Local and nonlocal biophysical effects of historical land use and land cover changes in CMIP6 models and the intermodel uncertainty

Land use and land cover changes (LULCCs) can influence surface temperature through local and nonlocal biophysical processes. However, the local and nonlocal effects of historical LULCCs have rarely been explicitly investigated. In this study, we separate the local and nonlocal effects of historical (1985–2014) human land use activities based on a set of simulations with and without LULCCs from the Coupled Model Intercomparison Project Phase 6. We also attempt to explore the sources of the intermodel difference in the LULCC effects using a variance decomposition method. The results show that the nonlocal effects (-0.06 ℃ at the global scale) dominate the cooling effect of historical LULCCs mainly via decreases in downward longwave radiation and increases in upward shortwave radiation. The local effects are relatively small at the global scale (0.01 ℃) and manifest as warming at low latitudes (driven by weakened sensible and latent heat fluxes) and cooling in the boreal regions (driven by enhanced upward shortwave radiation). There remains a large intermodel uncertainty in the total effects of historical LULCCs, most of which is contributed by the intermodel difference in nonlocal effects. Such intermodel inconsistency in nonlocal effects is mainly attributed to the intermodel difference in changes in downward longwave radiation and surface sensible heat flux. This study highlights the importance of nonlocal effects of historical LULCCs in terms of the magnitude and the contribution to the intermodel uncertainty in the LULCC effects.