biogeochemical and ecological processes from land-to-ocean have been perturbed by human interventions, including atmospheric composition change, climate change and land-use change. The extend to which these anthropogenic perturbations have altered regional and global CO2 budgets and trends along the LOAC are also presented and the knowledge gaps that are key to reduce uncertainties in future assessments of LOAC fluxes are identified. Finally, broader implications regarding the quantification of the terrestrial and open ocean sinks of anthropogenic carbon are briefly discussed

Significant progress has been made regarding the spatial distribution of sources and sinks of atmospheric CO2 within the global coastal ocean. The physical and biochemical drivers that govern these CO2 sources and sinks as well as their variability on seasonal timescales are, however, poorly understood. In this study we aim at filling this knowledge gap and assess to what extend the CO2 dynamics in the coastal ocean is controlled by the adjacent open ocean or conversely is dominated by continental influence. We use the global ocean biophysical model MOM6-COBALT, carefully evaluated against a well-established coastal data-product (Laruelle et al., 2017), to quantify the individual influences of the oceanic transport, thermal changes, freshwater influence, and biological activity on the annual mean spatial distribution of CO2 sources and sinks as well as their seasonal variability for the coastal ocean worldwide. Our analysis reveals complex spatial and temporal dynamics, depending on the coastal region and/or the time scale investigated (seasonal or annual) resulting from interactions and compensations between the different processes. We identify five typical behaviors of coastal systems (i.e., coastal regions under biological drawdown, vertical transport, imprint of land, dominated by intracoastal alongshore currents, and weak CO2 sources and sinks coastal regions) and propose a processed-based typology of the coastal ocean based on the processes that control the spatial and seasonal air-sea CO2 dynamics.

Lateral nitrogen (N) transport from land to the ocean through rivers is an important component of global N cycling. In this study, we present the implementation of fluvial transport of nitrogen into ORCHIDEE-CNP (Organising Carbon and Hydrology in Dynamic Ecosystems-CNP), which explicitly simulates N biogeochemistry in terrestrial ecosystems coupled with carbon, water and energy transfers. This new model branch called ORCHIDEE-Nlateral, simulates the lateral transport of water, dissolved inorganic N (DIN), dissolved organic N (DON) and particulate organic N (PON) from land to the ocean through river networks, the decomposition of DON and PON, and the denitrification of DIN in transit. ORCHIDEE-Nlateral was parameterized and evaluated based on global observations of water discharge (Global Runoff and Dara Centre, GRDC) and N concentration in the global river network (Global Water Quality Archive, GRQA). The model reproduces well the observed riverine discharges of water and total nitrogen (TN), and N exports from the land to the ocean. Globally, the TN flowing into rivers, denitrification of DIN and TN export to the ocean all increased from the year 1901 to 2015. The TN export to the ocean increased from 32 Tg N yr-1 to 37 Tg N yr-1 during 1901–2015, which is in good agreement with the corresponding global fluxes calculated from the well-established Global NEWS 2 model. In this study, we further re-assess the spatial and temporal distribution of global riverine N flows and stocks. Overall, our model approach represents a useful tool for simulating large-scale lateral N transfer and for predicting the future feedbacks between lateral N transfers and climate.

Over the past decade, the number of high-quality measurements of the sea surface partial pressure of CO2 (pCO2) has rapidly increased and large-scale community efforts have led to the compilation of these measurements into uniform quality-controlled databases. Moreover, the development of different robust interpolation techniques allowed one to circumvent the limitation of these datasets that remain discontinuous in time and space to create continuous spatiotemporal pCO2 maps. While significant progress has been made regarding the development of several global data-products for the global ocean, most of these products omit the coastal ocean and/or their spatial resolution is too coarse to fully capture the highly heterogeneous spatiotemporal pCO2 dynamics that occurs in these regions. As a result, the evaluation of the interannual variability and the long-term trends of the coastal air-sea CO2 exchange using a continuous CO2 flux (FCO2) product dedicated to the shallow portion of the global ocean has not yet been attempted and, hence, remains poorly understood. To address these limitations, this study updates the global coastal data-product of Laruelle et al. (2017) based on the coastal version of the Self Organizing Map and Feed Forward Network method and uses ~ 32 million observations to cover the longest period available for the coastal ocean (1982-2020). The good performance in space and time of this new data-product using several evaluation methods allows us to reconstruct the temporal evolution of the global coastal FCO2 sink based on observations. Our results indicate that today’s coastal ocean acts as a CO2 sink and that it has been a CO2 sink since the beginning of our study period (1982). This CO2 sink has however increased over time from a value of -0.25 Pg C yr-1 (for a total shelf surface area of 77 million km2) in the early 1980s to a current value of -0.6 Pg C yr-1. Our new product provides a new constraint for closing the global carbon cycle and its temporal evolution as well as for establishing regional carbon budgets requiring high resolution coastal flux estimates.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.

