Abstract
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 (~0.4 Pg C yr−1) or sequestered in sediments (~0.5 Pg C yr−1) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of ~0.1 Pg C yr−1 to the open ocean. According to our analysis, terrestrial ecosystems store ~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.
Highlights
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
Major challenges in the study of C in the aquatic continuum include the disentangling of the anthropogenic perturbations from the natural transfers, identifying the drivers responsible for the ongoing changes and, forecasting their future evolution, for example, by incorporating these processes in Earth system models
The organic carbon loop starts with the lateral leakage of some of the organic carbon that is fixed into the terrestrial biosphere by photosynthesis
Summary
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. Major challenges in the study of C in the aquatic continuum include the disentangling of the anthropogenic perturbations from the natural transfers, identifying the drivers responsible for the ongoing changes and, forecasting their future evolution, for example, by incorporating these processes in Earth system models Resolving these issues is necessary to refine the allocation of greenhouse-gas fluxes at the global and regional scale, and to establish policy-relevant regional budgets and mitigation strategies[29]. The term ‘boundless carbon cycle’ was introduced to designate the present-day lateral and vertical C fluxes to and from inland waters only[17] We extend this concept to all components of the global C cycle that are connected by the land–ocean aquatic continuum (Box 1) and discuss possible changes relative to the natural C cycle by providing new separate estimates for the present day and the anthropogenic perturbation. A brief justification of our proposed estimate for each of these fluxes is provided
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