Synthesis Report - Question 3

3.2

Carbon dioxide concentrations, globally averaged surface temperature, and sea level are projected to increase under all IPCC emissions scenarios during the 21st century.

3.3

All SRES emissions scenarios result in an increase in the atmospheric concentration of CO₂ . For the six illustrative SRES scenarios, the projected concentrations of CO₂ -- the primary anthropogenic greenhouse gas -- in the year 2100 range from 540 to 970 ppm, compared to about 280 ppm in the pre-industrial era and about 368 ppm in the year 2000 (see Figure 3-1f). These projections include the land and ocean climate feedbacks. The different socio-economic assumptions (demographic, social, economic, and technological) result in different levels of future greenhouse gases and aerosols. Further uncertainties, especially regarding the persistence of the present removal processes (carbon sinks) and the magnitude of the climate feedback on the terrestrial biosphere, cause a variation of about -10 to +30% in the year 2100 concentration, around each scenario. The total range is 490 to 1,260 ppm (75 to 350% above the year 1750 (pre-industrial) concentration).

WGI TAR Section 3.7.3.3

3.4

Model calculations of the concentrations of the primary non-CO₂ greenhouse gases by year 2100 vary considerably across the six illustrative SRES scenarios. For most cases, A1B, A1T, and B1 have the smallest increases, and A1FI and A2 the largest (see Figures 3-1g and 3-1h).

WGI TAR Section 4.4.5 & WGI TAR Box 9.1

3.5

The SRES scenarios include the possibility of either increases or decreases in anthropogenic aerosols, depending on the extent of fossil-fuel use and policies to abate polluting emissions. As seen in Figure 3-1i, sulfate aerosol concentrations are projected to fall below present levels by 2100 in all six illustrative SRES scenarios. This would result in warming relative to present day. In addition, natural aerosols (e.g., sea salt, dust, and emissions leading to sulfate and carbon aerosols) are projected to increase as a result of changes in climate.

WGI TAR Section 5.5 & SRES Section 3.6.4

3.6

The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100 (see Figure 3-1k). This is about two to ten times larger than the central value of observed warming over the 20th century and the projected rate of warming is very likely to be without precedent during at least the last 10,000 years, based on paleoclimate data (see Figure 9-1). For the periods 1990 to 2025 and 1990 to 2050, the projected increases are 0.4 to 1.1°C and 0.8 to 2.6°C, respectively. These results are for the full range of 35 SRES scenarios, based on a number of climate models.⁴ Temperature increases are projected to be greater than those in the SAR, which were about 1.0 to 3.5°C based on six IS92 scenarios. The higher projected temperatures and the wider range are due primarily to lower projected SO₂ emissions in the SRES scenarios relative to the IS92 scenarios, because of structural changes in the energy system as well as concerns about local and regional air pollution.

WGI TAR Section 9.3.3

3.7

By 2100, the range in the surface temperature response across different climate models for the same emissions scenario is comparable to the range across different SRES emissions scenarios for a single climate model. Figure 3-1 shows that the SRES scenarios with the highest emissions result in the largest projected temperature increases. Further uncertainties arise due to uncertainties in the radiative forcing. The largest forcing uncertainty is that due to the sulfate aerosols.

WGI TAR Section 9.3.3

Figure 3-1 (two parts): The different socio-economic assumptions underlying the SRES scenarios result in different levels of future emissions of greenhouse gases and aerosols. These emissions in turn change the concentration of these gases and aerosols in the atmosphere, leading to changed radiative forcing of the climate system. Radiative forcing due to the SRES scenarios results in projected increases in temperature and sea level, which in turn will cause impacts. The SRES scenarios do not include additional climate initiatives and no probabilities of occurrence are assigned. Because the SRES scenarios had only been available for a very short time prior to production of the TAR, the impacts assessments here use climate model results which tend to be based on equilibrium climate change scenarios (e.g., 2xCO₂ ), a relatively small number of experiments using a 1% per year CO₂ increase transient scenario, or the scenarios used in the Second Assessment Report (i.e., the IS92 series). Impacts in turn can affect socio-economic development paths through, for example, adaptation and mitigation. The highlighted boxes along the top of the figure illustrate how the various aspects relate to the integrated assessment framework for considering climate change (see Figure 1-1).

WGI TAR Figures 3.12, 4.14, 5.13, 9.13, 9.14, & 11.12, WGII TAR Figure 19-7, & SRES Figures SPM-2, SPM-5, SPM-6, & TS-10

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