A research highlight about the modelling of land-to-ocean aquatic continuum carbon fluxes. Based on the publication: Regnier, P., Resplandy, L., Najjar, R.G. et al. (2022) The land-to-ocean loops of the global carbon cycle. Nature 603, 401–410 . https://doi.org/10.1038/s41586-021-04339-9

At the interface between the continental and oceanic domains, estuaries are essential components of the land-ocean aquatic continuum that play a significant role in biogeochemical cycles, as they transform and export large amounts of carbon and nutrients from rivers to coastal waters. Because of this intense biogeochemical processing, they are significant ecosystems in terms of greenhouse gas exchange with the atmosphere. However, in spite of recent advances in remote sensing and the need for accurate estimates to calculate regional and global estuarine budgets, the quantification of their global surface area has not been updated in over a decade and remains poorly constrained. This is due to the lack of a global extensive database, the diversity of estuaries, and the controversial definition of their boundaries. To address these challenges, a hybrid approach was developed that combines the surface areas of over 700 estuaries worldwide (extracted from the literature or calculated using geographic information systems) with a novel extrapolation method to provide type-specific regional estimates for 45 regions. The upscaling formula applied is determined and calibrated using data from several regions where an extensive survey of total estuarine surface areas was available. The new global estimate of 733,801 ± 39,892 km2 is 31% lower than the previous global assessment and provides quantitative uncertainty estimates for regional and global estuarine surface areas as well as a breakdown between tidal systems and deltas (294,956 ± 30,780 km2), lagoons (179,946 ± 12,056 km2), and fjords (259,899 ± 22,328 km2).

Regional land carbon budgets provide insights into the spatial distribution of the land uptake of atmospheric carbon dioxide and can be used to evaluate carbon cycle models and to define baselines for land-based additional mitigation efforts. The scientific community has been involved in providing observation-based estimates of regional carbon budgets either by downscaling atmospheric CO2 observations into surface fluxes with atmospheric inversions, by using inventories of carbon stock changes in terrestrial ecosystems, by upscaling local field observations such as flux towers with gridded climate and remote sensing fields, or by integrating data-driven or process-oriented terrestrial carbon cycle models. The first coordinated attempt to collect regional carbon budgets for nine regions covering the entire globe in the RECCAP-1 project has delivered estimates for the decade 2000–2009, but these budgets were not comparable between regions due to different definitions and component fluxes being reported or omitted. The recent recognition of lateral fluxes of carbon by human activities and rivers that connect CO2 uptake in one area with its release in another also requires better definitions and protocols to reach harmonized regional budgets that can be summed up to a globe scale and compared with the atmospheric CO2 growth rate and inversion results. In this study, using the international initiative RECCAP-2 coordinated by the Global Carbon Project, which aims to be an update to regional carbon budgets over the last 2 decades based on observations for 10 regions covering the globe with a better harmonization than the precursor project, we provide recommendations for using atmospheric inversion results to match bottom-up carbon accounting and models, and we define the different component fluxes of the net land atmosphere carbon exchange that should be reported by each research group in charge of each region. Special attention is given to lateral fluxes, inland water fluxes, and land use fluxes.

Leaching of dissolved organic carbon (DOC) from soils into the river network is an important component of the land carbon (C) budget. At regional to global scales, its significance has been estimated through simple mass budgets, often using multi-year averages of observed fluvial DOC fluxes as a proxy of DOC leaching due to the limited availability of observations of the leaching flux itself. This procedure leads to a systematic underestimation of the leaching flux because of the decay of DOC during fluvial transport. Moreover, this procedure does not allow for revealing spatiotemporal variability in DOC leaching from soils, which is vital to better understand the drivers of DOC leaching and its impact on the local soil C budget. In this study, we use the land surface model (LSM) ORCHILEAK to simulate the terrestrial C budget, including leaching of DOC from the soil and its subsequent reactive transport through the river network of Europe. The model performance is evaluated not only against the sparse observations of the soil DOC leaching rate, but also against the more abundant observations of fluxes and reactivity of DOC in rivers, providing further evidence that our simulated DOC fluxes are realistic. The model is then used to simulate the spatiotemporal patterns of DOC leaching across Europe over the period 1972–2012, quantifying both the environmental drivers of these patterns and the impact of DOC leaching on the land C budget. Over the simulation period, we find that, on average, 14.3 Tg C yr−1 of DOC is leached from land into European rivers, which is about 0.6 % of the terrestrial net primary production (NPP), a fraction significantly lower than that reported for tropical river networks. On average, 12.3 Tg C yr−1 of the leached DOC is finally exported to the coast via the river network, and the rest is respired during transit. DOC leaching presents a large seasonal variability, with the maximum occurring in winter and the minimum in summer, except for most parts of northern Europe, where the maximum occurs in spring due to snowmelt. The DOC leaching rate is generally low in warm and dry regions, and high in the cold and wet regions of Europe. Furthermore, runoff and the ratio between runoff from shallower flow paths on one hand and deep drainage and groundwater flow on the other hand are the main drivers of the spatiotemporal variation of DOC leaching. Temperature, as a major control of DOC production and decomposition rates in the soils, plays only a secondary role.

