3.7.2.3. Rarely Considered Interactions

Most scenarios emphasize systemic interactions within nonhuman components of the climate system. These interactions are relatively well studied. The response of society to changes in the climate system is much less well studied. Land-use and land-cover change is an exception, but its treatment in climate scenarios still is far from ideal (see Section 3.3). The difficulty of including such interactions in scenario development is that many are not precisely specified and act indirectly. For example, warmer climates would change heating and cooling requirements of buildings. Such effects frequently are listed as impacts but are not factored in as adjustments to energy use and thus emission levels. Another example is population migration, which can be treated as an impact of environmental or socioeconomic change while also serving as a scenario of demographic change affecting future regional vulnerability (Döös, 1997).

A model that accounts for such societal interactions with the climate system is TARGETS (Rotmans and de Vries, 1997), which evolved from the WORLD model of Meadows et al. (1992). TARGETS is a highly aggregated model (only two regions and few resource classes) with simple relationships between population, economic development, and resource use; between environmental conditions and population/health/wealth; and between emissions, concentrations, climate, and impacts. The model generates globally averaged emissions and climate change scenarios. The strength of the model is that different interactions—including controversial ones, such as the effects of climate change on food availability and health and their interactions with population—can be explored easily, but its use for developing scenarios for impact assessment is limited.

Table 3-9: Some aspects of the SRES emissions scenarios and their implications for CO2 concentration, global temperature and sea-level rise by 2050 and 2100 compared to the IS92a emissions scenario (Leggett et al., 1992). Data in columns 2-4 are taken from Nakicenovic et al. (2000). Calculations in columns 6-7 are relative to 1990. T is change in mean annual temperature averaged across simple climate model runs emulating results of seven AOGCMs with average climate sensitivity of 2.8°C (Chapter 9, TAR WGI). CO2 concentrations were estimated by using the same model runs (data from S.C.B. Raper, Chapter 9, TAR WGI). Sea-level rise estimates are based on temperature changes (Chapter 11, TAR WGI). SRES-min and SRES-max are minimum and maximum estimates across all 40 SRES scenarios (35 fully quantified scenarios for CO2, T, and sea level). High and low estimates of CO2 concentration and temperature change account for uncertainties in climate sensitivity (across the range 1.7-4.2°C). Sea-level rise range also accounts for uncertainties in model parameters for land ice, permafrost, and sediment deposition. Note that scenario values are mutually consistent along all rows except for SRES-min and SRES-max.
Emissions Scenario	Global Population (billions)	Global GDPa (1012 US$ a-1)	Per Capita Income Ratiob	CO2 Concentrationc (ppm)	Global T (°C)	Global Sea- Level Rise (cm)
1990	5.3	21	16.1	354	0	0
2000	6.1-6.2	25-28d	12.3-14.2d	367e	0.2	2
2050
- SRESA1FI	8.7	164	2.8	573	1.9	17
- SRESA1B	8.7	181	2.8	536	1.6	17
- SRESA1T	8.7	187	2.8	502	1.7	18
- SRESA2	11.3	82	6.6	536	1.4	16
- SRESB1	8.7	136	3.6	491	1.2	15
- SRESB2	9.3	110	4.0	478	1.4	16
- IS92a	10.0	92	9.6	512	1.0	—
- SRES-min	8.4	59	2.4	463	0.8	5
- SRES-max	11.3	187	8.2	623	2.6	32
2100
- SRESA1FI	7.1	525	1.5	976	4.5	49
- SRESA1B	7.1	529	1.6	711	2.9	39
- SRESA1T	7.1	550	1.6	569	2.5	37
- SRESA2	15.1	243	4.2	857	3.8	42
- SRESB1	7.0	328	1.8	538	2.0	31
- SRESB2	10.4	235	3.0	615	2.7	36
- IS92a	11.3	243	4.8	721	2.4	—
- SRES-min	7.0	197	1.4	478	1.4	9
- SRES-max	15.1	550	6.3	1099	5.8	88
a Gross domestic product (1990 US$ trillion yr-1). b Ratio of developed countries and economies in transition (Annex I) to developing countries (Non-Annex I). c Modeled values are not the same as those presented by TAR WGI, Appendix II, which are based on simulations using two different carbon cycle models for the six illustrative SRES emissions scenarios. Both models produce very similar results to the model applied here for a mid-range climate sensitivity; discrepancies in the high and low estimates are attributable to differences in the modeled climate-carbon cycle feedback. d Modeled range across the six illustrative SRES scenarios. e Observed 1999 value (Chapter 3, WG I TAR).

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