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'deep-frozen' C into the atmosphere, when near-surface permafrost thaws. For the thawed
soils and their biogeochemistry, it is decisive whether dry or wet conditions predominate:
Aerobic decomposition is relatively fast and leads to the release of CO 2 , while anaerobic
decomposition is much slower and leads to the release of CH 4 as the main product of the
combustion of organic soil material. Therefore, not only the soil's temperature but also its
moisture status and specifically the presence of wetlands are important for the assessment
of the biogeochemical response to climatic conditions and thus should be represented in
climate or ESMs in a realistic and process-based manner. Thus, the adequate representation
of permafrost hydrology is a necessary and challenging task in climate modelling, which
will be considered in more detail in Sect. 3 .
2 Impact of Anthropogenic Land Use, Especially Irrigation, on Climate
Several studies (e.g., Gordon et al. 2005 ; Piao et al. 2007 ; Rost et al. 2008a , b ) demon-
strated that land cover conversions and water withdrawals have already noticeably changed
the partitioning of terrestrial precipitation into evapotranspiration and runoff. Gerten
( 2013 ) estimated that these direct human impacts have increased the global river discharge
by about 5 %, which is caused by the associated reduction in evapotranspiration.
Regionally, the implications of anthropogenic land use may be much larger. Partially, even
opposite effects (increased evapotranspiration, reduced runoff) may be induced by land-use
change (Destouni et al. 2013 ) or irrigation (Gerten et al. 2008 ).
Observations and model studies in tropical forests have shown effects of changing
surface energy and water balance on the state of the atmosphere. For example, Marengo
and Nobre ( 2001 ) found that the removal of vegetation led to decreases in precipitation,
evapotranspiration and moisture convergence in central and northern Amazonia. Oyama
and Nobre ( 2004 ) showed that the removal of vegetation in north-east Brazil would sub-
stantially decrease precipitation. Other model studies indicated that increased boreal forest
reduces the effects of snow albedo and causes regional warming (Denman et al. 2007 ).
Related to the latter, e.g., G ¨ttel et al. ( 2008 ) investigated the influence of changed veg-
etation fields on the projected regional climate over the Barents Sea region in an off-line
coupling experiment with the regional climate model (RCM) REMO and the dynamic
vegetation model LPJ-GUESS (Sitch et al. 2003 ). They projected a forest ratio increase
and a shift of the tree line to higher altitudes and latitudes caused by a warmer climate with
longer snow-free periods and growing season lengths. The feedback effects to the climate
of these changes were one order of magnitude lower than the effects of the greenhouse gas
forcing. A further warming in spring could be attributed to the snow-albedo effect, while a
cooling in summer was dedicated to changes in roughness length, enhanced transpiration
and changes in surface albedo. A more extreme study was conducted by Bathiany et al.
( 2010 ) who investigated the effect of large-scale changes in forest cover on global climate.
They completely removed tropical forest within the ESM of the Max Planck Institute for
Meteorology (MPI-ESM), which resulted in a simulated 0.4 K warming due to an increase
in CO 2 concentrations and a decrease in tropical evapotranspiration. A similar experiment
for the high northern latitudes led to a global cooling of 0.25 K in case of complete
deforestation and an equally large warming in case of afforestation. In both cases, the
involved albedo changes (snow-masking effect) are the main drivers of the temperature
change.
Land-use changes such as deforestation may have a substantial climate impact in areas
located close to strong climatic gradients, such as tropical regions as well as arid and
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