climate change on UAE - Stockholm Environment Institute-US Center

is based on a theoretical understanding of
system dynamics (bottom-up) or if it is built on
empirical observations (top-down).
Models built from the top down are more
common, if only because the line between an
experimental construct and a model is blurred
when using empirical data. Empirical models can
be as simple as deriving a function to describe
a relationship through a set of variables, or
can be as complex a system which predicts
spatial patterns of vegetation under changing
<strong>climate</strong> conditions (see the CASA model,
below). Empirical models may be methods of
interpreting or simplifying datasets with rich
temporal or spatial information, such as long
time series or satellite data; or may use dense
datasets to compile statistical relationships,
which may then be used for predictive purposes.
The advantages of empirically-based models is
that they can be relatively simple to construct
and interpret, are often highly explicit in their
assumptions, and, most importantly, are based
directly on data.
Broadly, the bottom-up modeling approach
relies on established theories on how individual
components of an ecosystem operate at the
micro-scale. These mechanistic models are
often built to be as general as possible, such
that they are not constrained by data (collected
by fallible observers) or limited by the way
communities and ecosystems are structured
today. The point of these models is to explore
relationships between ecosystem components
and forcing factors, understand dynamics, and
impose conditions on a simulated ecosystem
which may not exist today. Some of the most
developed versions of these models are able
to predict the structure and function of major
biomes from first principles (i.e. photosynthesis,
respiration, and nutrient requirements), and are
now being used to explore how <strong>climate</strong> <strong>change</strong>,
land use, and disturbance may impact future
biomes.
Model limitations
All ecosystem models are severely limited by
scale, scope, and assumptions. Key aspects of
each are briefly described below.
Scale: The most fundamental processes in
an ecosystem occur at micro-scales, where
photosynthesis and respiration occur,
nutrients are utilized, and water is cycled. There
are, however, also important processes which
happen at the scale of the leaf (for example,
growth, senescence, shading, herbivory), the
stem (individual mortality, light and water
availability), the patch (disturbances), the
community (competition), and the region
(<strong>climate</strong>, light availability). The levels to
which these processes are simulated are
computationally and data limited, and many
processes operate across scales.
Scope: The broader a model strives to be,
the more general its assumptions must
become. To simulate a single type of biome
effectively, one might choose to model or
simulate a modest number of individual
floristic species with known characteristics;
to then include yet more diverse biomes in
the model, one often has to reduce the level
of detail down to functional plant types
rather than individual species. Models which
are global in scope often reduce plant types
down to simple plant functional types which
distinguish between physiognomy (tree or
grass), leaf form (broadleaf or needle-leaf),
leaf longevity (evergreen or deciduous), and
photosynthetic pathway (C 3
or C 4
) (i.e. Wang
et al., 2004). Models which effectively capture
global-scale dynamics may be ineffective or
irrelevant for studies at the sub-biome scale.
Assumptions: Every form of ecosystem model
has (or should have) a well developed list of
general assumptions. Empirical models, for
example, often assume that relationships
between correlated variables are causal, or
at least replicable, and rarely model discrete
pathways or mechanisms. First-principles
(bottom-up) models are rarely entirely
mechanistic: at some level, even the most
rigorous models simplify processes and
mechanisms with empirical relationships.
9.2 Types of ecosystem models
Ecosystem models span a wide range of
functions, but several types may be useful for
evaluating the impacts of <strong>climate</strong> <strong>change</strong> on
ecosystems and biodiversity in the UAE.
Impacts, Vulnerability & Adaptation for
Dryland Ecosystems in Abu Dhabi
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