Abstract
Abstract. Global vegetation models traditionally treat anthropogenic land-use and land-cover changes (LULCCs) only as the changes in vegetation cover seen from one year to the next (net transitions). This approach ignores subgrid-scale processes such as shifting cultivation which do not affect the net vegetation distribution but which have an impact on the carbon budget. The differences in the carbon stocks feed back on processes like wildfires and desert formation. The simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) all describe LULCCs using the "Land-Use Harmonization Dataset". Though this dataset describes such subgrid-scale processes (gross transitions), some of the CMIP5 models still use the traditional approach. Using JSBACH/CBALANCE – the land carbon component of the Max Planck Institute Earth System Model (MPI-ESM), this study demonstrates how this potentially leads to a severe underestimation of the carbon emissions from LULCCs Using net transitions lowers the average land-use emissions from 1.44 to 0.90 Pg C yr−1 (38%) during the historical period (1850–2005) – a total lowering by 85 Pg C. The difference between the methods is smaller in the RCP scenarios (2006–2100) but in RCP2.6 and RCP8.5 still cumulates to 30–40 Pg C (on average 0.3–0.4 Pg C yr−1 or 13–25%). In RCP4.5 essentially no difference between the methods is found. Results from models using net transitions are furthermore found to be sensitive to model resolution.
Highlights
Since prehistoric times humans have been changing the surface of the Earth to suit their purposes (Williams, 2006)
This study addresses the differences arising from the inclusion or exclusion of subgrid-scale land-use and land-cover changes (LULCCs) which is the difference between net and gross LULCC algorithms
The present study addresses the differences in carbon emissions arising from applying net or gross transitions using the same model, JSBACH/CBALANCE, by re-running a sub-set of the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments performed with the Max Planck Institute for Meteorology Earth System Model (MPIESM) (Giorgetta et al, 2013)
Summary
Since prehistoric times humans have been changing the surface of the Earth to suit their purposes (Williams, 2006). Houghton et al (1983) included gross conversion effects in their bookkeeping model, Hurtt et al (2006) were the first to provide a data set which accounts for such cultivation practices by describing the anthropogenic land surface transformations within a grid cell bi-directionally in a form usable by grid-cell-based global vegetation models (GVMs) An update to this data set – the so-called Land-Use Harmonization Dataset – was presented in Hurtt et al (2011) and used by all CMIP5 models implementing LULCCs. The present study addresses the differences in carbon emissions arising from applying net or gross transitions using the same model, JSBACH/CBALANCE, by re-running a sub-set of the CMIP5 experiments performed with the Max Planck Institute for Meteorology Earth System Model (MPIESM) (Giorgetta et al, 2013).
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