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Few Earth system models adequately represent the unique permafrost soil biogeochemistry and its respective processes; this significantly contributes to uncertainty in estimating their responses, and that of the planet at large, to warming. Likewise, the riverine component of what is known as the "boundless carbon cycle" is seldom recognised in Earth system modelling. The hydrological mobilisation of organic material from a ∼ 1330-1580 PgC carbon stock to the river network results in either sedimentary settling or atmospheric "evasion", processes widely expected to increase with amplified Arctic climate warming. Here, the production, transport, and atmospheric release of dissolved organic carbon (DOC) from high-latitude permafrost soils into inland waters and the ocean are explicitly represented for the first time in the land surface component (ORCHIDEE) of a CMIP6 global climate model (Institut Pierre Simon Laplace - IPSL). The model, ORCHIDEE MICT-LEAK, which represents the merger of previously described ORCHIDEE versions MICT and LEAK, mechanistically represents (a) vegetation and soil physical processes for high-latitude snow, ice, and soil phenomena and (b) the cycling of DOC and CO2, including atmospheric evasion, along the terrestrial-aquatic continuum from soils through the river network to the coast at 0.5 to 2° resolution. This paper, the first in a two-part study, presents the rationale for including these processes in a high-latitude-specific land surface model, then describes the model with a focus on novel process implementations, followed by a summary of the model configuration and simulation protocol. The results of these simulation runs, conducted for the Lena River basin, are evaluated against observational data in the second part of this study., SCOPUS: ar.j, info:eu-repo/semantics/published

Natural lakes and reservoirs are important, yet not well constrained sources of greenhouse gasses to the atmosphere. In particular for N2O emissions, a huge variability is observed in the few, observation‐driven flux estimates that have been published so far. Recently, a process‐based, spatially explicit model has been used to estimate global N2O emissions from more than 6,000 reservoirs based on nitrogen (N) and phosphorous inflows and water residence time. Here, we extend the model to a dataset of 1.4 million standing water bodies comprising natural lakes and reservoirs. For validation, we normalized the simulated N2O emissions by the surface area of each water body and compared them against regional averages of N2O emission rates taken from the literature or estimated based on observed N2O concentrations. We estimate that natural lakes and reservoirs together emit 4.5±2.9 Gmol N2O‐N yr‐1 globally. Our global scale estimate falls in the far lower end of existing, observation‐driven estimates. Natural lakes contribute only about half of this flux, although they contribute 91% of the total surface area of standing water bodies. Hence, the mean N2O emission rates per surface area are substantially lower for natural lakes than for reservoirs with 0.8±0.5 mmol N m‐2yr‐1 vs. 9.6±6.0 mmol N m‐2yr‐1, respectively. This finding can be explained by on average lower external N inputs to natural lakes. We conclude that upscaling based estimates, which do not distinguish natural lakes from reservoirs, are prone to important biases. Funding agencies: European UnionEuropean Union (EU) [776810]; Swedish Research Council VRSwedish Research Council; Swedish Research Council FORMASSwedish Research CouncilSwedish Research Council Formas; European Research Council (ERC Grant)European Research Council (ERC) Accepted version on author's personal web page https://liu.se/dfsmedia/dd35e243dfb7406993c1815aaf88a675/35054-source/options/download/natural-lakes

Inland waters, including streams and rivers, are active components of the global carbon cycle. Despite the large areal extent of the world’s mountains, the role of mountain streams for global carbon fluxes remains elusive. Using recent insights from gas exchange in turbulent streams, we found that areal CO2 evasion fluxes from mountain streams equal or exceed those reported from tropical and boreal streams, typically regarded as hotspots of aquatic carbon fluxes. At the regional scale of the Swiss Alps, we present evidence that emitted CO2 derives from lithogenic and biogenic sources within the catchment and delivered by the groundwater to the streams. At a global scale, we estimate the CO2 evasion from mountain streams to 167 ± 1.5 Tg C yr−1, which is high given their relatively low areal contribution to the global stream and river networks. Our findings shed new light on mountain streams for global carbon fluxes., SCOPUS: ar.j, info:eu-repo/semantics/published

Nitrous oxide (N2 O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N2 O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N2 O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N2 O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half-degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N2 O emissions of 10.6-19.8GmolNyear-1 (148-277GgNyear-1 ), with reservoirs producing most N2 O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N2 O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.

The onset and expansion of agriculture has accelerated soil erosion by rainfall and runoff substantially, mobilizing vast quantities of soil organic carbon (SOC) globally. Studies show that at timescales of decennia to millennia this mobilized SOC can significantly alter previously estimated carbon emissions from land use change (LUC). However, a full understanding of the impact of erosion on land-atmosphere carbon exchange is still missing. The aim of our study is to better constrain the terrestrial carbon fluxes by developing methods compatible with Earth System Models (ESMs) in order to explicitly represent the links between soil erosion by rainfall and runoff and carbon dynamics. For this we use an emulator that represents the carbon cycle of a land surface model, in combination with the Revised Universal Soil Loss Equation model. We applied this modeling framework at the global scale to evaluate the effects of potential soil erosion (soil removal only) in the presence of other perturbations of the carbon cycle: elevated atmospheric CO2, climate variability, and LUC. We found that over the period 1850–2005 AD acceleration of soil erosion leads to a total potential SOC removal flux of 100 Pg C of which 80 % occurs on agricultural, pasture and natural grass lands. Including soil erosion in the SOC-dynamics scheme results in a doubling of the cumulative loss of SOC over 1850–2005 due to the combined effects of climate variability, increasing atmospheric CO2 and LUC. This additional erosional loss decreases the cumulative global carbon sink on land by 5 Pg for this specific period, with the largest effects found for the tropics, where deforestation and agricultural expansion increased soil erosion rates significantly. We also show that the potential effects of soil erosion on the global SOC stocks cannot be ignored when compared to the effects of climate change or land use change on the carbon cycle. We conclude that it is necessary to include soil erosion in assessments of LUC and evaluations of the terrestrial carbon cycle.

