Environmental Engineering Reference
In-Depth Information
climate models (for an overview, see McGuffie and Henderson-Sellers, 1997; IPCC,
1997). Initially, the concept of global warming potentials (GWPs) was introduced
(IPCC, 1990), which was an index that allowed the climate effects of emissions to
be compared, relative to those of carbon dioxide (CO
2
). However, the limitations of
GWPs became obvious for influences other than long-lived gases, such as CO
2
, meth-
ane (CH
4
) and the halocarbons. Subsequently, the concept of
radiative forcing of cli-
mate
was commonly used as a metric of climate change by the IPCC in its special
report (IPCC, 1995) for the first meeting of the Conference of Parties to the Con-
vention (United Nations Framework Convention on Climate Change) and the sec-
ond assessment report of working group 1 of the IPCC (IPCC, 1996).
Radiative forcing may be defined as a measure of the importance of perturbations
to the planetary radiation balance and is measured in watts per square metre (
Wm
-2
).
One of the main reasons for its use as a convenient metric is that there is an approx-
imately linear relationship between the change in global mean radiative forcing (
∆
F
)
and the global mean surface temperature change (
∆
T
s
):
∆
≈
λ
∆
(1)
where
λ
is the climate sensitivity parameter (
K
(
Wm
-2
)
-1
) (eg Shine and Forster, 1999).
Consequently, it can be seen that it is a very convenient metric, but nonetheless
a proxy for climate change. It is worthwhile putting values of radiative forcing in
context. Figure 5.1 summarizes the energy balance of the Earth's atmosphere system.
This shows that incoming and outgoing radiation is balanced (as a global average) at
343Wm
-2
. It should therefore be appreciated that relatively small perturbations can
make measurable and significant impacts on climate.
Figure 5.1
Radiative energy balance of the Earth's atmosphere
