The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly ...understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal ocean's carbon budget.
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.
Key Points
First global maps of river CO2 partial pressures and evasion at 0.5° resolution
Global river CO2 evasion estimated at 650 (483–846) Tg C yr−1
Latitudes between 10°N and 10°S contribute half of the global CO2 evasion
This review examines the current understanding of the global coastal ocean carbon cycle and provides a new quantitative synthesis of air-sea CO
2
exchange. This reanalysis yields an estimate for the ...globally integrated coastal ocean CO
2
flux of −0.25 ± 0.05 Pg C year
−1
, with polar and subpolar regions accounting for most of the CO
2
removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used to conceptualizecoastal carbon cycle processes. The carbon dynamics in three contrasting case study regions, the Baltic Sea, the Mid-Atlantic Bight, and the South China Sea, are compared in terms of the spatio-temporal variability of surface
p
CO
2
. Ocean carbon models that range from box models to three-dimensional coupled circulation-biogeochemical models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Common unresolved challenges remain for implementation of these models across RiOMar and OceMar systems. The long-term trends in coastal ocean carbon fluxes for different coastal systems under anthropogenic stress that are emerging in observations and numerical simulations are highlighted. Knowledge gaps in projecting future perturbations associated with before and after net-zero CO
2
emissions in the context of concurrent changes in the land-ocean-atmosphere coupled system pose a key challenge.
A new synthesis yields an estimate for a globally integrated coastal ocean carbon sink of −0.25 Pg C year
−1
, with greater than 90% of atmospheric CO
2
removal occurring in polar and subpolar regions.
The sustained coastal and open ocean carbon sink is vital in mitigating climate change and meeting the target set by the Paris Agreement.
Uncertainties in the future coastal ocean carbon cycle are associated with concurrent trends and changes in the land-ocean-atmosphere coupled system.
The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies.
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.
Land use practices alter the biomass and structure of soil microbial communities. However, the impact of land management intensity on soil microbial diversity (i.e. richness and evenness) and ...consequences for functioning is still poorly understood. Here, we addressed this question by coupling molecular characterization of microbial diversity with measurements of carbon (C) mineralization in soils obtained from three locations across Europe, each representing a gradient of land management intensity under different soil and environmental conditions. Bacterial and fungal diversity were characterized by high throughput sequencing of ribosomal genes. Carbon cycling activities (i.e., mineralization of autochthonous soil organic matter, mineralization of allochthonous plant residues) were measured by quantifying 12C- and 13C-CO2 release after soils had been amended, or not, with 13C-labelled wheat residues. Variation partitioning analysis was used to rank biological and physicochemical soil parameters according to their relative contribution to these activities. Across all three locations, microbial diversity was greatest at intermediate levels of land use intensity, indicating that optimal management of soil microbial diversity might not be achieved under the least intensive agriculture. Microbial richness was the best predictor of the C-cycling activities, with bacterial and fungal richness explaining 32.2 and 17% of the intensity of autochthonous soil organic matter mineralization; and fungal richness explaining 77% of the intensity of wheat residues mineralization. Altogether, our results provide evidence that there is scope for improvement in soil management to enhance microbial biodiversity and optimize C transformations mediated by microbial communities in soil.
•A moderate land use intensity may increase soil microbial diversity.•Microbial diversity is a good a predictor of the transformations of the C-cycling.•Changes in microbial diversity with land use intensity link to soil functioning.
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 < 100). Here we propose to derive the CO2 air‐sea exchange of the shelf seas by extracting 3 · 106 direct surface ocean CO2 measurements from the global database SOCAT (Surface Ocean CO2 Atlas), atmospheric CO2 values from GlobalVIEW and calculating gas transfer rates using readily available global temperature, salinity, and wind speed fields. We then aggregate our results using a global segmentation of the shelf in 45 units and 152 subunits to establish a consistent regionalized CO2 exchange budget at the global scale. Within each unit, the data density determines the spatial and temporal resolutions at which the air‐sea CO2 fluxes are calculated and range from a 0.5° resolution in the best surveyed regions to a whole unit resolution in areas where data coverage is limited. Our approach also accounts, for the first time, for the partial sea ice cover of polar shelves. Our new regionalized global CO2 sink estimate of 0.19 ± 0.05 Pg C yr−1 falls in the low end of previous estimates. Reported to an ice‐free surface area of 22 · 106 km2, this value yields a flux density of 0.7 mol C m−2 yr−1, ~40% more intense than that of the open ocean. Our results also highlight the significant contribution of Arctic shelves to this global CO2 uptake (0.07 Pg C yr−1).
Key Points
First data‐driven global regionalized CO2 budget for continental shelf seasArctic shelves contribute disproportionately to global coastal CO2 sinkSeasonality of coastal FCO2 can partly be explained by temperature and NPP
Nitrous oxide (N2O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N2O 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 N2O 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 N2O 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 N2O emissions of 10.6–19.8 Gmol N year−1 (148–277 Gg N year−1), with reservoirs producing most N2O 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 N2O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.
Nitrous oxide (N2O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. In this work, we develop a mechanistic modeling approach to explicitly predict N2O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. Our results indicate that IPCC emissions predictions are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's estimates is kinetically improbable in most river systems.
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.
In our study, we account for land‐derived increases in nutrient inputs to the ocean originating from both perturbed riverine and atmospheric transport for 1905–2010. These increases in nutrient supply affect the carbon cycle of both the coastal and open ocean while partly alleviating effects arising from changes in the physical climate.
CO sub(2) evasion from rivers (FCO sub(2)) is an important component of the global carbon budget. Here we present the first global maps of CO sub(2) partial pressures (pCO sub(2)) in rivers of stream ...orders 3 and higher and the resulting FCO sub(2) at 0.5 degree resolution constructed with a statistical model. A geographic information system based approach is used to derive a pCO sub(2) 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 pCO sub(2) are net primary production, population density, and slope gradient within the river catchment as well as mean air temperature at the sampling location (r super(2)=0.47). The predicted pCO sub(2) map was then combined with spatially explicit estimates of stream surface area A sub(river) and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO sub(2) map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO sub(2) of 2400 (2019-2826) mu atm and a FCO sub(2) of 650 (483-846) Tg C yr super(-1) (5th and 95th percentiles of confidence interval). Our global CO sub(2) evasion is substantially lower than the recent estimate of 1800 Tg C yr super(-1) although the training set of pCO sub(2) is very similar in both studies, mainly due to lower tropical pCO sub(2) estimates in the present study. Our maps reveal strong latitudinal gradients in pCO sub(2), A sub(river), and FCO sub(2). The zone between 10 degree N and 10 degree S contributes about half of the global CO sub(2) evasion. Collection of pCO sub(2) data in this zone, in particular, for African and Southeast Asian rivers is a high priority to reduce uncertainty on FCO sub(2). Key Points * First global maps of river CO sub(2) partial pressures and evasion at 0.5 degree resolution * Global river CO sub(2) evasion estimated at 650 (483-846) Tg C yr super(-1) * Latitudes between 10 degree N and 10 degree S contribute half of the global CO sub(2) evasion
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