Figure 1.6 Integrated Science Assessment Model (ISAM) estimated CO2 concentrations for the SRES marker scenarios.
forcing as a function of time for various SRES marker scenarios. Calculated radiative forcing increases to 7 Wm"2 by 2100 for high scenario A2 and 4.7 Wm"2 for low scenarios B1 and B2 relative to the beginning of the Industrial Era. As a result, each of the scenarios implies a significant warming tendency. Direct effects of aerosols are included in this analysis but indirect effects and effects on ozone are not considered.
The response of the climate system to the changes in radiative forcing is determined by the climate sensitivity, defined as the equilibrium surface temperature increase for doubling of atmospheric CO2 concentration. This parameter is intended to account for all the climate feedback processes not modeled explicitly. The middle panel of Figure 1.7 shows the model-calculated changes in global mean surface temperature for the various SRES scenarios assuming a central value for the climate sensitivity of 2.5°C. For the four scenarios, surface temperature is projected to increase by about 1.8°C to 2.6°C by 2100 relative to 1990 for this assumed climate sensitivity. The full range of uncertainty in the climate sensitivity would be presented by a broader range of 1.5 to 4.5°C for a doubling of CO2 concentration (IPCC, 1996a). Accounting for this uncertainty, the scenarios would give an increase in surface temperature of about 1.3 to 5°C for the four scenarios. The bottom panel of Figure 1.7 shows the derived sea level rise for the four scenarios. The difference in future sea level scenarios is much smaller as compared to the temperature scenarios. This is because the sea level rise has a much stronger memory effect due mainly
to the large thermal inertia of the ocean and hence long time scale of the ocean response.
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