climate change on UAE - Stockholm Environment Institute-US Center

considers that “there may be critical thresholds
beyond which some systems may not be able to
adapt to changing <strong>climate</strong> conditions without
radically altering their functional state and
system integrity” (IPCC, 2007). Practically
speaking, defining the criteria to reach a
threshold is very difficult to determine: either
it requires empirical evidence from similar
systems which have undergone some threshold
<strong>change</strong>, or a nearly flawless mechanistic model
(a potentially unachievable gold standard).
The theory of whether distinct, quantifiable
thresholds exist in complicated, natural
ecosystems continues to be debated in the
literature (Burkett et al., 2005; Briske et
al., 2006; Groffman et al., 2006), but there is
general agreement that the concept is at least
illustrative, if not functionally applicable in many
circumstances. To be susceptible to threshold
behavior, an ecosystem must have the potential
to obtain multiple stable states. Ecosystems
of all sizes are always in a state of flux, with
various components continually recovering
from disturbances; therefore, ecosystems do
not simply obtain a ‘climax’, but may reach a
stable equilibrium, in which the functional state
of the system remains essentially the same
despite disturbances. Some have described
the steady state as a suite of positive and
negative feedback loops, which balance to keep
ecosystem function relatively constant (van de
Koppel et al., 1997).
In some cases, either extreme disturbances
or (more often) chronic disturbances may be
enough to either enhance a positive feedback
cycle or attenuate a negative feedback cycle, and
the state of the ecosystem shifts dramatically.
The shift from one stable state to the next
requires that the system be pushed past a
particular threshold before settling into a new
steady state (Scheffer et al., 2001).
Current ecosystem theory suggests that multiple
steady states can be envisioned in a three
dimensional space (see Figure 8. 1) representing
the ecosystem state and conditions on a flat
plane, and a measure of ecosystem stability in
the vertical plane (Scheffer et al., 2001). The
“ecosystem state” is simply the functional form
of an ecosystem under a certain set of conditions.
The conditions may include <strong>climate</strong>, chemical,
and physical properties of the area. The line
represented on the flat plane traces the region
of maximum ecosystem stability, with two areas
of “attraction” (labeled as F 1
and F 2
).
Along the solid line, as conditions <strong>change</strong>
smoothly, the state of the ecosystem <strong>change</strong>s
smoothly. However, a significant perturbation
in either the state or the ambient conditions
may be enough to push the ecosystem into
a new steady state. This shift could be both
perceived and labeled as a catastrophic <strong>change</strong>.
In this representation, the depth of the wells
is a measure of the ecosystem’s resistance
to <strong>change</strong> (its intrinsic stability at particular
conditions), while the steepness of the well
walls represents the resilience of the ecosystem
(given a disturbance, how likely is the ecosystem
to return to its initial state).
In this case, resilience can be envisioned as the
magnitude of a disturbance required to push the
ecosystem into a new steady state (Carpenter et
al., 2001). In the context of <strong>climate</strong> <strong>change</strong> and
biodiversity, <strong>climate</strong> <strong>change</strong> would be a shift
in ambient conditions, driving the ecosystem
through a smooth transition along a steadystate
line. However, if land use <strong>change</strong>, habitat
destruction, or pollution weakens resilience, it
may be trivial to push an ecosystem into a new
steady state.
A threshold could be crossed by stressing
multiple state variables, or even a single
important state variable such as <strong>climate</strong> <strong>change</strong>
or precipitation availability. Changes in critical
loadings, such as nutrients, temperature, or
water availability can trigger a catastrophic
<strong>change</strong>, and external anthropogenic forcing can
also push a threshold envelope.
The concept of a threshold is a useful concept,
yet may lack a specific application in real world
management. Ecosystem models have been built
to explore the nature of ecosystem equilbria, and
in these process-based models, it is relatively
trivial to simulate threshold behavior. Since
even process-based models inevitably rely on
some degree of empirical numerical estimations,
thresholds are, to some degree, built into these
models. However, in practice, it is difficult to
define the boundaries of a natural ecosystem
state and the contributing conditions such
that a threshold can be quantitatively defined.
To define a threshold effectively, either the
behavior has to have been observed already in
a similar ecotype, or the understanding of the
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Climate Change Impacts, Vulnerability & Adaptation