Geoscience Reference
In-Depth Information
While the process of anthropogenic emissions due to fossil fuel burning is fairly well
established in state-of-the-art climate model simulations, up to now, the possible impact of
land-use changes on the climate is mostly neglected in long-term climate simulation. Dale
(
1997
) reviewed the literature dealing with the relationship between land-use change and
climate change and concluded that in recent centuries, land-use change has had much
greater effects on ecological variables than has climate change. Pielke et al. (
2002
) doc-
umented that land-use change impacts regional and global climate through the surface-
energy budget, as well as through the carbon cycle, whereat the surface-energy budget
effects may be more important than the carbon-cycle effects. While this is valid for the past
climate, results of Cox et al. (
2000
) indicated that carbon-cycle feedbacks could signifi-
cantly accelerate climate change over the twenty-first century and pointed out the necessity
to consider the potentially large direct human influences on terrestrial carbon uptake
through changes in land cover and land management. Changes in the land surface (veg-
etation, soils, water) resulting from human activities can affect the regional climate through
shifts in radiation, cloudiness and surface temperature. Changes in vegetation cover affect
surface energy and water balances at the regional scale, so that the impact of land-use
change may be very significant for the regional climate over time periods of decades or
longer (Denman et al.
2007
). The effects of a specific land-use change on the climate
depend on the surrounding environment and climate characteristics as a regional modelling
study of Gao et al. (
2003
) over China has shown.
An extreme anthropogenic impact on the local hydrology is the practice of irrigation.
Over 18 % of total cultivated land is irrigated (Fischer et al.
2007
); additionally, much
nonagricultural land has been substantially modified by human activities. Conversion of
land to agriculture not only impacts the local evaporation and hydrological response, but
may also influence the distribution of rainfall and evaporative demand in the surrounding
landscape as well as have remote impacts on the large-scale circulation. The latter will be
considered in more detail in Sect.
2
. Agriculture and urban development have increased
substantially in the past century and will continue to develop in the twenty-first century.
Therefore, any assessment of the world's water resources must take into account both the
direct and indirect influences of land-use changes and the exploitation of the riverine
system.
Earth's climate is determined to a large extent by greenhouse gases (GHG) in the
atmosphere, which influence the radiation budget and thus the energy balance of the planet.
Thus, fluxes that may change the atmospheric GHG content are of great importance in
climate change research. Apart from water vapour and anthropogenic GHG, various
components of the global carbon cycle, especially CO
2
and CH
4
, play a significant role. In
recent years, estimates for the amount of carbon stored in soils have attracted more and
more attention, and here especially the consideration of the vast permafrost regions
increased numbers drastically (Tarnocai et al.
2009
; Zimov et al.
2006
; Schuur et al.
2008
;
McGuire et al.
2009
). Permafrost, being defined as ground that is at or below zero degrees
Celsius for more than two consecutive years, affects roughly one-quarter of the northern
hemisphere (Brown et al.
1997
). It is believed to store between 1,400 and 1,800 Pg of C in
the upper few metres of the soil (Schuur et al.
2008
), which would be twice the amount of
the atmosphere's content. The high northern latitudes are one of the critical regions of
anthropogenic climate change, where the observed warming is clearly above average due
to the so-called Arctic Amplification (Solomon et al.
2007
; ACIA
2005
). Climate model
simulations project this trend to continue (Serreze and Barry
2011
). The combination of the
high C stocks in subarctic and arctic soils with the pronounced warming in the affected
regions could thus lead to a positive feedback through the release of formerly trapped,