CO2 evasion from rivers (FCO2) is an important component of the global carbon budget. Here we present the first global maps of CO2 partial pressures (pCO2) in rivers of stream orders 3 and higher and the resulting FCO2 at 0.5° resolution constructed with a statistical model. A geographic information system based approach is used to derive a pCO2 prediction function trained on data from 1182 sampling locations. While data from Asia and Africa are scarce and the training data set is dominated by sampling locations from the Americas, Europe, and Australia, the sampling locations cover the full spectrum from high to low latitudes. The predictors of pCO2 are net primary production, population density, and slope gradient within the river catchment as well as mean air temperature at the sampling location (r2 = 0.47). The predicted pCO2 map was then combined with spatially explicit estimates of stream surface area Ariver and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO2 map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO2 of 2400 (2019-2826) μatm and a FCO2 of 650 (483-846) Tg C yr-1 (5th and 95th percentiles of confidence interval). Our global CO2 evasion is substantially lower than the recent estimate of 1800 Tg C yr-1 although the training set of pCO2 is very similar in both studies, mainly due to lower tropical pCO2 estimates in the present study. Our maps reveal strong latitudinal gradients in pCO2, Ariver, and FCO2. The zone between 10°N and 10°S contributes about half of the global CO2 evasion. Collection of pCO2 data in this zone, in particular, for African and Southeast Asian rivers is a high priority to reduce uncertainty on FCO2.

This regional study quantifies the CO2 exchange at the air–water interface along the land–ocean aquatic continuum (LOAC) of the northeast North American coast, from streams to the shelf break. Our analysis explicitly accounts for spatial and seasonal variability in the CO2 fluxes. The yearly integrated budget reveals the gradual change in the intensity of the CO2 exchange at the air–water interface, from a strong source towards the atmosphere in streams and rivers (3.0 ± 0.5 TgC yr−1) and estuaries (0.8 ± 0.5 TgC yr−1) to a net sink in continental shelf waters (−1.7 ± 0.3 TgC yr−1). Significant differences in flux intensity and their seasonal response to climate variations is observed between the North and South sections of the study area, both in rivers and coastal waters. Ice cover, snowmelt, and intensity of the carbon removal efficiency through the estuarine filter are identified as important control factors of the observed spatiotemporal variability in CO2 exchange along the LOAC.

The high-latitude regions of the northern hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance – those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest – are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently-developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input data sets, are extensively evaluated against: (i) temperature gradients between the atmosphere and deep soils; (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.

Lakes (including reservoirs) are an important component of the global carbon (C) cycle, as acknowledged by the fifth assessment report of the IPCC. In the context of lakes, the boreal region is disproportionately important contributing to 27% of the worldwide lake area, despite representing just 14% of global land surface area. In this study, we used a statistical approach to derive a prediction equation for the partial pressure of CO

The damming of rivers represents one of the most far-reaching human modifications of the flows of water and associated matter from land to sea. Dam reservoirs are hotspots of sediment accumulation, primary productivity (P) and carbon mineralization (R) along the river continuum. Here we show that for the period 1970–2030, global carbon mineralization in reservoirs exceeds carbon fixation (P, https://www.nature.com/articles/ncomms15347, SCOPUS: ar.j, info:eu-repo/semantics/published

Lateral transfer of carbon (C) from terrestrial ecosystems into the inland water network is an important component of the global C cycle, which sustains a large aquatic CO2 evasion flux fueled by the decomposition of allochthonous C inputs. Globally, estimates of the total C exports through the terrestrial-aquatic interface range from 1.5 to 2.7 Pg C yr−1 (Cole et al., 2007; Battin et al., 2009; Tranvik et al., 2009), i.e. in the order of 2–5% of the terrestrial NPP. Earth System Models (ESM) of the climate system ignore these lateral transfers of C, and thus likely overestimate the terrestrial C sink. In this study, we present the implementation of fluvial transport of dissolved organic carbon (DOC) and CO2 into ORCHIDEE, the land surface scheme of the Institut Pierre-Simon Laplace ESM. This new model branch, called ORCHILEAK, represents DOC production from canopy and soils, DOC and CO2 leaching from soils to streams, DOC decomposition and CO2 evasion to the atmosphere during its lateral transport in rivers, as well as exchange with the soil carbon and litter stocks on floodplains and in swamps. We parameterized and validated ORCHILEAK for the Amazon basin, the world's largest river system with regard to discharge and one of the most productive ecosystems of the world. With ORCHILEAK, we are able to reproduce observed terrestrial and aquatic fluxes of DOC and CO2 in the Amazon basin, both in terms of mean values and seasonality. In addition, we are able to resolve the spatio-temporal variability in C fluxes along the canopy-soil-aquatic continuum at high resolution (1°, daily) and to quantify the different terrestrial contributions to the aquatic C fluxes. We simulate that more than 2/3 of the Amazon's fluvial DOC export is contributed by the decomposition of submerged litter. Throughfall DOC fluxes from canopy to ground are about as high as the total DOC inputs to inland waters. The latter, however, are mainly sustained by litter decomposition. Decomposition of DOC and submerged plant litter contributes slightly more than half of the CO2 evasion from the water surface, while the remainder is contributed by soil respiration. Total CO2 evasion from the water surface equals about 5 % of the terrestrial NPP. Our results highlight that ORCHILEAK is well suited to simulate carbon transfers along the terrestrial-aquatic continuum of tropical forests. It also opens the perspective that provided parameterization, calibration and validation is performed for other biomes, the new model branch could improve the quantification of the global terrestrial C sink and help better constrain carbon cycle-climate feedbacks in future projections.

