Datensatz

Zusammenfassung

Land cover change (LCC) influences surface temperature locally via biogeophysical effects by changing the water, energy, and momentum budget. In addition to these locally induced changes (local effects), LCC at a given location can cause changes in temperature elsewhere via advection and changes in circulation (nonlocal effects). This dissertation presents an approach to separate local and nonlocal effects in climate models. In three studies, the local and nonlocal effects on surface temperature are analyzed separately. First, local and nonlocal effects are separated in the land-atmosphere model ECHAM6/JSBACH3 by simulating LCC in some model grid cells while leaving vege- tation unchanged in others. The results show that the local effects do not depend on the number of LCC grid cells used in the separation approach. The local effects on surface temperature in the model agree reasonably well with observations. An energy balance decomposition reveals that the mechanisms differ strongly between the local and nonlocal effects. In the second part, a new look-up approach is developed to investigate the local effects on historical LCC and LCC in future scenarios. Historically, biogeophysical changes in surface temperature are dominated by land use while in the future, the combina- tion of warming background climate and subsequent natural shifts in the geographical distribution of forests may become of equal importance. The third part focuses on the nonlocal effects. Simulations with the fully coupled cli- mate model MPI-ESM reveal that the nonlocal cooling of large-scale LCC substantially contributes to the discrepancy between modeled and observed biogeophysical changes in surface temperature. When globally averaged, the deforestation-induced cooling from nonlocal effects outweighs the warming from local effects, and both local and nonlocal effects largely scale linearly with the spatial extent of LCC. The globally av- eraged nonlocal effects induce a cooling for deforestation in all latitudinal bands. In an inter-model comparison of plausible deforestation scenarios, the nonlocal effects induce a cooling also for most other investigated models. This thesis bridges the gap between idealized studies on large-scale LCC and studies on more plausible LCC extents. Furthermore, the separate analysis of local and nonlocal effects reconciles previous model-based studies that found a negative radiative forcing from deforestation and a global mean cooling, and observation-based studies that found a deforestation-induced local warming in most regions.