Rivers as leak in the terrestrial C sink (C-LEAK)

 

Introduction

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.

Therefore, the objectives of this research project were:

  1. to develop a process-based, integrated modelling framework that allows for the simulation of terrestrial carbon (C) budgets directly taking into account the lateral displacement of C along the inland water network, incl. biogeochemical transformation of C in transit, CO2 exchange with the atmosphere and burial of C in aquatic sediments.
  2. to use this modelling framework to analyse the direct (land-use change, hydrologic management) and indirect (climate change) anthropogenic drivers of these lateral C transfers and their effects on the anthropogenic CO2 budget.

 

Methodology

The main tool used in this project will be the Joint UK Land Environment Simulator (JULES). JULES simulates the exchange of C, energy and water between the land surface and the atmosphere as well as C budgets of vegetation and soils. JULES can be run as stand-alone model or coupled with an atmospheric circulation model. It has been used to quantify effects of climate-change and increasing atmospheric CO2 levels on the land carbon sink. JULES is developed by a large, open community, promoting the integration of process understanding from various fields of science. It has a modular structure, which allows replacing modules by upgraded, advanced modules or implementing new modules. The hydrology module of JULES provides sub-surface and surface runoff from the soil column at an hourly time-step, based on which a routing module simulates the lateral flows of water, as surface and sub-surface flows along the hillslope and in the stream channel. This is a unique feature that will allow performing detailed model-data comparison. A gridded routing scheme gives the flow directions from one grid cell to another.

In contrast to ORCHIDEE, routing schemes at very high resolution of up to 1 km² can be applied, allowing the simulation of lateral water flows even in smaller catchments at regional scale. In this project, the model code of JULES will be upgraded to simulate exports of C from soils to coasts. The implementation of non-conservative DOC and CO2 transport will be built on RL’s experience with ORCHIDEE, and complemented by the integration of POC fluxes, which requires simulation of soil erosion inputs to surface waters and particle trapping in lakes and reservoirs. These modifications will benefit greatly from access to the expertise in soil science and geomorphology at Exeter.

The model will be calibrated, applied and validated at regional scale for the UK, where high resolution (1 km²) forcing files and a dense monitoring network of river C fluxes for calibration and validation are available. Historical simulations (1850-2010) will be done with individual forcings (climate, CO2, land use, river damming) to attribute the observed changes in fluvial C transfers to anthropogenic perturbations and climate change. Future simulations (2010-2100) will be performed with contrasted scenarios of climate change (warmer drier, warmer wetter, more/less rainfall extremes, etc.) over the UK in order to assess the fate of fluvial C fluxes and impact on the land-ocean carbon balance.

 

Outcomes

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).

 

List of published, submitted or to be submitted articles

Hastie, A., Lauerwald, R., Weyhenmeyer, G., Sobek, S., Verpoorter, C. and Regnier, P.: CO2 evasion from boreal lakes: revised estimate, drivers of spatial variability, and future projections, Glob. Chang. Biol., 2018, doi: 10.1111/gcb.13902.

Nakhavali, M., Friedlingstein, P., Lauerwald, R., Tang, J., Chadburn, S., Camino-Serrano, M., Guenet, B., Harper, A., Walmsley, D., Peichl, M. and Gielen, B.: Representation of dissolved organic carbon in the JULES land surface model (vn4.4_JULES-DOCM), Geosci. Model Dev.., accepted, doi:10.5194/gmd-2017-172.

Camino-Serrano, M., Guenet, B., Luyssaert, S., Ciais, P., Bastrikov, V., De Vos, B., Gielen, B., Gleixner, G., Jornet-Puig, A., Kaiser, K., Kothawala, D., Lauerwald, R., Peñuelas, J., Schrumpf, M., Vicca, S., Vuichard, N., Walmsley, D. and Janssens, I. A.: ORCHIDEE-SOM: Modeling soil organic carbon (SOC) and dissolved organic carbon (DOC) dynamics along vertical soil profiles in Europe, Geosci. Model Dev., 11(3), 937–957, doi:10.5194/gmd-11-937-2018, 2018.

Guimberteau, M., Zhu, D., Maignan, F., Huang, Y., Yue, C., Dantec-Nédélec, S., Ottlé, C., Jornet-Puig, A., Bastos, A., Laurent, P., Goll, D., Bowring, S., Chang, J., Guenet, B., Tifafi, M., Peng, S., Krinner, G., Ducharne, A., Wang, F., Wang, T., Wang, X., Wang, Y., Yin, Z., Lauerwald, R., Joetzjer, E., Qiu, C., Kim, H. and Ciais, P.: ORCHIDEE-MICT (v8.4.1), a land surface model for the high latitudes: model description and validation, Geosci. Model Dev., 2018, 11, 121–163, doi: 10.5194/gmd-11-121-2018.