Over the past decade, estimates of the atmospheric CO2 uptake by continental shelf seas were constrained within the 0.18–0.45 Pg C yr−1 range. However, most of those estimates are based on extrapolations from limited data sets of local flux measurements (n

Local scale studies reported the silica recycling of salt marshes to substantially attenuate the dissolved silica (DSi) limitation in coastal waters during summer. To assess the importance of salt marshes in the silica budget of the North Sea, we extrapolate reported DSi exports by local scale studies to salt marsh areas adjacent to the North Sea. The resulting annual average contribution of salt marshes to the DSi budget of the North Sea is estimated to 0.8% of the annual river DSi export. During summer, this contribution may reach 2.4%. Thus, salt marshes likely impact the annual dissolved silica budget of the North Sea only weakly. However, for regions with favorable geographic conditions of low river DSi exports and large marsh areas, salt marsh DSi exports may substantially contribute to coastal DSi budgets. In the English Channel, salt marsh DSi exports are estimated to 16% of river DSi export in summer. However, the low data density calls for additional field research to improve extrapolations and the evaluation of the contribution of salt marsh DSi export to the coastal DSi budgets.

Estuaries act as strong carbon and nutrient filters and are relevant contributors to the atmospheric CO2 budget. They thus play an important, yet poorly constrained, role for global biogeochemical cycles and climate. This manuscript reviews recent developments in the modelling of estuarine biogeochemical dynamics. The first part provides an overview of the dominant physical and biogeochemical processes that control the transformations and fluxes of carbon and nutrients along the estuarine gradient. It highlights the tight links between estuarine geometry, hydrodynamics and scalar transport, as well as the role of transient and nonlinear dynamics. The most important biogeochemical processes are then discussed in the context of key biogeochemical indicators such as the net ecosystem metabolism (NEM), air–water CO2 fluxes, nutrient-filtering capacities and element budgets. In the second part of the paper, we illustrate, on the basis of local estuarine modelling studies, the power of reaction-transport models (RTMs) in understanding and quantifying estuarine biogeochemical dynamics. We show how a combination of RTM and high-resolution data can help disentangle the complex process interplay, which underlies the estuarine NEM, carbon and nutrient fluxes, and how such approaches can provide integrated assessments of the air–water CO2 fluxes along river–estuary–coastal zone continua. In addition, trends in estuarine biogeochemical dynamics across estuarine geometries and environmental scenario are explored, and the results are discussed in the context of improving the modelling of estuarine carbon and CO2 dynamics at regional and global scales.

In this study we analyzed the large scale spatial patterns of river pH, alkalinity, and CO2 partial pressure (PCO2) in North America and their relation to river catchment properties. The goal was to set up empirical equations which can predict these hydrochemical properties for non-monitored river stretches from geodata of e.g. terrain attributes, lithology, soils, land cover and climate. For an extensive dataset of 1120 river water sampling locations average values of river water pH, alkalinity and PCO2 were calculated. The catchment boundaries and catchment properties were calculated using GIS and different sets of geodata. The correlations between the hydrochemical properties and the catchment properties were explored using simple and multiple linear regression analysis. For each of the considered hydrochemical parameters, a multiple regression equation was fitted: for pH with the predictor's mean annual precipitation and areal proportions of carbonate rocks (r2 = 0.60); for alkalinity, in addition to these two predictors, with subsoil pH and areal proportions agricultural lands (r2 = 0.66); and for pPCO2 (i.e. the negative logarithm of PCO2) with mean air temperature, mean catchment slope gradient, and mean annual precipitation (r2 = 0.43). Based on these results, we argue that spatial patterns in river water pH and alkalinity are governed by catchment processes related to chemical rock weathering. For the PCO2, on the other hand, the spatial patterns are governed by in-river processes on which catchment properties can have an indirect effect. We conclude that our approach can be used to predict averages of these parameters for non-monitored river stretches, which in-turn allows for a better spatially explicit representation of the rivers' carbonate system at the regional to global scale, which will be needed for a refined analysis of rivers in the global carbon cycle.

