climate change on UAE - Stockholm Environment Institute-US Center

system across multiple spatial scales has to be
so comprehensive that it is reasonably certain a
threshold exists (Scheffer, 2001).
It is worth noting that in the literature, although
ecosystems are described as having multiple
steady states in almost all circumstances the
alternative state is one of severe degradation
relative to the original condition. So, for
example, while a lake ecosystem can either
persist as a clear water system with a diverse
range of invertebrates, benthic life, and fish, or
as a eutrophic, nutrient overloaded pool with
heavy algal cover, clearly the original condition
is preferred for biodiversity, function, and
environmental quality.
8.2. Implications of <strong>climate</strong> <strong>change</strong>
on UAE dryland ecosystems
The implications of <strong>climate</strong> <strong>change</strong> for the
arid ecosystems of the UAE are several fold,
and we explore several case examples in the
next section. However, we can potentially
state several widely applicable principles for
ecosystem <strong>change</strong>s in hot, arid regions.
Increases in aridity and reductions in
soil moisture: In general, if precipitation
remains low but both daytime and
evening temperatures increase in the
UAE, we would expect that soil moisture
will decline significantly. The amount of
moisture available to plants is a function
of precipitation, infiltration into soils, and
evapotranspiration, or the amount of water
which evaporates from surfaces or is released
by the stomata of plants. As temperatures
increase, evapotranspiration rises, and
soil moisture declines (Xu et al., 2004). If
precipitation events are more severe yet less
frequent, we would expect more water to run
off soil surfaces and thus remain unavailable
to plants.
Narrower margin of semi-arid grasslands
and scrublands: The shift towards more
arid soils will render it increasingly difficult
for even highly drought-adapted plants to
survive along desert margins. In the African
Sahara, extended drought periods (decadelength,
or longer), regularly extend sandy
desert margins into previously productive
landscapes (ref).
Mountaintop and wadi ecosystems reduced
or disappear: Similarly to desert margin
ecosystems, both natural and managed
ecosystems in Wadis and on mountains are
at risk as temperatures rise and precipitation
patterns <strong>change</strong>. In many parts of the world,
species which survive on the mountains of
arid regions persist because the mountains
receive either marginally more precipitation,
or have slightly cooler temperatures than
lowlands. These so-called “sky-islands” are
able to support a higher diversity of both
plant and animal species, and many will be
threatened by <strong>change</strong>s in <strong>climate</strong>. As global
temperatures increase, the only available
climatically acceptable area for these species
will be at higher elevations, which almost
always have less area available; if temperatures
increase as dramatically as expected in some
scenarios, many of these ecosystems will be
displaced altogether (i.e. made locally extinct;
Halpin, 1994). In seasonally moist wadis, a
similar balance could be threatened by more
intermittent runoff. Plants which require
continual soil moisture could be exposed to
extended droughts, and experience elevated
rates of erosion as heavy precipitation floods
these ecosystems.
Drylands shift towards invasive annuals or
shrubby perennials: Dryland ecosystems
are highly responsive to the frequency and
magnitude of rain pulses (Xu et al., 2004;
Huxman et al., 2004; Sponseller, 2007). At the
smallest of precipitation pulses, infiltration
may not extend more than several millimeters
into the soil, and can stimulate photosynthetic
activity in biological soil crusts (an important
micro-ecosystem of fungi, bacteria, and algae
discussed in more depth later) while remaining
ineffective in triggering growth in vascular
plants. Annual grasses with dormant seed
bank may germinate with heavy early-season
precipitation, while the growth of perennial
shrubs can be enhanced by sufficient midseason
rainfall (Huxman et al., 2004). Shallowrooting
grasses are able to take advantage
of short mid-season rain pulses as small
as 5 mm (Sala and Lauenroth, 1982), while
deeply rooted perennial shrubs rely on lowlying
water reserves and larger precipitation
events, but are able to withstand extended
droughts. Once established, grasslands
can become dominant through repeated
brushfires, which destroy perennial shrubs.
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Climate Change Impacts, Vulnerability & Adaptation