Environmental Engineering Reference
In-Depth Information
The global-mean radiative forcing (
Δ
F
) is approximately related to the equilib-
rium global-mean surface temperature change (
Δ
T
) by (IPCC
1994
) (Eq.
2.1
):
∆
T
= λ∆
F
(2.1)
where
λ
is the climate sensitivity parameter. Although there is a large discrepancy
in the actual value of
λ
in different models, its values are assumed to be approxi-
mately independent of the agent causing the forcing. The spread in the model
estimates of
λ
varies from about 0.4-1.2 K (W m
−
2
)
-1
, that is, approximately by
a factor of 3 (IPCC
1990
,
2001
). The models also indicate generic deviations of
λ
from the case of global CO
2
perturbations: increases of O
3
in the upper tropo-
sphere generally produce lower values of
λ
, while O
3
perturbations in the lower
stratosphere lead to higher values of
λ
(Joshi et al.
2003
).
Global average radiative forcings in 2005 (best estimates with 5-95 % uncer-
tainty ranges) with respect to 1750 for atmospheric constituents are
+
1.66 (range:
+
1.49 to
+
1.83) W m
−
2
for CO
2
,
+
0.48 (
+
0.43 to
+
0.53) W m
−
2
for CH
4
,
+
0.16
(
+
0.14 to
+
0.18) W m
−
2
for N
2
O,
+
0.34 (
+
0.31 to
+
0.37) W m
−
2
for halocar-
bons,
+
0.35 (
+
0.25 to
+
0.65) W m
−
2
for tropospheric O
3
, and
+
0.12 (
+
0.06 to
+
0.30) W m
−
2
for changes in solar irradiance (Fig.
1
) (IPCC
2007a
). On the other
hand, anthropogenic contributions to aerosols (primarily sulphate aerosol, organic
carbon, black carbon, nitrate and dust) produce an overall cooling effect, with a
total direct radiative forcing of
−
0.5 (
−
0.9 to
−
0.1) W m
−
2
and an indirect cloud
albedo forcing of
−
0.7 (
−
1.8 to
−
0.3) W m
−
2
(IPCC
2007a
). The CO
2
radiative
forcing increased by 20 % from 1995 to 2005, the largest change for any decade in
at least the last 200 years (IPCC
2007a
).
The global warming potential (GWP) is used within the Kyoto Protocol to the
United Nations Framework Convention on Climate Change (UNFCCC) as a met-
ric for weighting the climate impact of the emission of different greenhouse gases
(IPCC
1990
,
2001
; Shine et al.
2005
). The GWP is the time-integrated radiative
forcing due to a pulse emission of a given gas, over some given time period (or
horizon), relative to a pulse emission of carbon dioxide (IPCC
2001
). GWPs are
an index for estimating relative global warming contributions, due to the atmos-
pheric emission of a kg of a particular greenhouse gas compared to the emis-
sion of a kg of carbon dioxide. For instance, CH
4
and N
2
O have relatively long
atmospheric residence times (12 and 114 years, respectively), which combined
with their ability to efficiently absorb infrared radiation results into GWPs of 23
and 296 times, respectively, that of CO
2
on a per-kg basis and a 100 years time
horizon (IPCC
2001
). In addition, the perfluorocarbons (e.g. CF
4
and C
2
F
6
) and
sulphur hexafluoride (SF
6
) have really long atmospheric residence times (50000,
10000, and 3200 years, respectively) and are strong absorbers of infrared radia-
tion. The resulting GWPs are 5700, 11900, and 22200 times, respectively, that of
CO
2
on a per-kg basis for 100 years time horizon (Table
1
) (IPCC
2001
). Most
of the halocarbons recently used (halogenated methanes and ethanes) show high
GWPs ranging from 12 to 12000 times that of CO
2
on a per-kg basis for 100 years
time horizon. Their atmospheric lifetimes vary from 0.3 to 260 years (Table
1
)
(IPCC
2001
).
