Climate Change and the European Water Dimension - Agri ...

of the impact of anthropogenic absorbing aerosols on regional climate change (see
Figure I.5).
I.I. Future projections
Large efforts have been made in recent years to construct reliable global climate
models and several Atmosphere-Ocean General Circulation Models (AOCGM) are
now available. Regional climate models with higher spatial resolution supplement
these global models, having a relatively coarse resolution of hundreds of kilometres
or more. Not all of the forcings discussed above were included in the models used for
the TAR projections; in most cases only well-mixed greenhouse gases and the direct
effect of sulphate aerosols were considered. Models, used for the simulation of future
global climate change scenarios in the TAR, were evaluated against observations;
i.e. their ability to simulate present and past climates was tested and it was
concluded that they provide credible simulations of climate, at least down to subcontinental
scales and over temporal scales from seasonal to decadal.
The projection of future climate changes requires not only an understanding of the
physical (and possibly biogeochemical) aspects but also predictions of future
emissions scenarios. The IPCC has defined a number of scenarios for the world
economic and demographical development, grouped into different storylines and
scenario families, that differs e.g. by the rate of economic growth, use of non-fossil
energy sources and population growth. The TAR has compared the results of these
different scenarios. The variations caused by choice of scenario were found to be of
the same order as those caused by the choice of model.
The TAR concentrates particularly on two scenarios from the IPCC Special Report of
Emission Scenarios (SRES), A2 and B2, that differ with respect to population growth
rates (A2 has the highest population growth rate) and socio-economic development.
A2 represents a scenario with relatively high greenhouse gas emissions while B2 has
relatively low emissions. For the global average surface air temperature (SAT),
applying the A2 scenario, the models predict an increase of 1.1 0 C with a range from
0.5 to 1.7 0 C for the difference between average 1961-1990 and average 2021 to
2050. For the B2 scenario, the mean is 1.2 0 C with a range from 0.5 to 1.4 0 C. If the
1961-1990 average is compared to the average global surface temperature at the
end of the century, the differences become somewhat more pronounced: A2 gives a
mean value of 3.0 0 C with a range of 1.3 to 4.5 0 C while B2 gives a mean of 2.2 0 C with
a range of 0.9 to 3.4 0 C. The projected temperature increases over the 21 st century
(1990 to 2100) have been calculated for the full set of scenarios described in the
SRES, using a simplified procedure compared to the full AOCGM runs; the global
average surface temperature increases predicted were in range 1.4 to 5.8 0 C in 2050.
Inclusion of sulphate aerosols in the models lead to a slower warming in interval up
to the mean mid-21 st century; e.g. for one of the typical scenarios (IS92a) the
average temperature increase goes down from 1.6 to 1.3 0 C when sulphate aerosols
are included. Since fossil fuel burning is a source of sulphate aerosol, the inclusion of
such aerosol tend to mitigate the greenhouse warming caused by use of these fuels
and thus make alternative scenarios less favourable in the short run. However, since
the removal of CO2 from the atmosphere is much slower than the deposition of
aerosols, at longer time scales the warming caused by the accumulated CO2
dominates over the cooling by the sulphate aerosols, and the scenarios with high
fossil fuel consumption becomes the most unfavourable.
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