The temporal variability of the sea surface partial pressure of CO2 (pCO2) and the underlying processes driving this variability are poorly understood in the coastal ocean. In this study, we tailor an existing method that quantifies the effects of thermal changes, biological activity, ocean circulation and freshwater fluxes to examine seasonal pCO2 changes in highly variable coastal environments. We first use the Modular Ocean Model version 6 (MOM6) and biogeochemical module Carbon Ocean Biogeochemistry And Lower Trophics version 2 (COBALTv2) at a half-degree resolution to simulate coastal CO2 dynamics and evaluate them against pCO2 from the Surface Ocean CO2 Atlas database (SOCAT) and from the continuous coastal pCO2 product generated from SOCAT by a two-step neuronal network interpolation method (coastal Self-Organizing Map Feed-Forward neural Network SOM-FFN, Laruelle et al., 2017). The MOM6-COBALT model reproduces the observed spatiotemporal variability not only in pCO2 but also in sea surface temperature, salinity and nutrients in most coastal environments, except in a few specific regions such as marginal seas. Based on this evaluation, we identify coastal regions of “high” and “medium” agreement between model and coastal SOM-FFN where the drivers of coastal pCO2 seasonal changes can be examined with reasonable confidence. Second, we apply our decomposition method in three contrasted coastal regions: an eastern (US East Coast) and a western (the Californian Current) boundary current and a polar coastal region (the Norwegian Basin). Results show that differences in pCO2 seasonality in the three regions are controlled by the balance between ocean circulation and biological and thermal changes. Circulation controls the pCO2 seasonality in the Californian Current; biological activity controls pCO2 in the Norwegian Basin; and the interplay between biological processes and thermal and circulation changes is key on the US East Coast. The refined approach presented here allows the attribution of pCO2 changes with small residual biases in the coastal ocean, allowing for future work on the mechanisms controlling coastal air–sea CO2 exchanges and how they are likely to be affected by future changes in sea surface temperature, hydrodynamics and biological dynamics.

Quantification of land surface-atmosphere fluxes of carbon dioxide (CO2) fluxes and their trends and uncertainties is essential for monitoring progress of the EU27+UK bloc as it strives to meet ambitious targets determined by both international agreements and internal regulation. This study provides a consolidated synthesis of fossil sources (CO2 fossil) and natural sources and sinks over land (CO2 land) using bottom-up (BU) and top-down (TD) approaches for the European Union and United Kingdom (EU27+UK), updating earlier syntheses (Petrescu et al., 2020, 2021b). Given the wide scope of the work and the variety of approaches involved, this study aims to answer essential questions identified in the previous syntheses and understand the differences between datasets, particularly for poorly characterized fluxes from managed ecosystems. The work integrates updated emission inventory data, process-based model results, data-driven sectoral model results, and inverse modeling estimates, extending the previous period 1990–2018 to the year 2020 to the extent possible. BU and TD products are compared with European National Greenhouse Gas Inventories (NGHGIs) reported by Parties including the year 2019 under the United Nations Framework Convention on Climate Change (UNFCCC). The uncertainties of the EU27+UK NGHGI were evaluated using the standard deviation reported by the EU Member States following the guidelines of the Intergovernmental Panel on Climate Change (IPCC) and harmonized by gap-filling procedures. Variation in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), originate from within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, key sources of differences between estimates arise primarily in activities. System boundaries and emission categories create differences in CO2 fossil datasets, while different land use definitions for reporting emissions from Land Use, Land Use Change and Forestry (LULUCF) activities result in differences for CO2 land. The latter has important consequences for atmospheric inversions, leading to inversions reporting stronger sinks in vegetation and soils than are reported by the NGHGI. For CO2 fossil emissions, after harmonizing estimates based on common activities and selecting the most recent year available for all datasets, the UNFCCC NGHGI for the EU27+UK accounts for 3392 ± 49 Tg CO2 yr-1 (926 ± 13 Tg C yr-1), while eight other BU sources report a mean value of 3340 [3238,3401] [25th,75th percentile] Tg CO2 yr-1 (948 [937,961] Tg C yr-1). The sole top-down inversion of fossil emissions currently available accounts for 3800 Tg CO2 yr-1 (1038 Tg C yr-1), a value close to that of the NGHGI, but for which uncertainty estimates are not yet available. For the net CO2 land fluxes, during the most recent five-year period including the NGHGI estimates, the NGHGI accounted for -91 ± 32 Tg C yr-1 while six other BU approaches reported a mean sink of -62 [-117,-49] Tg C yr-1 and a 15-member ensemble of dynamic global vegetation models (DGVMs) reported -69 [-152,-5] Tg C yr-1. The five-year mean of three TD regional ensembles combined with one non-ensemble inversion of -73 Tg C yr-1 has a slightly smaller spread (0th–100th percentile of [-135,45] Tg C yr-1), and was calculated after removing land-atmosphere CO2 fluxes caused by lateral transport of carbon (crops, wood trade and inland waters) resulting in increased agreement with the the NGHGI and bottom-up approaches. Results at the sub-sector level (Forestland, Cropland, Grassland) show generally good agreement between the NGHGI and sub-sector-specific models, but results for a DGVM are mixed. Overall, for both CO2 fossil and net CO2 land fluxes, we find current independent approaches are consistent with the NGHGI at the scale of the EU27+UK. We conclude that CO2 emissions from fossil sources have decreased over the past 30 years in the EU27+UK, while large uncertainties on net uptake of CO2 by the land surface prevent trend identification. In addition, a gap on the order of 1000 Tg C yr-1 between CO2 fossil emissions and net CO2 uptake by the land exists regardless of the type of approach (NGHGI, TD, BU), falling well outside all available estimates of uncertainties. However, uncertainties in top-down approaches to estimate CO2 fossil emissions remain uncharacterized and are likely substantial. The data used to plot the figures are available at https://doi.org/10.5281/zenodo.7365863.

