From land use to land cover: restoring the afforestation signal in a coupled integrated assessment–earth system model and the implications for CMIP5 RCP simulations

摘要

Climate projections depend on scenarios of fossil fuel emissions and land usechange, and the Intergovernmental Panel on Climate Change (IPCC) AR5 parallel process assumes consistent climatescenarios across integrated assessment and earth system models (IAMs andESMs). The CMIP5 (Coupled Model Intercomparison Project Phase 5) project used a novel "land use harmonization" based onthe Global Land use Model (GLM) to provide ESMs with consistent 1500–2100 landuse trajectories generated by historical data and four IAMs. A directcoupling of the Global Change Assessment Model (GCAM), GLM, and the CommunityESM (CESM) has allowed us to characterize and partially address a major gapin the CMIP5 land coupling design: the lack of a corresponding land coverharmonization. For RCP4.5, CESM global afforestation is only 22% ofGCAM's 2005 to 2100 afforestation. Likewise, only 17% of GCAM's 2040afforestation, and zero pasture loss, were transmitted to CESM within thedirectly coupled model. This is a problem because GCAM relied onafforestation to achieve RCP4.5 climate stabilization. GLM modifications andsharing forest area between GCAM and GLM within the directly coupled modeldid not increase CESM afforestation. Modifying the land use translator inaddition to GLM, however, enabled CESM to include 66% of GCAM'safforestation in 2040, and 94% of GCAM's pasture loss as grassland andshrubland losses. This additional afforestation increases CESM vegetationcarbon gain by 19 PgC and decreases atmospheric CO₂ gain by 8 ppmvfrom 2005 to 2040, which demonstrates that CESM without additionalafforestation simulates a different RCP4.5 scenario than prescribed by GCAM.Similar land cover inconsistencies exist in other CMIP5 model results,primarily because land cover information is not shared between models.Further work to harmonize land cover among models will be required toincrease fidelity between IAM scenarios and ESM simulations and realize thefull potential of scenario-based earth system simulations.