Understanding the global carbon (C) cycle is of crucial importance to map current and future climate dynamics relative to global environmental change. A full characterization of C cycling requires detailed information on spatiotemporal patterns of surface-atmosphere fluxes. However, relevant C cycle observations are highly variable in their coverage and reporting standards. Especially problematic is the lack of integration of vertical oceanic, inland freshwaters and terrestrial carbon dioxide (CO2) exchange. Here we adopt a data-driven approach to synthesize a wide range of observation-based spatially explicit surface-atmosphere CO2 fluxes from 2001 and 2010, to identify the state of today’s observational opportunities and data limitation. The considered fluxes include vertical net exchange of open oceans, continental shelves, estuaries, rivers, and lakes, as well as CO2 fluxes related to gross primary productivity, terrestrial ecosystem respiration, fire emissions, loss of tropical aboveground C, harvested wood and crops, as well as fossil fuel and cement emissions. Spatially explicit CO2 fluxes are obtained through geostatistical and/or remote sensing-based upscaling; minimizing biophysical or biogeochemical assumptions encoded in process-based models. We estimate a global bottom-up net C exchange (NCE) between the surface (land, ocean, and coastal areas) and the atmosphere. Uncertainties for NCE and its components are derived using resampling. In most continental regions our NCE estimates agree well with independent estimates from other sources. This holds for Europe (mean ±1 SD: 0.80 ± 0.16 PgC/yr, positive numbers are sources to the atmosphere), Russia (−0.02 ± 0.49 PgC/yr), East Asia (1.76 ± 0.38 PgC/yr), South Asia (0.25 ± 0.16 PgC/yr), and Australia (0.22 ± 0.47 PgC/yr). Our NCE estimates also suggest large C sink in tropical areas. The global NCE estimate is −6.07 ± 3.38 PgC/yr. This global bottom-up value is the opposite direction of what is expected from the atmospheric growth rate of CO2, and would require an offsetting surface C source of 4.27±0.10 PgC/yr. This mismatch highlights large knowledge and observational gaps in tropical areas, particularly in South America, Africa, and Southeast Asia, but also in North America. Our uncertainty assessment provides the basis for designing new observation campaigns. In particular, we lack seasonal monitoring of shelf, estuary and inland water-atmosphere C exchange. Also, extensive pCO2 measurements are missing in the Southern Ocean. Most importantly, tropical land C fluxes suffer from a lack of in-situ observations. The consistent derivation of data uncertainties could serve as prior knowledge in multi-criteria optimization such as the Carbon Cycle Data Assimilation System (CCDAS) without overstating data credibility. Furthermore, the spatially explicit flux estimates may be used as a starting point to assess the validity of countries’ claims of reducing net C emissions in climate change negotiations.

CO 2 consumption by chemical weathering is an integral part of the boundless carbon cycle, whose spatial patterns and controlling factors on continental scale are still not fully understood. A dataset of 338 river catchments throughout North America was used to empirically identify predictors of bicarbonate fluxes by chemical weathering and interpret the underlying controlling factors. Detailed analysis of major ion ratios enables distinction of the contributions of silicate and carbonate weathering and thus quantifying CO 2 consumption. Extrapolation of the identified empirical model equations to North America allows the analysis of the spatial patterns of the CO 2 consumption by chemical weathering. Runoff, lithology and land cover were identified as the major predictors of the riverine bicarbonate fluxes and the associated CO 2 consumption. Other influence factors, e.g. temperature, could not be established in the models. Of the distinguished land cover classes, artificial surfaces, dominated by urban areas, increase bicarbonate fluxes most, followed by shrubs, grasslands, managed lands, and forests. The extrapolation results in an average specific bicarbonate flux of 0.3 Mmol km −2 a −1 by chemical weathering in North America, of which 64% originates from atmospheric CO 2 , and 36% from carbonate mineral dissolution. Chemical weathering in North America thus consumes 50 Mt atmospheric CO 2 -C per year. About half of that originates from 10% of the area of North America. The estimated strength of individual predictors differs from previous studies. This highlights the need for a globally representative set of regionally calibrated models of CO 2 consumption by chemical weathering, which apply very detailed spatial data to resolve the heterogeneity of earth surface processes.

Silicate weathering mobilizes “fresh” dissolved silica (DSi). The major factors governing DSi mobilization by chemical weathering on continental or global scales have not been satisfactorily quantified. Furthermore, influence of regional variations of proposed factors on large scale DSi mobilization is not properly assessed. A continental-scale, process-oriented, empirical model for DSi mobilization is developed to assess this research gap. The model is calibrated on river chemistry data from 142 monitoring stations from the conterminous USA, selected for minimal anthropogenic and water–body influence in their catchments. The average area of the catchments is 3890 km 2 . The average observed DSi yield of the catchments is 2.68 t SiO 2 km − 2 a − 1 . The model calculates DSi yield as subject to catchment attributes, i.e. climate, lithology, land cover and morphology. As lithological source data for the model, a new lithological map of North America was developed. The high spatial resolution of the new map allows assessment of lithology classes with mapped extents as small as 0.5 km 2 . The average lithology polygon size is 75 km 2 . The developed multi-lithological, non-linear, lumped model for annual DSi mobilization describes 89% of the observed variance of DSi yield. It uses lithology proportion and runoff as predictors. With runoff, DSi yield increases differently for individual lithological classes. Basic igneous rocks show the highest DSi yields with respect to a given runoff. Consolidated sedimenary lithological classes yield DSi in the reversed order of their defined silicon contents. Of all lithological classes, the least DSi per runoff is mobilized from acid plutonic rocks. Apart from the two major predictors, analysis of the model also provides evidence for an influence of temperature and land cover on DSi mobilization. Comparison with existing studies shows that controlling factors on DSi mobilization vary regionally. Thus, studies calibrated in different regions result in significantly different DSi yields for comparable lithological classes. This emphasizes the need for global DSi mobilization models to be regionally calibrated.

