Climate change - Atmosphere, Weather and Climate - page 400

and interactions to be simulated, based on our current
understanding of how they operate. Mathematical
simulation of the white box kind is potentially
very powerful but involves the need for a
thorough understanding of the variables, states,
feedbacks, transfers and forcings of the complex
system (i.e. the parameters), together with the laws
of physics of the atmosphere and oceans on which
they are based. The most powerful of such models
are coupled atmosphere-ocean general circulation
models (GCM) (see Chapter 8). GCMs require
very detailed calibration and adjustment. They are
thus run with actual empirical data relating to past
decades, in the light of which adjustments are made
to model parameters (such as those relating to fluxes,
feedbacks and forcing) in order to improve the
overall performance of the model. The advantage
of the GCM is that it attempts to take into account
the total structure and dynamism of the earth-
atmosphere-ocean system. Its disadvantages are
its obvious failure to do this completely, the huge
amount of data required to establish, test and run
(i.e. force) this white box predictive model and
also, extremely importantly, the lack of knowledge
regarding the future forcing conditions.

4 An immediate shift to renewable and nuclear energy

sources with CO 2 emissions reduced to 50 per cent
of 1985 levels by 2050.

The Third Assessment Report of the IPCC issued in
2001 adopts a range of emission scenarios (contained
in a Special Report on Emission Scenarios (SRES))
based on differing views of the global future. One set
(A1) considers rapid economic growth, global popu-
lation peaking in mid-century and then declining,
and introduction of more efficient technologies. Three
variants are: A1F1 fossil fuel intensive; A1T non-fossil
energy sources, and A1B a balance across all energy
sources. Scenario A2 considers global heterogeneity,
increasing population, fragmented and slower tech-
nological change. A second set includes B1 where
population trends are as in A1, but the global economy
is service and information based, with clean, resource-
efficient technologies. B2 envisages slower population
increase, intermediate levels of economic development,
and diverse, regionally oriented technological change.
Figure 13.17 shows the projected changes in CO 2 , CH 4
and CFC-11 concentrations during the twenty-first
century, based on these scenarios. CO 2 concentrations
are projected to rise to between 540 and 970 ppm by
2100, corresponding to increases of 90 and 250 per cent
above the pre-industrial level. Methane concentration
changes will range between -190 ppb and 1970 ppb
above 1998 levels by 2100.
The associated projected increases in anthropogenic
radiative forcing corresponding to the SRES cases of
Figure 13.17 are shown in Figure 13.18. The projected
range is 4 to 9 W m -2 by 2100. In 1995 it was estimated
that to stabilize the concentration of greenhouse gases
at 1990 levels would require the following percentage
reductions in emissions resulting from human activities:
CO 2 >60 per cent; CH 4 15 to 20 per cent; N 2 O 70 to
80 per cent; CFCs 70 to 85 per cent. The 2001 IPCC
report notes that to stabilize CO 2 concentrations at 450
(650) ppm would require anthropogenic emissions to
drop below 1990 levels within a few decades (about a
century).
The IPCC Scientific Assessment for 1990 predicted,
with business as usual, an overall global temperature
rise of 3.5°C by the end of the twenty-second century
with a wide error band. Twenty numerical simulation
models were run to equilibrium (reached at c . AD 2030)
for the period 1850 to 2050, assuming a doubling of CO 2
during that period but employing only a relatively

F THE IPCC MODELS

The most sophisticated coupled atmosphere-ocean
general circulation models in current use were devel-
oped under the aegis of the Intergovernmental Panel
on Climate Change (IPCC). The panel issued three
comprehensive reports in 1990, 1995 and 2001. These
sought to assess the impacts of predicted increases in
greenhouse gas concentrations during the twenty-first
century. In the earlier reports the projected increases
were based on four possible scenarios:

1 Business as usual (BAU). This envisaged modest

controls and efficiency improvements over industrial
emissions; uncontrolled agricultural emissions; deple-
tion of tropical forests at the present rate. This would
result in the world CO 2 concentration being more than
twice the pre-industrial era level by the year 2070.

2 A shift towards lower-carbon fuels and natural gas.

3 Scenario B; plus immediate phasing out of CFCs,

limitation of agricultural emissions; plus a gradual
shift to renewable and nuclear energy sources.