Knowledge of the spatial distribution of the fluxes of greenhouse gases (GHGs) and their temporal variability as well as flux attribution to natural and anthropogenic processes is essential to monitoring the progress in mitigating anthropogenic emissions under the Paris Agreement and to inform its global stocktake. This study provides a consolidated synthesis of CH4 and N2O emissions using bottom-up (BU) and top-down (TD) approaches for the European Union and UK (EU27 + UK) and updates earlier syntheses (Petrescu et al., 2020, 2021). The work integrates updated emission inventory data, process-based model results, data-driven sector model results and inverse modeling estimates, and it extends the previous period of 1990–2017 to 2019. BU and TD products are compared with European national greenhouse gas inventories (NGHGIs) reported by parties under the United Nations Framework Convention on Climate Change (UNFCCC) in 2021. Uncertainties in NGHGIs, as reported to the UNFCCC by the EU and its member states, are also included in the synthesis. Variations in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), arise from diverse sources including within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, the activities included are a key source of bias between estimates, e.g., anthropogenic and natural fluxes, which in atmospheric inversions are sensitive to the prior geospatial distribution of emissions. For CH4 emissions, over the updated 2015–2019 period, which covers a sufficiently robust number of overlapping estimates, and most importantly the NGHGIs, the anthropogenic BU approaches are directly comparable, accounting for mean emissions of 20.5 Tg CH4 yr−1 (EDGARv6.0, last year 2018) and 18.4 Tg CH4 yr−1 (GAINS, last year 2015), close to the NGHGI estimates of 17.5±2.1 Tg CH4 yr−1. TD inversion estimates give higher emission estimates, as they also detect natural emissions. Over the same period, high-resolution regional TD inversions report a mean emission of 34 Tg CH4 yr−1. Coarser-resolution global-scale TD inversions result in emission estimates of 23 and 24 Tg CH4 yr−1 inferred from GOSAT and surface (SURF) network atmospheric measurements, respectively. The magnitude of natural peatland and mineral soil emissions from the JSBACH–HIMMELI model, natural rivers, lake and reservoir emissions, geological sources, and biomass burning together could account for the gap between NGHGI and inversions and account for 8 Tg CH4 yr−1. For N2O emissions, over the 2015–2019 period, both BU products (EDGARv6.0 and GAINS) report a mean value of anthropogenic emissions of 0.9 Tg N2O yr−1, close to the NGHGI data (0.8±55 % Tg N2O yr−1). Over the same period, the mean of TD global and regional inversions was 1.4 Tg N2O yr−1 (excluding TOMCAT, which reported no data). The TD and BU comparison method defined in this study can be operationalized for future annual updates for the calculation of CH4 and N2O budgets at the national and EU27 + UK scales. Future comparability will be enhanced with further steps involving analysis at finer temporal resolutions and estimation of emissions over intra-annual timescales, which is of great importance for CH4 and N2O, and may help identify sector contributions to divergence between prior and posterior estimates at the annual and/or inter-annual scale. Even if currently comparison between CH4 and N2O inversion estimates and NGHGIs is highly uncertain because of the large spread in the inversion results, TD inversions inferred from atmospheric observations represent the most independent data against which inventory totals can be compared. With anticipated improvements in atmospheric modeling and observations, as well as modeling of natural fluxes, TD inversions may arguably emerge as the most powerful tool for verifying emission inventories for CH4, N2O and other GHGs. The referenced datasets related to figures are visualized at https://doi.org/10.5281/zenodo.7553800 (Petrescu et al., 2023).

