Environmental Engineering Reference
In-Depth Information
4
Physical Climate Change
in the 21st Century
4.1 REGIONAL PATTERNS OF WARMING AND RELATED FACTORS
An approximation on which we will rely in this report is “pattern
scaling”, in which a robust pattern of climate change is assumed to exist
that describes the geographical and seasonal structure of the temperature
response that does not depend on climate sensitivity or on details of the
forcing or emission scenario. All of the scenario and sensitivity dependence
is captured within the time evolving global mean surface temperature, T
G
(t).
Letting the symbol
x
stand for the two horizontal spatial coordinates and the
time of year, the assumption is that
T(t,
x
) = T
G
(t)
t
(
x
)
The pattern
t
(
x
) has a spatial-annual mean of unity by definition. We
focus on temperature here and discuss pattern scaling for precipitation in
Section 4.2.
The validity of this approximation is discussed by Santer et al. (1990),
Mitchell et al. (1999), and Mitchell (2003). It has been used extensively
for regional temperature (and precipitation) change projections (Dessai et
al., 2005; Murphy et al., 2007; Watterson, 2008) and impacts studies, as a
substitute to running fully coupled simulations under different scenarios or
with different models with a range of climate sensitivities.
The pattern is derived from experiments with fully coupled Global Cli-
mate Models (GCMs), with validation from efforts to isolate the well-mixed
greenhouse gas signal from the historical temperature record. The value of
the method relies on the pattern remaining fairly constant during a simula-
tion, across different concentration pathway scenarios and across different
model settings. Regionally and temporally differentiated results under dif-
ferent scenarios or climate sensitivities can be derived by first character-
izing the stable geographical pattern of warming (and its spatial variability
105