Empirical matter-flux models calibrated on a large number of catchments (“distributed lumped models”) are regularly used to analyze fluvial HCO 3 - fluxes. Despite the demonstrated applicability for spatial predictions, the applicability of distributed lumped models for historic reconstructions or future projections needs validation. Here, as a first evaluation, predictions of a published distributed lumped HCO 3 - flux model will be compared to observed HCO 3 - flux time series of individual catchments and additionally to models which were calibrated using those time series. The distributed lumped model, which was calibrated by regional data, predicts the time-series of single catchments well. Additionally, the results and parameters of models calibrated on time series of the single catchments are within the range that is covered by the distributed lumped model. The evaluation hints at the applicability of distributed lumped models, calibrated by annual flux data of a large number of catchments, for historic reconstructions and future projections of fluvial HCO 3 - flux at the regional scale.

Because there remains a lack of knowledge about the spatially explicit distribution of chemical weathering rates at the global scale, a model that considers prominent first-order factors is compiled step by step and the implied spatial variability in weathering is explored. The goal is to fuel the discussion about the development of an "Earth System" weathering function. We use as a starting point an established model of the dependence of chemical weathering on lithology and runoff, calibrated for an island arc setting, which features very high chemical weathering rates and a strong dependence on lithology and runoff. The model is enhanced stepwise with further factors accounting for soil shielding and temperature, and the observed variation of fluxes is discussed in context of observed data from large rivers globally.Results suggest that the global soil shielding reduces chemical weathering (CW) fluxes by about 44%, compared to an Earth surface with no deeply weathered soils but relatively young rock surfaces (e.g. as in volcanic arc and other tectonically active areas). About 70% of the weathering fluxes globally derive from 10% of the land area, with Southeast Asia being a primary "hot spot" of chemical weathering. In contrast, only 50% of runoff is attributed to 10% of the land area; thus the global chemical weathering curve is to some extent disconnected from the global runoff curve due to the spatially heterogeneous climate as well as rock and soil properties. The analysis of carbonate dissolution reveals that about half of the flux is not delivered from labeled carbonate sedimentary rocks, but from trace carbonates in igneous rocks as well as from siliciclastic sediment areas containing matrix carbonate.In addition to total chemical weathering fluxes, the release of P, a nutrient that controls biological productivity at large spatial scales, is affected by the spatial correlation between runoff, lithology, temperature and soil properties. The areal abundance of deeply weathered soils in Earth's past may have influenced weathering fluxes and P-fuelled biological productivity significantly, specifically in the case of larger climate shifts when high runoff fields shift to areas with thinner soils or areas with more weatherable rocks and relatively increased P-content. This observation may be particularly important for spatially resolved Earth system models targeting geological time scales. The model is discussed against current process knowledge and geodata with focus on improving future global chemical weathering model attempts.Identified key processes and geodata demanding further research are a) the representation of flowpaths to distinguish surface runoff, interflow and baseflow contributions to CW-fluxes, b) freeze-thaw effects on chemical weathering, specifically for the northern latitudes, c) a more detailed analysis to identify to what extent the spatially heterogeneous distribution of Earth surface properties causes a decoupling of the Earth system rating functions between CW-fluxes and global runoff, as well as d) an improved understanding of where and to what extent trace or matrix carbonates in silicate-dominated rocks and sediments contribute to carbonate weathering. The latter demands e) an improved representation of carbonate content in lithological classes in the lithological representation of the Earth surface. Further improvement of the lithological database is needed for f) the age of rocks and g) the geochemistry of sediments with focus on unconsolidated sediments in the large basins. And clearly h) an improved global soil database is needed for future improvements with reliable soil depth, mineralogical composition as well as physical properties. © 2013 The Authors., SCOPUS: ar.j, info:eu-repo/semantics/published

This contribution reviews the role of the terrestrial-aquatic interface in the global carbon cycle. We highlight that carbon leakage through this interface has profound ramifications for the terrestrial carbon balance and for the anthropogenic CO2 budget. Our budget analysis identifies the need for an integrated process-based quantitative understanding of the terrestrial-aquatic interface that includes its response to anthropogenic perturbations. Complementary observational and modeling efforts in this direction are presented.

International audience; Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8(-0.25)(+0.25) petagrams of carbon (Pg C) per year from streams and rivers and 0.32(-0.26)(+0.52) Pg C yr(-1) from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr(-1) is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.

Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

[1] Fluvial transport of dissolved organic carbon (DOC) is an important link in the global carbon cycle. Previous studies largely increased our knowledge of fluvial exports of carbon to the marine system, but considerable uncertainty remains about in-stream/in-river losses of organic carbon. This study presents an empirical method to assess the nonconservative behavior of fluvial DOC at continental scale. An empirical DOC flux model was trained on two different subsets of training catchments, one with catchments smaller than 2,000 km2 (n = 246, avg. 494 km2) and one with catchments larger than 2,000 km2 (n = 207, avg. 26,525 km2). A variety of potential predictors and controlling factors of fluvial DOC fluxes is discussed. The predictors retained for the final DOC flux models are runoff, slope gradient, land cover, and areal proportions of wetlands. According to the spatially explicit extrapolation of the models, in North America south of 60°N, the total fluvial DOC flux from small catchments (25.8 Mt C a−1, std. err.: 12%) is higher than that from large catchments (19.9 Mt C a−1, std. err.: 10%), giving a total DOC loss of 5.9 Mt C a−1 (std. err.: 78%). As DOC losses in headwaters are not represented in this budget, the estimated DOC loss is rather a minimum value for the total DOC loss within the fluvial network.

Nature 503, 355–359 (2013); doi:10.1038/nature12760 In Fig. 1a of this Article, the highest value on the colour scale should be 11,772 instead of 1,772. This has been corrected in the HTML and PDF versions of the paper online.

