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Rivers are pivotal in the global carbon cycle, transporting terrestrial carbon to the ocean while emitting significant amount of CO2 ${\mathrm{C}\mathrm{O}}_{2}$ to the atmosphere. However, the partitioning of fluvial inorganic carbon (IC) between downstream transport and atmospheric evasion remains uncertain due to intricate hydrodynamic and biogeochemical processes. Inspired by Budyko's hydrological work, this study introduces a dimensionless framework to identify critical factors in fluvial IC partitioning: the IC fraction in equilibrium with the atmosphere and the ratio of advection to evasion timescales. River catchment analyses and modeling reveal that the equilibrium ratio determines the fraction of IC stably transported downstream. The hydrodynamic‐driven timescale ratio determines the fate of out‐of‐equilibrium IC, with low‐order streams favoring atmospheric evasion and higher‐order streams promoting downstream transport. This framework provides a simple yet robust approach to predicting river carbon dynamics, with implications for land‐to‐ocean transport, fluvial emissions, and climate mitigation strategies such as enhanced weathering. Plain Language Summary: Rivers contain large amounts of dissolved inorganic carbon originating from the decomposition of organic matter and soil water fluxes. This carbon is partly transported downstream across the river network to the ocean and partly emitted as CO2 ${\mathrm{C}\mathrm{O}}_{2}$ to the atmosphere. Quantifying the partitioning between these fluxes is crucial for accurate carbon budgets from the local to the global scale, but the complex interplay of biogeophysical processes makes it challenging. Inspired by Budyko's seminal work in hydrology, we present a framework that determines the main chemical and physical processes governing the fluvial inorganic carbon partitioning. We identify two dimensionless numbers (or indexes) that can be defined with only a few parameters and provide quantitative and robust predictions on fluvial carbon fate. These numbers are linked to water turbulence and chemistry, and their values change across the river network. Key Points: Rivers partition inorganic carbon between downstream transport and CO2 ${\mathrm{C}\mathrm{O}}_{2}$ emissions to the atmosphereIncorporating key hydrodynamic and biogeochemical processes, we identify the dimensionless numbers that govern the inorganic carbon partitioningWater hydrodynamics plays a critical role in the fate of the carbon that is out of equilibrium with the atmosphere [ABSTRACT FROM AUTHOR]

Hydrogen (H2) is a promising low-carbon alternative to fossil fuels for many applications. However, significant gaps in our understanding of the atmospheric H2 budget limit our ability to predict the impacts of greater H2 usage. Here we use NOAA H 2 dry air mole fraction observations from air samples collected from ground-based and ship platforms during 2010–2019 to evaluate the representation of H 2 in the NOAA GFDL-AM4.1 atmospheric chemistry-climate model. We find that the base model configuration captures the observed interhemispheric gradient well but underestimates the surface concentration of H2 by about 10 ppb. Additionally, the model fails to reproduce the 1–2 ppb yr -1 mean increase in surface H 2 observed at background stations. We show that the cause is most likely an underestimation of current anthropogenic emissions, including potential leakages from H 2 -producing facilities. We also show that changes in soil moisture, soil temperature, and snow cover have most likely caused an increase in the magnitude of the soil sink, the most important removal mechanism for atmospheric H2 , especially in the Northern Hemisphere. However, there remains uncertainty due to fundamental gaps in our understanding of H 2 soil removal, such as the minimum moisture required for H 2 soil uptake, for which we performed extensive sensitivity analyses. Finally, we show that the observed meridional gradient of the H2 mixing ratio and its seasonality can provide important constraints to test and refine parameterizations of the H2 soil sink. [ABSTRACT FROM AUTHOR]

Hydrogen (H2) is being considered for many applications as an alternative to fossil fuels. Robust assessment of the climate implications of increased H2 usage in the global economy is partly hindered by uncertainties in its biogeochemical cycle. Here we use NOAA H2 dry air mole fraction observations from air samples collected from ground-based and ship platforms from 2010 to 2019 to evaluate the representation of H2 in the NOAA GFDL-AM4.1 atmospheric chemistry-climate model. We find that the model captures the observed interhemispheric gradient well but underestimates the surface concentration of H2 by about 10 ppbv. Observations show a 1–2 ppbv/year mean increase in surface H2 at background stations, while the simulated H2 exhibits no significant change over the 2010–2019 period. We show that this model bias is primarily driven by the estimated decrease of anthropogenic emissions, mostly from transportation, and that including leakage from H2-producing facilities can improve the simulated trend. We find that changes in soil moisture, soil temperature, and snow cover likely increase the magnitude and modify spatial distribution of the soil sink, the most important removal mechanism for atmospheric H2. However, the magnitude and even the sign of such changes is uncertain due to fundamental gaps in our understanding of H2 soil removal, such as the minimum soil moisture for H2 soil uptake. We show that the observed meridional gradient of H2 mixing ratio and its seasonality provide important constraints to test and refine parameterizations of H2 soil removal. [ABSTRACT FROM AUTHOR]

Soil uptake by H2 -oxidizing bacteria is the main sink of the global hydrogen cycle, accounting for nearly 80% of the atmospheric H2 consumption. Although the H2 uptake is strongly influenced by soil moisture, little attention has been paid to coherently couple the water and hydrogen dynamics in soils. Toward this goal, we improve the mechanistic representation of the H2 uptake as a function of soil moisture and highlight the role of the moisture temporal fluctuations on the biotic consumption of H2 . The results show that, due to the strongly nonlinear relationship between soil moisture and H2 uptake, addressing the dry-wet sequences is necessary to characterize the H2 uptake in semi-arid regions correctly. From novel analytical relationships validated with field data, we also infer the biotic and abiotic limitations in the global soil H2 uptake. It is shown that, diffusion generally limits the uptake in humid temperate and tropical regions, while biotic limitations tend to occur in very arid or cold soils. Finally, we discuss the implications that climate change may have on the H2 soil sink. [ABSTRACT FROM AUTHOR]

Enhanced weathering (EW) is a promising negative-emission technology that artificially accelerates the dissolution of natural minerals, promotes biomass growth, and alleviates the acidification of soils and natural waters. EW aims to increase the alkalinity of natural waters (alkalinization) to promote a transfer of CO 2 from the atmosphere to the water. Here we provide a quantification of the alkalinization carbon-capture efficiency (ACE) as a function of the water chemistry. ACE can be used for any alkaline mineral in various natural waters. We show that ACE strongly depends on the water pH, with a sharp transition from minimum to maximum in a narrow interval of pH values. We also quantify ACE in three compartments of the land-to-ocean aquatic continuum: the world topsoils, the lakes of an acid-sensitive area, and the global surface ocean. The results reveal that the efficiency of terrestrial EW varies markedly, from 0 to 100 %, with a significant trade-off in acidic conditions between carbon-capture efficiency and enhanced chemical dissolution. The efficiency is more stable in the ocean, with a typical value of around 80 % and a latitudinal pattern driven by differences in seawater temperature and salinity. Our results point to the importance of an integrated hydrological and biogeochemical theory to assess the fate of the weathering products across the aquatic continuum from land to ocean., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Amilcare Porporato reports financial support was provided by BP Plc. Amilcare Porporato reports financial support was provided by National Science Foundation., (Copyright © 2022 Elsevier B.V. All rights reserved.)