Knowledge of the spatial distribution of the fluxes of greenhouse gases and their temporal variability as well as flux attribution to natural and anthropogenic processes is essential to monitoring the progress in mitigating anthropogenic emissions under the Paris Agreement and to inform its Global Stocktake. This study provides a consolidated synthesis of CH4 and N2O emissions using bottom-up (BU) and top-down (TD) approaches for the European Union and UK (EU27+UK) and updates earlier syntheses (Petrescu et al., 2020, 2021). The work integrates updated emission inventory data, process-based model results, data-driven sector model results, inverse modelling estimates, and extends the previous period 1990–2017 to 2020. BU and TD products are compared with European National GHG Inventories (NGHGI) reported by Parties under the United Nations Framework Convention on Climate Change (UNFCCC) in 2021. The uncertainties of NGHGIs were evaluated using the standard deviation obtained by varying parameters of inventory calculations, reported by the EU Member States following the guidelines of the Intergovernmental Panel on Climate Change (IPCC) and harmonized by gap-filling procedures. Variation in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), arise from diverse sources including within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, the activities included are a key source of bias between estimates e.g. anthropogenic and natural fluxes, which, in atmospheric inversions are sensitive to the prior geospatial distribution of emissions. For CH4 emissions, over the updated 2015–2019 period, which covers a sufficiently robust number of overlapping estimates, and most importantly the NGHGIs, the anthropogenic BU approaches are directly comparable, accounting for mean emissions of 20.5 Tg CH4 yr−1 (EDGAR v5v6.0, last year 2018) and 18.4 Tg CH4 yr−1 (GAINS, 2015), close to the NGHGI estimates of 17.5 ± 2.1 Tg CH4 yr−1. TD inversions estimates give higher emission estimates, as they also detect natural emissions. Over the same period, high resolution regional TD inversions report a mean emission of 34 Tg CH4 yr−1. Coarser-resolution global-scale TD inversions result in emission estimates of 23 Tg CH4 yr−1 and 24 Tg CH4 yr−1 inferred from GOSAT and surface (SURF) network atmospheric measurements, respectively. The magnitude of natural peatland and mineral soils emissions from the JSBACH-HIMMELI model, natural rivers, lakes and reservoirs emissions, geological sources and biomass burning together could account for the gap between NGHGI and inversions and account for 8 Tg CH4 yr−1. For N2O emissions, over the 2015–2019 period, both BU products (EDGAR v5v6.0 and GAINS) report a mean value of anthropogenic emissions of 0.9 Tg N2O yr−1, close to the NGHGI data (0.8 ± 55 % Tg N2O yr−1). Over the same period, the mean of TD global and regional inversions was 1.4 Tg N2O yr−1 (excluding TOMCAT which reported no data). The TD and BU comparison method defined in this study can be "operationalized" for future annual updates for the calculation of CH4 and N2O budgets at the national and EU27+UK scales. Future comparability will be enhanced with further steps involving analysis at finer temporal resolutions and estimation of emissions over intra-annual timescales, of great importance for CH4 and N2O, which may help identify sector contributions to divergence between prior and posterior estimates at the annual/inter-annual scale. Even if currently comparison between CH4 and N2O inversions estimates and NGHGIs is highly uncertain because of the large spread in the inversion results, TD inversions inferred from atmospheric observations represent the most independent data against which inventory totals can be compared. With anticipated improvements in atmospheric modelling and observations, as well as modelling of natural fluxes, TD inversions may arguably emerge as the most powerful tool for verifying emissions inventories for CH4, N2O and other GHGs. The referenced datasets related to figures are visualized at https://doi.org/10.5281/zenodo.6992472 (Petrescu et al., 2022).

In this study, we present the first combined open and coastal ocean pCO2 mapped monthly climatology (Landschützer et al. (2020), doi:10.25921/qb25-f418, https://www.nodc.noaa.gov/ocads/oceans/MPI-ULB-SOM_FFN_clim.html) constructed from observations collected between 1998 and 2015 extracted from the Surface Ocean CO2 Atlas (SOCAT) database. We combine two neural network-based pCO2 products, one from the open ocean and the other from the coastal ocean, and investigate their consistency along their common overlap areas. While the difference between open and coastal ocean estimates along the overlap area increases with latitude, it remains close to 0 μatm globally. Stronger discrepancies, however, exist on the regional level resulting in differences that exceed 10 % of the climatological mean pCO2, or an order of magnitude larger than the uncertainty from state of the art measurements. This also illustrates the potential of such analysis to inform the measurement community about the locations where additional measurements are essential to better represent the aquatic continuum and improve our understanding of the carbon exchange at the air water interface. A regional analysis further shows that the seasonal carbon dynamics at the coast-open interface are well represented in our climatology. While our combined product is only a first step towards a true representation of both the open ocean and the coastal ocean air-sea CO2 flux in marine carbon budgets, we show it is a feasible task and the present data product already constitutes a valuable tool to investigate and quantify the dynamics of the air-sea CO2 exchange consistently for oceanic regions regardless of its distance to the coast.

International audience; Carbon storage by the ocean and by the land is usually quantified separately, and does not fully take into account the land-to-ocean transport of carbon through inland waters, estuaries, tidal wetlands and continental shelf waters—the `land-to-ocean aquatic continuum' (LOAC). Here we assess LOAC carbon cycling before the industrial period and perturbed by direct human interventions, including climate change. In our view of the global carbon cycle, the traditional `long-range loop', which carries carbon from terrestrial ecosystems to the open ocean through rivers, is reinforced by two `short-range loops' that carry carbon from terrestrial ecosystems to inland waters and from tidal wetlands to the open ocean. Using a mass-balance approach, we find that the pre-industrial uptake of atmospheric carbon dioxide by terrestrial ecosystems transferred to the ocean and outgassed back to the atmosphere amounts to 0.65 ± 0.30 petagrams of carbon per year (±2 sigma). Humans have accelerated the cycling of carbon between terrestrial ecosystems, inland waters and the atmosphere, and decreased the uptake of atmospheric carbon dioxide from tidal wetlands and submerged vegetation. Ignoring these changing LOAC carbon fluxes results in an overestimation of carbon storage in terrestrial ecosystems by 0.6 ± 0.4 petagrams of carbon per year, and an underestimation of sedimentary and oceanic carbon storage. We identify knowledge gaps that are key to reduce uncertainties in future assessments of LOAC fluxes.

International audience; Changes in winter and spring temperatures have been widely used to explain the diverse responses of spring phenology to climate change. However, few studies have quantified their respective effects. Using 386,320 in situ observations of leaf unfolding date (LUD) of six tree species in Europe, we show that accelerated spring thermal accumulation and changes in winter chilling explain, on average, 61% and 39%, respectively, of the advancement in LUD for the period 1951–2019. We find that winter warming may not have delayed bud dormancy release, but rather it has increased the thermal requirement in reaching leaf unfolding. This increase in thermal requirement and the decreased efficiency of spring warming for thermal accumulation partly explain the weakening response of leaf unfolding to warming. Our study stresses the need to better assess the antagonistic and heterogeneous effects of winter and spring warming on leaf phenology, which is key to projecting future vegetation–climate feedbacks.