A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr(-1) since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (similar to 0.4 Pg C yr(-1)) or sequestered in sediments (similar to 0.5 Pg C yr(-1)) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of similar to 0.1 Pg C yr(-1) to the open ocean. According to our analysis, terrestrial ecosystems store similar to 0.9 Pg C yr(-1) at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr(-1) previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land-ocean aquatic continuum need to be included in global carbon dioxide budgets.

Over the last decade the number of regional to global scale studies of river chemical fluxes and their steering factors increased rapidly, entailing a growing demand for appropriate databases to calculate mass budgets, to calibrate models, or to test hypotheses. We present a short overview of the recently established GLObal RIver CHemistry database GLORICH, which combines an assemblage of hydrochemical data from varying sources with catchment characteristics of the sampling locations. The information provided include e.g. catchment size, lithology, soil, climate, land cover, net primary production, population density and average slope gradient. The data base comprises 1.27 million samples distributed over 17,000 sampling locations.

The ongoing development of the Global Carbon Project (GCP) global methane (CH4 ) budget shows a continuation of increasing CH4 emissions and CH4 accumulation in the atmosphere during 2000-2017. Here, we decompose the global budget into 19 regions (18land and 1 oceanic) and five key source sectors to spatially attribute the observed global trends. A comparison of top-down (TD) (atmospheric and transport model-based) and bottom-up (BU) (inventory- and process model-based) CH4 emission estimates demonstrates robust temporal trends with CH4 emissions increasing in 16 of the 19 regions. Five regions-China, Southeast Asia, USA, South Asia, and Brazil-account for >40% of the global total emissions (their anthropogenic and natural sources together totaling >270Tg CH4 yr-1 in 2008-2017). Two of these regions, China and South Asia, emit predominantly anthropogenic emissions (>75%) and together emit more than 25% of global anthropogenic emissions. China and the Middle East show the largest increases in total emission rates over the 2000 to 2017 period with regional emissions increasing by >20%. In contrast, Europe and Korea and Japan show a steady decline in CH4 emission rates, with total emissions decreasing by ~10% between 2000 and 2017. Coal mining, waste (predominantly solid waste disposal) and livestock (especially enteric fermentation) are dominant drivers of observed emissions increases while declines appear driven by a combination of waste and fossil emission reductions. As such, together these sectors present the greatest risks of further increasing the atmospheric CH4 burden and the greatest opportunities for greenhouse gas abatement.

Lakes (including reservoirs) are an important component of the global carbon (C) cycle, as acknowledged by the fifth assessment report of the IPCC. In the context of lakes, the boreal region is disproportionately important contributing to 27% of the worldwide lake area, despite representing just 14% of global land surface area. In this study, we used a statistical approach to derive a prediction equation for the partial pressure of CO2 (pCO2 ) in lakes as a function of lake area, terrestrial net primary productivity (NPP), and precipitation (r2 = .56), and to create the first high-resolution, circumboreal map (0.5°) of lake pCO2 . The map of pCO2 was combined with lake area from the recently published GLOWABO database and three different estimates of the gas transfer velocity k to produce a resulting map of CO2 evasion (FCO2 ). For the boreal region, we estimate an average, lake area weighted, pCO2 of 966 (678-1,325) μatm and a total FCO2 of 189 (74-347) Tg C year-1 , and evaluate the corresponding uncertainties based on Monte Carlo simulation. Our estimate of FCO2 is approximately twofold greater than previous estimates, as a result of methodological and data source differences. We use our results along with published estimates of the other C fluxes through inland waters to derive a C budget for the boreal region, and find that FCO2 from lakes is the most significant flux of the land-ocean aquatic continuum, and of a similar magnitude as emissions from forest fires. Using the model and applying it to spatially resolved projections of terrestrial NPP and precipitation while keeping everything else constant, we predict a 107% increase in boreal lake FCO2 under emission scenario RCP8.5 by 2100. Our projections are largely driven by increases in terrestrial NPP over the same period, showing the very close connection between the terrestrial and aquatic C cycle.

