9.3.1.2 Projections of future climate from forcing scenario experiments (IS92a)
 
Figure 9.5: (a) The time evolution of the globally averaged temperature
change relative to the years (1961 to 1990) of the DDC simulations (IS92a).
G: greenhouse gas only (top), GS: greenhouse gas and sulphate aerosols (bottom).
The observed temperature change (Jones, 1994) is indicated by the black
line. (Unit: °C). See Table 9.1 for more
information on the individual models used here. (b) The time evolution of
the globally averaged precipitation change relative to the years (1961 to
1990) of the DDC simulations. GHG: greenhouse gas only (top), GS: greenhouse
gas and sulphate aerosols (bottom). (Unit: %). See Table
9.1 for more information on the individual models used here. |
Please note that the use of projections for forming climate
scenarios to study the impacts of climate change is discussed in Chapter
13.
These experiments include changes in greenhouse gases plus the direct effect
of sulphate aerosol using IS92a type forcing (see Chapter
6 for a complete discussion of direct and indirect effect forcing from sulphate
aerosols). The temperature change (Figures 9.5a and 9.7a,
top) for the 30-year average 2021 to 2050 compared with 1961 to 1990 is +1.3°C
with a range of +0.8 to +1.7°C as opposed to +1.6°C with a range of
+1.0 to +2.1°C for greenhouse gases only (Cubasch and Fischer-Bruns, 2000).
The experiments including sulphate aerosols show a smaller temp-erature rise
compared to experiments without sulphate aerosols due to the negative radiative
forcing of these aerosols. Additionally, in these simulations CO2
would double around year 2060. Thus for the averaging period being considered,
years 2021 to 2050, the models are still short of the CO2 doubling
point seen in the idealised 1%/yr CO2 increase simulations. These
sensitivity ranges could be somewhat higher (about 30%) if the positive feedback
effects from the carbon cycle are included interactively but the magnitude of
these feedbacks is uncertain (Cox et al., 2000; Friedlingstein, 2001). The globally
averaged precipitation response for 2021 to 2050 for greenhouse gases plus sulphates
is +1.5% with a range of +0.5 to +3.3% as opposed to +2.3% with a range of +0.9
to +4.4% for greenhouse gases only (Figures 9.5b and 9.7a,
bottom).
9.3.1.3 Marker scenario experiments (SRES)
As discussed in Section 9.1.2, only the draft marker
SRES scenarios A2 and B2 have been integrated with more than one AOGCM, because
the scenarios were defined too late to have experiments ready from all the modelling
groups in time for this report. Additionally, some new versions of models have
been used to run the A2 and B2 scenarios that have not had time to be evaluated
by Chapter 8. Therefore, we present results from all the
model simulations and consider them all as possible realisations of future climate
change, but their ranges are not directly comparable to the simple model results
in Section 9.3.3 (range: 1.4 to 5.8°C), because
in the simple model analysis seven somewhat different versions of the nine models
have been considered. Additionally, for the AOGCMs the temperature changes are
evaluated for an average of years 2071 to 2100 compared with 1961 to 1990, while
the simple model results are differences of the year 2100 minus 1990.
The average temperature response from nine AOGCMs using the SRES A2 forcing
(Figures 9.6a and 9.7b, top) for the
30-year average 2071 to 2100 relative to 1961 to 1990 is +3.0°C with a range
of +1.3 to +4.5°C, while using the SRES B2 scenarios it amounts to +2.2°C
with a range of +0.9 to +3.4°C. The B2 scenario produces a smaller warming
which is consistent with its lower positive radiative forcing at the end of
the 21st century. For the 30-year average 2021 to 2050 using the A2 scenario,
the globally averaged surface air temperature increase compared with 1961 to
1990 is +1.1°C with a range of +0.5 to +1.4°C, while using the SRES
B2 scenarios it amounts to +1.2°C with a range of +0.5 to +1.7°C. The
values for the SRES scenarios for the mid-21st century are lower than for the
IS92a scenarios for the corresponding period due to differences in the forcing.
The average precipitation response using the SRES A2 forcing (Figures
9.6b and 9.7b, bottom) for the 30-year average 2071
to 2100 compared with 1961 to 1990 is an increase of 3.9% with a range of 1.3
to 6.8% , while using the SRES B2 scenarios it amounts to an increase of 3.3%
with a range of 1.2 to 6.1%. The lower precipitation increase values for the
B2 scenario are consistent with less globally averaged warming for that scenario
at the end of the 21st century compared with A2. For the 30-year average 2021
to 2050 the globally averaged precipitation increases 1.2% for the A2 scenario,
and 1.6% for B2 which is again consistent with the slightly greater global warming
in B2 for mid-21st century compared with A2. Globally averaged changes of temperature
and precipitation are summarised in Figure 9.7b. A more
extensive analysis of globally averaged temperature changes for a wider range
of SRES forcing scenarios using a simple climate model is given in Section
9.3.3.
 
Figure 9.6: (a) The time evolution of the globally averaged
temperature change relative to the years (1961 to 1990) of the SRES
simulations A2 (top) and B2 (bottom) (Unit: °C). See Table
9.1 for more information on the individual models used here. (b)
The time evolution of the globally averaged precipitation change relative
to the years (1961 to 1990) of the SRES simulations A2 (top) and B2
(bottom) (Unit: %). See Table 9.1 for
more information on the individual models used here. |
|
 
Figure 9.7: (a) The global mean, the maximum and minimum simulated
by the respective models and the standard deviation for the CMIP2
experiments at the time of CO2-doubling and for the DDC experiments
during the years 2021 to 2050 relative to the years 1961 to 1990 for
temperature (top) (Unit: °C) and precipitation (bottom) (Unit:
%). G: greenhouse gases only, GS: greenhouse gases and sulphate aerosols.
See Table 9.1 for more information on
the individual models used here. (b) The global mean, the maximum
and minimum simulated by the respective models and the standard deviation
for the SRES scenario experiments A2 and B2 performed by the AOGCMs,
for the years 2021 to 2050 and 2071 to 2100 relative to the years
1961 to 1990 for temperature (top) (Unit: °C) and precipitation
(bottom) (Unit: %). See Table 9.1 for
more information on the individual models used here. |
|
