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Global carbon dioxide (CO2) evasion from inland waters (rivers, lakes, and reservoirs) and carbon (C) export from land to oceans constitute critical terms in the global C budget. However, the magnitudes, spatiotemporal patterns, and underlying mechanisms of these fluxes are poorly constrained. Here, we used a coupled terrestrial–aquatic model to assess how multiple changes in climate, land use, atmospheric CO2 concentration, nitrogen (N) deposition, N fertilizer and manure applications have affected global CO2 evasion and riverine C export along the terrestrial‐aquatic continuum. We estimate that terrestrial C loadings, riverine C export, and CO2 evasion in the preindustrial period (1800s) were 1,820 ± 507 (mean ± standard deviation), 765 ± 132, and 841 ± 190 Tg C yr−1, respectively. During 1800–2019, multifactorial global changes caused an increase of 25% (461 Tg C yr−1) in terrestrial C loadings, reaching 2,281 Tg C yr−1 in the 2010s, with 23% (104 Tg C yr−1) of this increase exported to the ocean and 59% (273 Tg C yr−1) being emitted to the atmosphere. Our results showed that global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans, highlighting the important role of inland waters in the global C balance, an amount that should be taken into account in future C budgets. Our analysis supports the view that a major feature of the global C cycle–the transfer from land to ocean–has undergone a dramatic change over the last two centuries as a result of human activities. Plain Language Summary: Despite occupying only 1% of the Earth's surface, inland waters (rivers, lakes, and reservoirs) play a critical role in global carbon (C) cycling by linking two of the Earth's largest C pools, terrestrial and marine ecosystems, as well as by exchanging CO2 with the atmosphere. Inland waters emit and bury C before it reaches the oceans, with important implications for the global C budget. Although global estimates of lateral C fluxes have been made previously, much uncertainty exists in their magnitudes, spatiotemporal patterns, and underlying controls (anthropogenic vs. natural processes). By improving a coupled terrestrial–aquatic model, we assess how climate, land use, atmospheric CO2, and nitrogen enrichment affected global CO2 evasion and riverine C export along the terrestrial–aquatic continuum since the 1800s. We estimate a 25% increase in terrestrial C loading since the 1800s, of which 59% was emitted to the atmosphere and 23% was exported to the ocean. The increased riverine C exports were primarily due to increasing atmospheric CO2 level and nitrogen inputs; additionally, climate and land conversion dominated interannual and decadal variations in CO2 evasion. Our findings indicate that anthropogenic‐induced climate change and multiple environmental stresses since the preindustrial era have resulted in significant increases in terrestrial C export to oceans and CO2 evasion. Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and ocean, underscoring the importance of inland waters for closing the global carbon budget. Key Points: Terrestrial carbon loading since 1800 has increased by 25%, with 23% of this increase exported to the ocean and 59% being emitted to the atmosphereAtmospheric CO2 and N inputs dominated C export increase, while the climate and land use change dominated the decadal variations in CO2 evasionGlobal inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans in the 2010s [ABSTRACT FROM AUTHOR]

The evasion of CO2 from inland waters, a major carbon source to the atmosphere, depends on dissolved inorganic carbon (DIC) concentrations. Our understanding of DIC dynamics across gradients of climate, geology, and vegetation conditions however have remained elusive. To understand its large‐scale patterns and drivers, we collated instantaneous and mean (multiyear average) DIC concentrations from about 100 rivers draining minimally‐impacted watersheds in the contiguous United States. Within individual sites, instantaneous concentrations (C) measured at daily to seasonal scales exhibit a near‐universal response to changes in river discharge (Q) in a negative power law form. High concentrations occur at low discharge when DIC‐enriched groundwater dominates river discharge; low concentrations occur under high flow when relatively DIC‐poor shallow soil water predominates river discharge. Such patterns echo the widely observed increase of soil CO2 and DIC with depth and the shallow‐and‐deep hypothesis that emphasizes the importance of flow paths and source water chemistry. Across sites, mean concentrations (Cm) decrease with increasing mean discharge (Qm), a long‐term climate measure, and reachs maxima at around 200 mm/yr. A parsimonious model reveals that high mean DIC arises from soil CO2 accumulation when rates of DIC‐generating reactions are relatively high compared to its export fluxes in arid climates. Although instantaneous and mean DIC concentrations similarly decrease with increasing discharge, results here highlight their distinct drivers: daily to seasonal‐scale instantaneous concentration variations (C) are controlled by subsurface CO2 distribution over depth (from below), whereas long‐term mean concentrations (Cm) are regulated by climate (from above). The results emphasize the significance of land‐river connectivity via subsurface flow paths. They also underscore the importance of characterizing subsurface CO2 distribution to illuminate belowground processes in order to project the future of water and carbon cycles in a warming climate. Key Points: Within individual sites, instantaneous dissolved inorganic carbon (DIC) (C) exhibits a near‐universal dilution pattern with lower concentrations at high discharge (Q)Across sites, DIC (Cm) similarly exhibits a dilution pattern with a lower mean concentration at high mean discharge (Qm) in humid climatesAlbeit similar patterns, results highlight distinct drivers of instantaneous and mean concentrations by subsurface CO2 profiles and climate, respectively [ABSTRACT FROM AUTHOR]