The temporal variability of the sea surface partial pressure of CO2 (pCO2) and the underlying processes driving this variability are poorly understood in the coastal ocean. In this study, we tailor an existing method that quantifies the effects of thermal changes, biological activity, ocean circulation and fresh water fluxes to examine seasonal pCO2 changes in highly-variable coastal environments. We first use the Modular Ocean Model version 6 (MOM6) and biogeochemical module Carbon Ocean Biogeochemistry And Lower Trophics version 2 (COBALTv2) at a half degree resolution to simulate the coastal CO2 dynamics and evaluate it against pCO2 from the Surface Ocean CO2 Atlas database (SOCAT) and from the continuous coastal pCO2 product generated from SOCAT by a two-step neuronal network interpolation method (coastal-SOM-FFN, Laruelle et al., 2017). The MOM6-COBALT model not only reproduces the observed spatio-temporal variability in pCO2 but also in sea surface temperature, salinity, nutrients, in most coastal environments except in a few specific regions such as marginal seas. Based on this evaluation, we identify coastal regions of ‘high’ and ‘medium’ model skill where the drivers of coastal pCO2 seasonal changes can be examined with reasonable confidence. Second, we apply our decomposition method in three contrasted coastal regions: an Eastern (East coast of the U.S) and a Western (the Californian Current) boundary current and a polar coastal region (the Norwegian Basin). Results show that differences in pCO2 seasonality in the three regions are controlled by the balance between ocean circulation, biological and thermal changes. Circulation controls the pCO2 seasonality in the Californian Current, biological activity controls pCO2 in the Norwegian Basin, while the interplay between biology, thermal and circulation changes is key in the East coast of the U.S. The refined approach presented here allows the attribution of pCO2 changes with small residual biases in the coastal ocean, allowing future work on the mechanisms controlling coastal air-sea CO2 exchanges and how they are likely to be affected by future changes in sea surface temperature, hydrodynamics and biological dynamics.

Leaching of dissolved organic carbon (DOC) from soils to the river network is an important component of the land carbon (C) budget. At regional to global scales, its significance has been estimated through simple mass budgets, often using multi-year averages of observed fluvial DOC fluxes as proxy of DOC leaching due to the limited availability of observations of the leaching flux itself. This procedure leads to a systematic underestimation of the leaching flux because of the reactivity of DOC during fluvial transport. Moreover, this procedure does not allow to reveal spatio-temporal variability in DOC leaching from soils, which is needed to better understand the drivers of DOC leaching and its impact on the local soil C budget. In this study, we use the land surface model ORCHILEAK to simulate the terrestrial C budget including leaching of DOC from the soil and its subsequent reactive transport through the river network of Europe. The model performance is not only evaluated against the sparse observations of DOC leaching, but also against the more abundant observations of fluxes and reactivity of DOC in rivers, providing further evidence that our simulated DOC leaching fluxes are realistic. The model is then used to simulate the spatio-temporal patterns of DOC leaching across Europe over the period 1972 to 2012, quantifying both the environmental drivers of these patterns as well as the impact of DOC leaching on the land C budget. Over the simulation period, we find that on average 14.3 TgC yr−1 of DOC is leached from land to European rivers, which is only about 0.6 % of the terrestrial net primary production, a fraction about one order of magnitude lower than reported for tropical river networks. Of the DOC leaching, on average 12.3 TgC yr−1 is exported to the coast via the river network, the rest being respired in transit. DOC leaching presents a large seasonal variability, with a maximum occurring in winter and a minimum in summer, except for the Northern most part of Europe where the maximum occurs in spring due to the snow melt. DOC leaching rate is generally lower in warm and dry regions, and higher in cold and wet regions of Europe. Furthermore, runoff, and the ratio between runoff from shallower flow paths vs. deep drainage and groundwater flow, is the main driver of the spatial variation of DOC leaching. Temperature, as a major control of DOC production and decomposition rates in the soils, plays only a secondary role.

Changes in winter and spring temperatures have been widely used to explain the diverse responses of spring phenology to climate change. However, our understanding of their respective roles remain incomplete. Using >300,000 in situ observations of leaf unfolding date (LUD) in Europe, we show that the advancement of LUD since 1950 is due both to accelerated spring thermal accumulation and changes in winter chilling which explain 61% and 39% of the LUD shifts, respectively. Winter warming did not substantially retard the releasing of bud dormancy, but increased the thermal requirement to reach leaf unfolding. The increase of thermal requirement and decreased efficiency of spring warming on accelerating thermal accumulation partly explained the temporally (1950s-2010s) decreasing response of LUD to warming. Our study stresses the need to better assess the antagonistic and heterogeneous effects of winter and spring warming on leaf phenology, which is key to projection of future vegetation-climate feedbacks.

