Abstract
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 atmosphere
Atmospheric CO2 and N inputs dominated C export increase, while the climate and land use change dominated the decadal variations in CO2 evasion
Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans in the 2010s