Since the industrial revolution, emissions of carbon dioxide (CO2) due to human activities have drastically increased carbon concentration in the atmosphere, perturbing the natural cycle of carbon (C). Oceans and the land biosphere have seen their C stocks increase. In order to better understand the dynamics of those sinks, it is essential to understand the link between them, the inland waters. In my thesis, I used three different methods to improve our understanding if the C dynamics in the European inland waters, with a focus on the fate of dissolved organic C (DOC) in the river network. First I applied a land surface model at the European scale to estimate and study the spatio-temporal variability of DOC leaching from land to rivers. I estimated that around 14 TgC per year are leached into the European river network, about 0.6% of the net primary production (NPP). I observed an important spatio-temporal variability with a maximum during winter and minimum in summer with the exception of nordic region where the maximum occurs in spring after the snow melt. Mes results showed that the fraction of NPP that is leached as DOC in the river primarly depends on the runoff and drainage while temperature only plays a secondary role. Secondly, I sampled the Meuse in order to study the biodegradability of DOC in the river. I estiamted a half-life time around 10 days, value inferior to the calculated water residence time in the Meuse, meaning that most of the DOC will be degraded before reaching the sea. Thirdly, based on literature, I built a C budget for European inland waters at the country scale in order to evaluate the import and export of C through border via rivers. I estimated that over Europe around 2.3 m-2 per year an-1 are imported and 4.4 gC m-2 per year exported leading to a net river C balance (RNCB) of 2.1 gC m-2 per year. With the exception of the Netherlands, Portugal, Estonia and Ukraine, all countries have a positive RNCB meani ng that the export more C than they import. I compared the RNCB against other components of the national C budget and against a other lateral flux of C between countries, the emissions related to wood and crop harvest trades. I showed that for some countries, the RNCB can be around the same order of magnitudes as harvest trades and thus should be included in national budget., Depuis la révolution industrielle, les émissions de dioxyde de carbone (CO2) vers l’atmosphère dues à l’activité humaine ont fortement augmentées, perturbant le cycle naturel du carbone (C). Les océans et l’écosystème terrestre ont vu leur stock en C augmenter. Afin de mieux comprendre la dynamique de ces puits, il est essentiel de s’intéresser au lien antre le puits terrestre et les océans, c’est-à-dire les eaux continentales. Dans ma thèse, j’ai utilisé trois méthodes différentes afin d’améliorer notre compréhension de la dynamique du C dans le réseau hydrographique Européen et avec un focus sur le C organique dissous (COD). Tout d’abord, j’ai appliqué un modèle du système terre à l’échelle Européenne pour estimer et étudier la variabilité spatio-temporelle du transfert du C des terres jusqu’aux rivières. J’ai estimé qu’en moyenne environ 14.3 TgC par an sont transférés des terres vers le système hydrographique Européen, ce qui représente envrion 0.6% de la productivité primaire nette (NPP). J’ai observé également une importante variabilité spatio-temporelle avec un maximum en hiver et un minimum en été sauf dans les régions nordiques où le maximum a lieu au printemps lors de la fonte des neiges. Mes résultats montrent que la fraction de NPP transférée en tant que COD vers les rivières est principalement contrôlé par le ruissellement et le drainage. Ensuite, j’ai effectué des campagnes d’échantillonnage sur la Meuse afin d’étudier la biodégradation du COD. J’ai estimé un temps de demi-vie à environ 10 jours, valeur inférieure au temps de résidence de l’eau de la Meuse estimé sur tout le bassin à 24 jours, ce qui signifie que la majorité du COD aura été décomposé avant d’atteindre l’estuaire. Et finalement, sur base de la littérature, j’ai construit un budget C pour les eaux continentales pour chaque pays Européen pour évalue les imports et exports de C à travers les frontières via les rivières. J’ai estimé que sur toute l’Europe en m oyenne environ 2.3 gC m-2 an-1 sont importés et 4.4 gC m-2 an-1 sont exportés entraînant un bilan net de C dans les rivières (RNCB) de 2.1 gC m-2 an-1. A l’exception des Pays-Bas, du Portugal, de l’Estonie et de l’Ukraine, tous les pays ont un RNCB positif, ils exportent plus de C qu’ils n’en importent. J’ai comparé le RNCB avec d’autres composants du budget national de C et ainsi qu’avec un autre flux latéral de C d’un pays vers un autre, les émissions liées aux échanges de récoltes de bois et d’agriculture. J’ai montré que certains pays le RNCB est du même ordre de grandeur que les échanges de récoltes et devraient donc être inclus dans les budget nationaux de C.

Lateral carbon transport from soils to the ocean through rivers has been acknowledged as a key component of the global carbon cycle, but it is still neglected in most global land surface models (LSMs). Fluvial transport of dissolved organic carbon (DOC) and CO2 has been implemented in the ORCHIDEE LSM, while erosion-induced delivery of sediment and particulate organic carbon (POC) from land to river was implemented in another version of the model. Based on these two developments, we take the final step towards the full representation of biospheric carbon transport through the land–river continuum. The newly developed model, called ORCHIDEE-Clateral, simulates the complete lateral transport of water, sediment, POC, DOC, and CO2 from land to sea through the river network, the deposition of sediment and POC in the river channel and floodplains, and the decomposition of POC and DOC in transit. We parameterized and evaluated ORCHIDEE-Clateral using observation data in Europe. The model explains 94 %, 75 %, and 83 % of the spatial variations of observed riverine water discharges, bankfull water flows, and riverine sediment discharges in Europe, respectively. The simulated long-term average total organic carbon concentrations and DOC concentrations in river flows are comparable to the observations in major European rivers, although our model generally overestimates the seasonal variation of riverine organic carbon concentrations. Application of ORCHIDEE-Clateral for Europe reveals that the lateral carbon transfer affects land carbon dynamics in multiple ways, and omission of this process in LSMs may lead to an overestimation of 4.5 % in the simulated annual net terrestrial carbon uptake over Europe. Overall, this study presents a useful tool for simulating large-scale lateral carbon transfer and for predicting the feedbacks between lateral carbon transfer and future climate and land use changes.

