Scenarios Based on Outputs from Climate Models
Main Implications of SRES Scenarios
- The four scenario families each have a narrative storyline and consist of 40 scenarios developed by six modeling groups.
- The 40 scenarios cover the full range of GHGs and SO2 emissions consistent with the underlying range of driving forces from scenario literature.
- The 40 SRES scenarios fall into different groups – the three scenario families A2, B1, and B2, plus four groups within the A1 scenario family, two of which (A1C and A1G) have been combined into one fossil-intensive group A1FI in the Summary for Policymakers. The four A1 groups are distinguished by their technological emphasis – on coal (A1C), oil and gas (A1G), non-fossil energy sources (A1T), or a balance across all sources (A1).
- The scenarios are grouped into four categories of cumulative CO2 emissions, which indicate that scenarios with different driving forces can lead to similar cumulative emissions and those with similar driving forces can branch into different categories of cumulative emissions.
- Four of the 40 scenarios are designated as marker scenarios that are characteristic of the four scenarios families. Together with the two additional illustrative scenarios selected from the scenario groups in the A1 family, they capture most of the emissions and driving forces spanned by the full set of the scenarios.
- There is no single central or “best guess” scenario, and probabilities or likelihood are not assigned to individual scenarios. Instead, the writing team recommends that the smallest set of scenarios used should include the four designated marker scenarios and the two additional illustrative scenarios selected from the scenario groups in the A1 family.
- Distinction between scenarios that envisage stringent environmental policies and those that include direct climate policies was very difficult because of many definitional and other ambiguities.
- All scenarios describe futures that are generally more affluent than today. Many scenarios envisage a more rapid convergence in per capita income ratios in the world compared to the IS92 scenarios; at the same time, they jointly cover a wide range of GHG and SO2 emissions.
- Emissions profiles are more dynamic than the IS92 scenarios, which reflects changes in future emissions trends for some scenarios and GHG species.
- The levels of GHG emissions are generally lower than the IS92 levels, especially toward the end of the 21st century, while SO2 emissions, which have a cooling effect on the atmosphere, are significantly lower than in IS92.
- Alternative combinations of main scenario driving forces can lead to similar levels of GHG emissions by the end of the 21st century. Scenarios with different underlying assumptions can result in very similar climate changes.
- Technology is at least as important a driving force of GHG emissions as population and economic development across the set of 40 SRES scenarios.
Figure 4: Model results: global mean temperature projections for the six illustrative SRES scenarios using a simple climate model tuned to a number of complex models with a range of climate sensitivities. The darker shading represents the envelope of the full set of thirty-five SRES scenarios using the average of the model results (mean climate sensitivity is 2.8°C, or 5.0°F). The lighter shading is the envelope based on all seven model projections (with climate sensitivity in the range 1.7 to 4.2°C, or 3.1 to 8.1°F). The bars show, for each of the six illustrative SRES scenarios, the range of simple model results in 2100 for the seven AOGCM model tunings.
Figure 5: Analysis of intra-model consistency in regional relative warming (warming relative to each model's global average warming). Regions are classified as showing either agreement on warming in excess of 40% above the global average ('Much greater than average warming'), agreement on warming greater than the global average ('Greater than average warming'), agreement on warming less than the global average ('Less than average warming'), or disagreement among models on the magnitude of regional relative warming ('Inconsistent magnitude of warming') There is also a category for agreement on cooling (which never occurs). A consistent result from at least seven of the nine models is deemed necessary for agreement. The global annual average warming of the models used span 1.2 to 4.5°C, or 2.2 to 8.1°F for A2 and 0.9 to 3.4°C, or 1.6 to 6.1°F for B2, and therefore a regional 40% amplification represents warming ranges of 1.7 to 6.3°C, or 3.1 to 11.3°F for A2 and 1.3 to 4.7°C, or 2.3 to 8.5 °F for B2.
Figure 6: Analysis of intra-model consistency in regional precipitation change. Regions are classified as showing either agreement on increase with an average change of greater than 20% ('Large increase'), agreement on increase with an average change between 5 and 20% ('Small increase'), agreement on a change between –5 and +5% or agreement with an average change between –5 and 5% ('No change'), agreement on decrease with an average change between –5 and -20% ('Small decrease'), agreement on decrease with an average change of less than -20% ('Large decrease'), or disagreement ('Inconsistent sign'). A consistent result from at least seven of the nine models is deemed necessary for agreement.
Table 2
Figure 7: Global average sea level rise 1990 to 2100 for the SRES scenarios. Thermal expansion and land ice changes were calculated using a simple climate model calibrated separately for each of seven AOGCMs, and contributions from changes in permafrost, the effect of sediment deposition and the long-term adjustment of the ice sheets to past climate change were added. Each of the six lines appearing in the key is the average of AOGCMs for one of the six illustrative scenarios. The region in dark shading shows the range of the average of AOGCMs for all thirty five SRES scenarios. The region in light shading shows the range of all AOGCMs for all thirty five scenarios. The region delimited by the outermost lines shows the range of all AOGCMs and scenarios including uncertainty in land-ice changes, permafrost changes and sediment deposition. Note that this range does not allow for uncertainty relating to ice-dynamic changes in the West Antarctic ice sheet.