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Abstract:
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.