For much of the Pleistocene, regions of the Earth underlain by permafrost have been net accumulators of terrestrially-fixed plant carbon (C), known as organic C, to the extent that in the present day the soils of the northern circumpolar permafrost region alone contain a C mass outweighing that which exists in the modern atmosphere by a factor of over two. At the same time, the rivers of the Arctic permafrost region discharge about 11% of the global volumetric river water flux into oceans, doing so into an ocean (the Arctic) with 1% of global ocean water volume and a very high surface area: volume ratio, making it comparatively sensitive to influxes of terrestrially derived matter. This river flux is sourced from precipitation as either rain or snow, which, upon initial contact with the landscape has the immediate potential to interact with C in one of two ways: Water running over carbonate or silicate –bearing rocks will cause a reaction whose reactant requires the uptake of atmospheric CO2, which is subsequently transported in river water. This ‘inorganic’ C derived from interaction of water, atmosphere and lithosphere thus represents a C storage or ‘sink’ vector. In addition, water interacting with organic matter in tree canopies, litter or soil can dissolve C contained therein, and transfer it via surface and subsurface water flows into rivers, whereupon it may either be metabolised to the atmosphere or exported to the sea. Recent improvements in understanding of terrestrial C dynamics indicate that this hydrologic transfer of organic matter represents the dominant fate of organic carbon, after plant and soil respiration are accounted for. In the context of amplified Arctic anthropogenic warming, the thermal exposure imposed on the permafrost C stock with expectations of enhanced future precipitation point toward substantial shifts in the lateral flux-mediated organic and inorganic C cycle. However, the complex totality of the processes involved make prediction of this shift difficult. Addressing this gap in instrumental power and theoretical understanding, this collection of studies builds upon previous advances in earth system modelling to include the production and lateral transport of dissolved organic C (DOC), respiration-derived CO2, and rock-weathering derived alkalinity in a global land surface model previously developed to specifically resolve permafrost-region processes. By subjecting the resulting model to state of the art soil, water, vegetation and climatology datasets, we are able to reproduce existing lateral transport processes and fluxes, and project them into the future. In what follows, we show that while Pan-Arctic alkalinity exports and attendant CO2 uptake increase over the 20th and 21st Centuries under warming, DOC fluxes decline largely as a result of deeper soil water flow-paths and the resulting changes in carbon-water interactions. Rather than displaying a clear continuous (linear or non-linear) temperature sensitivity, future Arctic DOC release can increase or decrease with temperature depending on changes in the thermal state and hydrologic flow paths in the deep soil. The net marine effect of these fluxes is to decrease future terrestrially derived seawater acidification. Future model improvements to include representations of particulate C, methane generation, pyrogenic DOC, peat generation, soil ice/land surface subsidence are required to increase the rigor of the results generated by these models.; Pendant la majeure partie du Pléistocène, les régions de la Terre recouvertes de pergélisol ont été des accumulateurs nets de carbone (C) d’origine végétal et transféré au sol. L’accumulation de ce C organique dans les sols de la région de pergélisol circumpolaire nord a conduit à des stocks qui contiennent actuellement une masse C supérieure à celle qui existe dans l'atmosphère par un facteur de plus de deux. Dans le même temps, les rivières du pergélisol arctique rejettent environ 11% du flux d’eau fluvial global dans les océans, et ce dans un océan (l’Arctique) correspondant à 1% du volume d’eau total des océans et une très grande surface ce qui le rend relativement sensible aux afflux de matières dérivées des surfaces terrestres. Ce flux fluvial provient de précipitations sous forme de pluie ou de neige qui, lors du contact initial avec la surface, ont le potentiel immédiat d'interagir avec le C de l'une des deux manières suivantes: d’une part, l'eau qui coule sur des roches carbonatées ou silicatées provoquera une réaction dont le réactif nécessite l'absorption de CO2 atmosphérique, qui est ensuite transporté dans l'eau des rivières. Ce C inorganique issu de l’interaction de l’eau, de l’atmosphère et de la lithosphère représente donc un vecteur de stockage ou de «puits» du C. D’autre part, l’eau qui interagit avec la matière organique présente dans les arbres, la litière ou le sol peut dissoudre le C qu’elle contient et le transférer par les eaux de surface et souterraines dans les rivières. Ce carbone peut ensuite être métabolisée vers l’atmosphère ou exportée dans la mer. Des améliorations récentes dans la compréhension de la dynamique du C terrestre indiquent que ce transfert hydrologique de matière organique représente le devenir dominant du carbone organique, après prise en compte de la respiration des plantes et du sol. Dans le contexte du réchauffement climatique d’origine anthropique amplifié de l'Arctique, l'exposition thermique imposée au stock de pergélisol de C, associé à d'une augmentation des précipitations futures, laisse présager des changements importants dans le cycle du carbone organique et inorganique induit par les flux latéraux. Cependant, la totalité des processus impliqués rend difficile la prévision de ce changement. Partant de ce constat, cette thèse s’appuie sur les avancées antérieures en matière de modélisation du système terrestre pour inclure la production et le transport latéral de carbone organique dissous (COD), de CO2 dérivé de la respiration et d’alcalinité dérivée au sein d’un modèle global de surface terrestre développé précédemment pour résoudre spécifiquement les processus des régions boréales. Al’aide de données de pointe sur le sol, l'eau, la végétation et la climatologie pour forcer les conditions aux limites nous sommes en mesure de reproduire les processus et les flux de transport latéraux existants ainsi que faire des projections futures. Dans cette thèse, nous montrons que les exportations d'alcalinité panarctique et l'absorption du CO2 qui l'accompagne augmentent avec le réchauffement, que les flux de COD diminuent en grande partie à cause des circuits d'écoulement d'eau plus profonds dans le sol et des changements qui en résultent dans les interactions carbone-eau. Enfin, nous observons que la libération de COD dans l’Articque n’est pas linéairement liée à la temperaturre. Par conséquent, la future libération de COD dans l'Arctique peut augmenter ou diminuer avec la température en fonction des modifications de l'état thermique et des trajectoires hydrologiques dans les sols profonds. L'effet net de ces flux sur les océans est de réduire l'acidification future de l'eau de mer d'origine terrestre. Les améliorations futures apportées au modèle pour inclure des représentations du carbone particulaire, de génération de méthane, de COD pyrogénique, de subsidence de glace / surface du sol sont nécessaires pour accroître la rigueur des résultats générés par ce modèle.