2023, Heinke, Jens, Rolinski, Susanne, Müller, Christoph

To represent the impact of grazing livestock on carbon (C) and nitrogen (N) dynamics in grasslands, we implement a livestock module into LPJmL5.0-tillage, a global vegetation and crop model with explicit representation of managed grasslands and pastures, forming LPJmL5.0-grazing. The livestock module uses lactating dairy cows as a generic representation of grazing livestock. The new module explicitly accounts for forage quality in terms of dry-matter intake and digestibility using relationships derived from compositional analyses for different forages. Partitioning of N into milk, feces, and urine as simulated by the new livestock module shows very good agreement with observation-based relationships reported in the literature. Modelled C and N dynamics depend on forage quality (C:N ratios in grazed biomass), forage quantity, livestock densities, manure or fertilizer inputs, soil, atmospheric CO2 concentrations, and climate conditions. Due to the many interacting relationships, C sequestration, GHG emissions, N losses, and livestock productivity show substantial variation in space and across livestock densities. The improved LPJmL5.0-grazing model can now assess the effects of livestock grazing on C and N stocks and fluxes in grasslands. It can also provide insights about the spatio-temporal variability of grassland productivity and about the trade-offs between livestock production and environmental impacts.

2020, Goelzer, Heiko, Nowicki, Sophie, Payne, Anthony, Larour, Eric, Seroussi, Helene, Lipscomb, William H., Gregory, Jonathan, Abe-Ouchi, Ayako, Shepherd, Andrew, Simon, Erika, Agosta, Cécile, Alexander, Patrick, Aschwanden, Andy, Barthel, Alice, Calov, Reinhard, Chambers, Christopher, Choi, Youngmin, Cuzzone, Joshua, Dumas, Christophe, Edwards, Tamsin, Felikson, Denis, Fettweis, Xavier, Golledge, Nicholas R., Greve, Ralf, Humbert, Angelika, Huybrechts, Philippe, Le clec'h, Sebastien, Lee, Victoria, Leguy, Gunter, Little, Chris, Lowry, Daniel P., Morlighem, Mathieu, Nias, Isabel, Quiquet, Aurelien, Rückamp, Martin, Schlegel, Nicole-Jeanne, Slater, Donald A., Smith, Robin S., Straneo, Fiammetta, Tarasov, Lev, van de Wal, Roderik, van den Broeke, Michiel

The Greenland ice sheet is one of the largest contributors to global mean sea-level rise today and is expected to continue to lose mass as the Arctic continues to warm. The two predominant mass loss mechanisms are increased surface meltwater run-off and mass loss associated with the retreat of marine-terminating outlet glaciers. In this paper we use a large ensemble of Greenland ice sheet models forced by output from a representative subset of the Coupled Model Intercomparison Project (CMIP5) global climate models to project ice sheet changes and sea-level rise contributions over the 21st century. The simulations are part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6).We estimate the sea-level contribution together with uncertainties due to future climate forcing, ice sheet model formulations and ocean forcing for the two greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results indicate that the Greenland ice sheet will continue to lose mass in both scenarios until 2100, with contributions of 90-50 and 32-17mm to sea-level rise for RCP8.5 and RCP2.6, respectively. The largest mass loss is expected from the south-west of Greenland, which is governed by surface mass balance changes, continuing what is already observed today. Because the contributions are calculated against an unforced control experiment, these numbers do not include any committed mass loss, i.e. mass loss that would occur over the coming century if the climate forcing remained constant. Under RCP8.5 forcing, ice sheet model uncertainty explains an ensemble spread of 40 mm, while climate model uncertainty and ocean forcing uncertainty account for a spread of 36 and 19 mm, respectively. Apart from those formally derived uncertainty ranges, the largest gap in our knowledge is about the physical understanding and implementation of the calving process, i.e. the interaction of the ice sheet with the ocean. © Author(s) 2020.