Environmental Engineering Reference
In-Depth Information
Source:
IPCC, 2001
Figure 5.2
Global annual mean radiative forcing (Wm
-2
) from a number of agents for
the period from the pre-industrial (1750) to the present (2000)
The recent summary for policy-makers of the third assessment report (SPM-TAR)
of the IPCC working group 1 (IPCC, 2001) provides the most up-to-date assess-
ment of radiative forcing of climate that may be conveniently given in one figure,
shown in Figure 5.2.
Figure 5.2 shows the radiative forcing from different components: the aircraft 'con-
trails' and 'enhanced cloudiness' are identified as they are signatures unique to avia-
tion, other aviation effects being subsumed within the other radiative forcing bars. It
is incorrect to simply sum the radiative forcing values because of shorter- or longer-
lasting effects. The radiative forcings are from a number of agents since 1750, expressed
as global annual averages. Some species are more long-lived than others; thus, if
emissions of a long-lived species stopped tomorrow, the effect would continue for
centuries. Other effects are shorter term - for example, sulphate aerosol would cease
within approximately one or two months under the same hypothetical scenario of
the complete cessation of emissions. Nonetheless, it is useful to point out a total
radiative forcing of approximately 2.5Wm
-2
from the long-lived greenhouse gases in
the first bar (ie CO
2
, CH
4
, nitrous oxide (N
2
O), chlorofluorocarbons (CFCs)).
Before moving on to estimates of radiative forcing from aviation, it is worth
considering the question of what this total perturbation of climate means. The sum-
mary for policy-makers for the IPCC's third assessment report (IPCC, 2001) states
that the increase in global average surface temperature over the 20th century was 0.6
± 0.2 °C. From a range of emissions scenarios and model assessments, the globally
averaged surface temperature is projected to further increase by between 1.4 and
5.8 °C from 1990 to 2100, depending upon the scenario assumed.