Accurate monitoring of vegetation stress is required for better modelling and forecasting of primary production, in a world where heatwaves and droughts are expected to become increasingly prevalent. Variability in formaldehyde (HCHO) concentrations in the troposphere is dominated by local emissions of short-lived biogenic (BVOC) and pyrogenic volatile organic compounds. BVOCs are emitted by plants in a rapid protective response to abiotic stress, mediated by the energetic status of leaves (the excess of reducing power when photosynthetic light and dark reactions are decoupled, as occurs when stomata close in response to water stress). Emissions also increase exponentially with leaf temperature. New analytical methods for the detection of spatiotemporally contiguous extremes in remote-sensing data are applied here to satellite-derived atmospheric HCHO columns. BVOC emissions are shown to play a central role in the formation of the largest positive HCHO anomalies. Although vegetation stress can be captured by various remotely sensed quantities, spaceborne HCHO emerges as the most consistent recorder of vegetation responses to the largest climate extremes, especially in forested regions.

BACKGROUND Sarcopenia is suspected to influence the complication rates in patients undergoing radical cystectomy (RC). The aim of our study was to assess variations in sarcopenia in patients scheduled for neoadjuvant cisplatin-based chemotherapy (NAC) and RC for muscle invasive bladder cancer (MIBC) and to explore the impact of sarcopenia on complications linked to NAC or surgery. METHODS Between 2012 and 2017, 82 consecutive patients who underwent NAC and RC for cT2-T4 N0 MIBC were retrospectively selected. Using CT scan before and after NAC, Lumbar Skeletal Muscle Index (SMI) was assessed by two observers. We defined severe sarcopenia as SMI

Past century increases in terrigenous N and P inputs to the ocean due to industrialization, agricultural practices and wastewater have been reported to have dramatic consequences for ecosystems in various coastal regions. Yet, the impacts of increased nutrient inputs through river transports and atmospheric deposition on the coastal and open ocean carbon cycle have yet to be quantitatively investigated at the global scale. To address this gap of knowledge, we enhanced the ocean biogeochemical model HAMOCC at a horizontal resolution of around 0.4° in order to improve the representation of temporal changes of riverine fluxes and of coastal ocean dynamics in the model. Through a series of simulations with differing model forcings, we isolated individual effects arising from (1) increasing atmospheric CO2 levels, (2) a changing physical climate and (3) alterations in oceanic inputs of terrigenous P and N inputs, all over the 1905 to 2010 period. Our results indicate a strong response of the coastal ocean ecosystem to increased terrestrial nutrient inputs, which induce the global coastal Net Primary Production (NPP) to increase by 14% over the simulation time span. This eutrophication signal is, furthermore, partly exported to the open ocean, which undergoes an increase in NPP of 1.75 Pg C yr-1, or 4 % in relative terms, in the simulations, owing to the cross-shelf export of 33-46% of the anthropogenic P and N inputs to the coastal ocean. As a whole, increased P and N inputs lead to an overall global ocean NPP rise of around 2.15 Pg C yr-1, or 5% (combined coastal and open ocean). This net increase attributed to land-ocean couplings exceeds the simulated global oceanic NPP decrease of 4 % associated with stronger upper ocean thermal stratification over the time span, a feedback that been under stronger scrutiny in published literature. Our results suggest that increased riverine nutrient concentrations due to anthropogenic activities may also have substantial impacts for ecosystems in the open ocean, in contrary to what was assumed until now, although this is dependent on the rate of transfer of the nutrients from the coastal to the open ocean.

The spatio-temporal variability and the underlying drivers of the carbon dioxide (CO2) exchange at the air-water interface (FCO2) of the global coastal ocean are still poorly understood and their quantification remains highly uncertain. Here, we present an analysis of the spatial and seasonal variability of FCO2 using a high-resolution (0.25 degree) monthly climatology (1998-2015 period) for coastal sea surface partial pressure in CO2 (pCO2), globally.Overall, a clear latitudinal pattern emerges from our analysis regarding sources/sinks distribution of atmospheric CO2 and we find that in most regions, annual mean CO2 flux densities are comparable in sign and magnitude to those of the adjacent open ocean except for river dominated systems. Globally, coastal regions act as a CO2 sink with a more intense uptake occurring in summer because of the disproportionate influence of high latitude coastal seas in the Northern Hemisphere. The majority of the coastal seasonal FCO2 variations stems from the air-sea pCO2 gradient, although changes in wind speed and sea-ice cover can also be significant regionally. To investigate further the drivers of the spatio-seasonal variability, our observation-based pCO2 climatology is used in conjunction with global ocean biogeochemistry model MOM6-COBALT. The model outputs allow us to quantify the respective contributions of thermal effects, biology, and non-thermal physical processes (circulation and freshwater inputs) to seasonal variations in coastal pCO2. Generally, biological activity is the dominant driver of the pCO2 seasonal variability in temperate and high latitudes while thermal and non-thermal physical processes dominate in low latitudes.

