Geography Reference
In-Depth Information
included in the calculations (Houghton 1997).
The theoretical surface temperature has been
calculated to be—18°C based on a 30 per cent
value for the average reflectivity of the Earth and
atmosphere. The actual global surface temperature
is about 15°C— the difference represents the
natural greenhouse effect. Clouds play an
important but complex role in this radiation
balance (IPCC1990).
It is customary in studying possible scenarios
of a future global climate to model a doubling of
the pre-industrial (1750) carbon dioxide
concentration of 280 ppmv, or its radiative
equivalent. At the current rate of increase in
greenhouse gas emissions under a business-as-
usual scenario, doubling might occur between
2070 and 2100 (IPCC 1995). Conservative
radiation models of the climate system indicate an
initial rise of 1.3°C for a doubling of carbon
dioxide, which is equivalent to 4 W m-
2
(Watts
per square metre). This would mean an initial fall
in outgoing radiation from 240 to 236 W m-
2
,
which would need to be restored to 240 W m-
2
to
maintain radiation equilibrium. This assumes that
there is no change in the average amount of solar
energy reaching the outer edge of the atmosphere,
which is about 1,370 W m-
2
(IPCC 1990). This
value is referred to as 'the solar constant'.
One way of indicating the effect on the radiative
balance of greenhouse gases and other components
is to express this in terms of radiative forcing (W
m-
2
) from pre-industrial times to the present. Figure
2.3 shows estimates of the annually averaged
radiative forcing due to human activity over this
period. It also shows the natural changes in solar
output from 1850 to the present (IPCC WGI1996).
The first column portrays the summed effect of
carbon dioxide, methane, nitrous oxide and the
halocarbons (CFCs). The radiative forcing is about
2.6 W m-
2
or 0.8°C, with an error bar of 0.8 W m-
2
. The error bar indicates the range of current
estimates by models, while the confidence level is
shown as a subjective assessment by Houghton
(1997). It can be seen that the level of confidence is
low for both tropospheric aerosols and solar
variability. Expressing the radiative forcing effect in
this way has led to the concept of global warming
potential (GWP), which is defined as the ratio of
the enhanced greenhouse effect of any gas
compared with that of carbon dioxide. GWP is
then used as a basis for 'trade-offs' in which the
increase of the GWP of one gas can be offset against
Figure 2.3
Estimates of the
globally and annually averaged
anthropogenic radiative forcing
in W m-
2
due to changes in
concentrations of greenhouse
gases and aerosols from a pre-
industrial base (CO2
concentration of 280 ppmv) to
1992. Estimated solar output
changes are from 1850 to 1990
(0.02% of the mean solar
constant). The shaded
rectangular bars represent
mid-range estimates of
radiative forcing—both positive
and negative.
Source:
Houghton 1997.
