Geoscience Reference
In-Depth Information
Figure 13.18
Model predicted atmos-
pheric radiative forcing (W m
-2
) for the
emission scenarios shown in Figure
13.17.
Source
: Adapted from Houghton
et al
.
(2001). Reproduced by permission of the
IPCC and Cambridge University Press.
(Summary for policy-makers. Report of
WG 1, IPCC, p. 66, fig. 19.)
10
A1FI
A1B
B1
B2
9
8
7
Model ensemble
all SRES
envelope
6
5
4
3
2
1
0
1800
1900
2000
2100
Year
autumn and winter associated with reduced sea ice
and snow cover than was predicted in 1990, and little
warming over the Arctic in summer. They indicated a
reduction in diurnal temperature range over land in most
seasons and most regions. The important prediction
of an enhanced global hydrological cycle included
increased rainfall over southern Europe, increased pre-
cipitation and soil moisture in high latitudes in winter,
and, most important, the possibility that Asian summer
monsoonal rainfall might be decreased due to the
anthropogenic aerosol effect. More consideration was
given to possible extreme climatic events. Predictions
included an increase in extremely high temperatures and
a decrease in winter days with extremely low temper-
atures; a decrease in diurnal temperature variability in
certain regions; an increase in precipitation intensities
and extreme rainfall events; and possibly more frequent
or severe drought periods in warmer climates.
Since 1992, changes to the models include improved
treatment of anthropogenic aerosols (direct and indirect
effects) and non-CO
2
greenhouse gases. Overall, anthro-
pogenic aerosols are believed to reduce the effects of
greenhouse gas forcing by an average of one-third
(range 20 to 40 per cent). The incorporation of an
improved knowledge of ocean dynamics into the models
has, however, been limited by the fact that although
mesoscale ocean eddies can now be well modelled, the
computational complexity involved limits their full
integration into coupled atmosphere-ocean models.
Most experiments are now run in a transient mode
whereby the steady increase of greenhouse gas con-
centrations is simulated. Figure 13.20 illustrates the
predicted warming expected when CO
2
concentrations
will have doubled.
The 1995 estimate for global warming by the year
2100 was in the range 1.0-3.5°C based on six scenarios
developed in 1992. Figure 13.21 shows the geographical
distribution of the projected change in annual surface
air temperature by 2100. The 2001 estimates project
somewhat greater warming in the range 1.4 to 5.8°C
by 2100 (Figure 13.22). The principal reason for this is
that the expected emissions of sulphur dioxide are lower
in the SRES scenarios. Warming will be greatest in
northern high-latitude land areas in winter, particularly
in North America. The global atmospheric moisture
content and precipitation will increase, especially over
northern mid- to high latitudes and these same regions
can expect more intense precipitation events over land.
Over most areas where mean precipitation increases,
there will also be greater interannual variability.
G OTHER ENVIRONMENTAL
IMPACTS OF CLIMATE CHANGE
1 Sea-level
The mechanisms influencing sea-level over the globe
are extremely complex. Following the Last Glacial
Maximum about 20,000 years ago, sea-level rose
rapidly as the major ice sheets of North America and