Resolving regional carbon budgets is critical for informing land-based mitigation policy. For nine regions covering nearly the whole globe, we collected inventory estimates of carbon-stock changes complemented by satellite estimates of biomass changes where inventory data are missing. The net land–atmospheric carbon exchange (NEE) was calculated by taking the sum of the carbon-stock change and lateral carbon fluxes from crop and wood trade, and riverine-carbon export to the ocean. Summing up NEE from all regions, we obtained a global ‘bottom-up’ NEE for net land anthropogenic CO2 uptake of –2.2 ± 0.6 PgC yr−1 consistent with the independent top-down NEE from the global atmospheric carbon budget during 2000–09. This estimate is so far the most comprehensive global bottom-up carbon budget accounting, which set up an important milestone for global carbon-cycle studies. By decomposing NEE into component fluxes, we found that global soil heterotrophic respiration amounts to a source of CO2 of 39 PgC yr−1 with an interquartile of 33–46 PgC yr−1—a much smaller portion of net primary productivity than previously reported., info:eu-repo/semantics/published

International audience; We synthesized N2O emissions over North America using 17 bottom-up (BU) estimates from 1980–2016 and five top-down (TD) estimates from 1998 to 2016. The BU-based total emission shows a slight increase owing to U.S. agriculture, while no consistent trend is shown in TD estimates. During 2007–2016, North American N2O emissions are estimated at 1.7 (1.0–3.0) Tg N yr−1 (BU) and 1.3 (0.9–1.5) Tg N yr−1 (TD). Anthropogenic emissions were twice as large as natural fluxes from soil and water. Direct agricultural and industrial activities accounted for 68% of total anthropogenic emissions, 71% of which was contributed by the U.S. Our estimates of U.S. agricultural emissions are comparable to the EPA greenhouse gas (GHG) inventory, which includes estimates from IPCC tier 1 (emission factor) and tier 3 (process-based modeling) approaches. Conversely, our estimated agricultural emissions for Canada and Mexico are twice as large as the respective national GHG inventories.Nitrous oxide (N2O) is the third most important greenhouse gase (GHG) after CO2 and CH4 causing global warming. Among world regions, North America (defined herein as U.S., Canada, and Mexico) is the second largest source of N2O emissions globally, and previous source estimates for this region vary widely. This study aims to provide a comprehensive N2O assessment over North America including all available estimates based on a number of approaches. We report total emissions, and emissions from four anthropogenic source sectors, over the past four decades. Agriculture and industry are two major N2O sources in North America. Our results show a minor increase in the total N2O emission due to agricultural trends in the U.S. Our bottom-up estimate of U.S. agricultural N2O emissions are close to those in the EPA national GHG inventory that includes both empirical and model results. The high consistency suggests the need to take process-based modeling results into account for future national GHG inventories.

The contribution of continental shelves to the marine carbon cycle is still poorly understood. Their preindustrial state is, for one, essentially unknown, which strongly limits the quantitative assessment of their anthropogenic perturbation. To date, approaches developed to investigate and quantify carbon fluxes on continental shelves have strongly simplified their physical and biogeochemical features. In this study, we enhance the global ocean biogeochemistry model HAMburg Ocean Carbon Cycle by explicitly representing riverine loads of carbon and nutrients, as well as improving the representation of organic matter dynamics in the coastal ocean. Our simulations, at a resolution of similar to 0.4 degrees, reveal a globally averaged shelf water residence time (RT) of 12-17 months, which is much shorter than the global RTs previously assumed in benchmark studies (>4 years). This shorter global RT, induced primarily through outer shelf regions with large oceanic inflows, promotes an efficient offshore transport of terrestrial and marine organic carbon (0.44 PgCyr(-1)) and a dissolved inorganic carbon sink from the organic cycling of carbon on the global shelf (net ecosystem productivity [NEP] equal to +0.20 PgCyr(-1)). In turn, this autotrophic state of continental shelves contributes to a weak global preindustrial sink of atmospheric CO2 (0.04 PgCyr(-1)), dominated by extensive regions with large oceanic inflows and positive NEPs, such as the Patagonian shelf, the East China Sea and the outer North Sea. The contemporary global shelf CO2 uptake of 0.15 PgCyr(-1) furthermoresuggests that the anthropogenic CO2 uptake (0.11 PgCyr(-1)) on the global continental shelf is less efficient with respect to the open ocean.

The implications of climate change and other human perturbations on the oceanic carbon cycle are still associated with large uncertainties. Global-scale modelling studies are essential to investigate anthropogenic perturbations of oceanic carbon fluxes but, until now, they have not considered the impacts of temporal changes in riverine and atmospheric inputs of P and N on the marine net biological productivity (NPP) and air–sea CO2 exchange (FCO2). To address this, we perform a series of simulations using an enhanced version of the global ocean biogeochemistry model HAMOCC to isolate effects arising from (1) increasing atmospheric CO2 levels, (2) a changing physical climate and (3) alterations in inputs of terrigenous P and N on marine carbon cycling over the 1905–2010 period. Our simulations reveal that our first-order approximation of increased terrigenous nutrient inputs causes an enhancement of 2.15 Pg C year−1 of the global marine NPP, a relative increase of +5% over the simulation period. This increase completely compensates the simulated NPP decrease as a result of increased upper ocean stratification of −3% in relative terms. The coastal ocean undergoes a global relative increase of 14% in NPP arising largely from increased riverine inputs, with regional increases exceeding 100%, for instance on the shelves of the Bay of Bengal. The imprint of enhanced terrigenous nutrient inputs is also simulated further offshore, inducing a 1.75 Pg C year−1 (+4%) enhancement of the NPP in the open ocean. This finding implies that the perturbation of carbon fluxes through coastal eutrophication may extend further offshore than that was previously assumed. While increased nutrient inputs are the largest driver of change for the CO2 uptake at the regional scale and enhance the global coastal ocean CO2 uptake by 0.02 Pg C year−1, they only marginally affect the FCO2 of the open ocean over our study's timeline., SCOPUS: ar.j, info:eu-repo/semantics/published