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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - C-LEAK (Rivers as leak in the terrestrial C sink)

Teaser

Summary

Inland waters play an important role in the global carbon (C) cycle, both as land-ocean transport routes and as ecosystems where large amounts of organic C derived from terrestrial ecosystems are processed, generating substantial net-emissions of CO2 to the atmosphere, or buried and sequestered in aquatic sediments. Researchers from the field of aquatic biogeochemistry have been pointing out the substantial role the inland water plays for the terrestrial C and greenhouse gas budget for over a decade (Cole et al., 2007; Lauerwald et al., 2015; Raymond et al., 2013; Tranvik et al., 2009). It was however only in their latest report (Ciais et al., 2013) that the IPCC acknowledged the role of inland waters for the global C cycle, and Earth System models (ESMs) used to project the terrestrial sink for anthropogenic CO2 emissions still ignore the role of inland waters as a lateral link between land and ocean and as a greenhouse gas source to the atmosphere. There is evidence that anthropogenic actions increase C exports from the terrestrial systems through inland waters, e.g. by increasing erosion from agricultural soils, sewage injections or indirectly by thawing permafrost (Regnier et al., 2013). Thus, ignoring the leakage of C from terrestrial ecosystems through the inland water network potentially leads to an overestimation of the terrestrial land C sink and, consequently, to the underestimation of atmospheric CO2 concentrations and air temperatures in future projections (Ciais et al., in revision for Nature). Representing inland water C fluxes in an ESM framework would improve the assessment of the anthropogenic perturbation of the global C cycle and climate projections.
The objective of this research project was to overcome the limitations of recent ESMs and global C budget estimates by implementing the cycling of terrestrial C through inland waters into an integrated modelling framework. This allows simulation of the evolution of inland water C fluxes under climate change, anthropogenic CO2 emissions, land use change, land use related soil erosion, and river damming, and their implications for the terrestrial C budgets.

Work performed

Over the last 2 years, RL completed his pioneering effort to implement the representation of fluvial C transport and CO2 budgets of the river-floodplain systems into the land surface scheme of an ESM. This model was successfully validated against observed C fluxes in the Amazon basin (Lauerwald et al., 2017, GMD). Recently, the model has been used to project the coupled evolution of inland water C fluxes and the terrestrial C sink over the 21st century, predicting a substantial intensification of C cycling along the river-floodplain network, mainly caused by increasing atmospheric CO2 concentrations and only partly being offset by the negative effect of climate change (Lauerwald et al., in revision for Nature Geoscience). At the same time, in cooperation with Adam Hastie, a PhD student at ULB under his supervision, he contributed to a future projection of CO2 emissions from boreal lakes, which also predicts a substantial increase over the 21st century (Hastie et al., 2018, GCB).

In cooperation with Mahdi Nakhavali, a PhD student with Pierre Friedlingstein (line manager of RL) at the University of Exeter, he valorised his special modelling experience for the technical implementation of dissolved organic carbon (DOC) cycling in soils and DOC leaching from soils to the inland water network into the land surface model JULES. This led to one publication on the model development, testing and calibration for various observational sites across Europe (Nakhavali et al., 2018). Three more manuscripts on the global scale application of this model, on the simulation of spatio-temporal trends in DOC leaching over the historical period and over the 21st century have been submitted (1, to GBC) or are in preparation (in a state close to submission). Also in these studies, the importance of increasing atmospheric CO2 concentrations and climate change on lateral exports of terrestrial C have been proven.

In addition, RL participated in international research projects on the simulation of soil erosion effects on soil organic carbon (SOC) storage (Naipal et al., 2018, BG) and on fluvial exports of sediments and the related flux of particulate organic carbon (POC) (Zhang et al., in preparation) using a ESM model framework. These studies involve simulations over the historical period at European to global scale.

Finally, RL was one of the main contributors to a model study on the effect of river damming on C fluxes through the global inland water network from 1970 to 2050 (Maavara et al., 2017, Nature Comm.). For this study, he implemented the data bases for existing and planned dam reservoirs into a river routing scheme, which allows simulation of the inputs of POC and DOC from the river catchment into a dam reservoir and routing of the outflow of DOC and POC from a dam downstream to the next reservoir or river mouth. This study demonstrates how the amounts of terrestrially derived C which are processed or buried within the inland water network increase with the intensification of river damming. The river-dam routing scheme has recently been applied to re-estimate the N2O emissions from the inland water network, including rivers, reservoirs and estuaries, at the global scale(Maavara et al., in revision for GCB).

Final results

With his scientific achievements over the project period, RL contributed substantially to the progress in his research field. He achieved the major paradigm shift in ESM development from the representation of merely vertical exchange of C between atmosphere, vegetation and soil to the boundless representation of C cycling along the land-inland water continuum from the tree tops to the coast. He demonstrated how global change affects the C budgets not only of terrestrial ecosystems, but also inland waters, and how this affects the projected land sink of the anthropogenic CO2 emissions. The newly developed modelling framework allows evaluation of the impacts of soil erosion and river damming as anthropogenic impacts beside anthropogenic CO2 emissions, land use change and climate change, which are taken into account in a traditional ESM framework. By that, the research of RL allows more comprehensive assessment of the anthropogenic perturbation of the global C cycle, and by that a refined assessment of the anthropogenic greenhouse gas budget.