Lauerwald, R., Regnier, P., Camino-Serrano, M., Guenet, B., Guimberteau, M., Ducharne, A., Polcher, J. and Ciais, P.: ORCHILEAK (revision 3875): A new model branch to simulate carbon transfers along the terrestrial-aquatic continuum of the Amazon basin. Geosci. Model Dev., 2017, 10, 3821–3859, doi: 10.5194/gmd-10-3821-2017.

Maavara, T., Lauerwald, R., Regnier, P. and Van Cappellen, P.: Global perturbation of organic carbon cycling by river damming, Nat. Commun., 2017, 8, doi:10.1038/ncomms15347.

Articles in revision

Naipal, V., Ciais, P., Wang, Y., Lauerwald, R., Guenet, B. and Van Oost, K.: Global soil organic carbon removal by water erosion under climate change and land use change during 1850-2005, Biogeosciences, (bg-2017-527), 2018.

Articles submitted

Mahdi Nakhavali, Ronny Lauerwald, Pierre Regnier, Bertrand Guenet,Sarah ChadburnandPierre Friedlingstein: A global distribution of dissolved organic carbon in soil and in leaching. Submitted to Biogeosciences.

Articles in preparation

R. Lauerwald, P. Regnier, B. Guenet, P. Friedlingstein, P. Ciais: Coupled evolution of riverine carbon fluxes and the terrestrial C sink in the Amazon basin 1861-2099, in preparation for Nature Climate Change

Taylor Maavara, Ronny Lauerwald, Goulven Laruelle, Zahra Akbarzadeh, Nicholas J. Bouskill, Philippe Van Cappellen, Pierre Regnier: Nitrous oxide emissions from inland waters: Are the IPCC estimates too high?, in preparation from Global Biogeochemical Cycles.

 

References

Lauerwald, R., Regnier, P., Camino-Serrano, M., Guenet, B., Guimberteau, M., Ducharne, A., Polcher, J. and Ciais, P.: ORCHILEAK (revision 3875): A new model branch to simulate carbon transfers along the terrestrial-aquatic continuum of the Amazon basin, Geosci. Model Dev., 10(10), doi:10.5194/gmd-10-3821-2017, 2017.

Lauerwald, R., Regnier, P., Guenet, B., Friedlingstein, P. and Ciais, P.: Coupled evolution of riverine carbon fluxes and the terrestrial C sink in the Amazon basin over the historical period and the 21st century, Nat. Geosci., in revision

Maavara, T., Lauerwald, R., Regnier, P. and Van Cappellen, P.: Global perturbation of organic carbon cycling by river damming, Nat. Commun., 8, doi:10.1038/ncomms15347, 2017.

Mayorga, E., Seitzinger, S. P., Harrison, J. A., Dumont, E., Beusen, A. H. W., Bouwman, A. F., Fekete, B. M., Kroeze, C. and Van Drecht, G.: Global Nutrient Export from WaterSheds 2 (NEWS 2): Model development and implementation, Environ. Model. {&} Softw., 25(7), 837–853, doi:10.1016/j.envsoft.2010.01.007, 2010.

Naipal, V., Ciais, P., Wang, Y., Lauerwald, R., Guenet, B. and Van Oost, K.: Global soil organic carbon removal by water erosion under climate change and land use change during 1850-2005, Biogeosciences, (bg-2017-527), 2018.

Nakhavali, M., Friedlingstein, P., Lauerwald, R., Tang, J., Chadburn, S., Camino-Serrano, M., Guenet, B., Harper, A., Walmsley, D., Peichl, M. and Gielen, B.: Representation of dissolved organic carbon in the JULES land surface model (vn4.4-JULES-DOCM), Geosci. Model Dev., 11(2), doi:10.5194/gmd-11-593-2018, 2018.

 

Additional information and contact

‌This project was funded through an EU Horizon 2020 Marie Sklodowska-Curie Individual Fellowship 703813, awarded to Ronny Lauerwald, hosted by the University of Exeter and supervised by Professor Pierre Friedlingstein in the Climate Dynamics Research Group (part of Exeter Climate Systems - XCS, Mathematics) between 1 July 2016 and 30 June 2018.

Professor Tim Quine, Geography, as an additional mentor, and Leo de Sousa-Webb, as Training Manager, supported the Research Fellow and the project.

This is a legacy website and will remain publicly available on the University of Exeter website.

 

Legacy email contact: C-LEAK@exeter.ac.uk

 

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