LUC-LCC scenarios are all sensitive to underlying assumptions of future changes in, for example, agricultural productivity and demand. This can lead to large differences in scenario conclusions. For example, the FAO scenario (Alexandratos, 1995) demonstrates that land as a resource is not a limiting factor, whereas the IMAGE scenarios (Alcamo et al., 1996) show that in Asia and Africa, land rapidly becomes limited over the same time period. In the IMAGE scenarios, relatively rapid transitions toward more affluent diets lead to rapid expansion of (extensive) grazing systems. In contrast, the FAO study does not specify the additional requirement for pastureland. The main difference in assumptions is that animal productivity becomes increasingly dependent on cereals (FAO) compared to pastures (IMAGE). This illustrates how varying important assumptions may lead to discrepancies and inconsistencies between scenario conclusions. In interpreting LUC-LCC scenarios, their scope, underlying assumptions, and limitations should be carefully and critically evaluated before resulting land-cover patterns are declared suitable for use in other studies. A better perspective on how to interpret LUC-LCC both as a driving force and as a means for adaptation to climate change is strongly required. One of the central questions is, "How can we better manage land and land use to reduce vulnerability to climate change and to meet our adaptation and mitigation needs?" Answering this question requires further development of comprehensive LUC-LCC scenarios.

Table 3-2: Some illustrative estimates of reference and future levels of atmospheric constituents that typically are applied in model-based and experimental impact studies. Global values are presented, where available. European values also are shown to illustrate regional variations at the scale of many impact studies.
Scenario	[CO2]a (ppm )	[SO2]b (mg m-3)	S-Depositionc (meq m-2 a-1)	N-Depositionc (meq m-2 a-1)	Ground-Level [O3]d (ppb)
Reference/Control
- Global/hemispheric	367	0.1-10	26	32	40
- Europe	—	5-100+	12-165 (572)	11-135 (288)	28-50 (72)
- Experiments	290-360	0-10	—	—	10-25
Future
- Experiments	490-1350	50-1000	—	—	10-200
2010/2015
- Global/hemispheric	388-395	—	26	36	—
- Europe	—	—	7-63 (225)	5-95 (163)	—
2050/2060
- Global/hemispheric	463-623	—	—	—	~60
- Europe	—	—	8-80 (280)	5-83 (205)	—
2100
- Global/hemispheric	478-1099	—	—	—	>70
- Europe	—	—	6-49 (276)	4-60 (161)	—
a Carbon dioxide concentration. Reference: Observed 1999 value (Chapter 3, WG I TAR). Experiments: Typical ranges used in enrichment experiments on agricultural crops. Some controls used ambient levels; most experiments for future conditions used levels between 600 and 1000 ppm (Strain and Cure, 1985; Wheeler et al., 1996). Future: Values for 2010, 2050, and 2100 are for the range of emissions from 35 SRES scenarios, using a simple model (data from S.C.B. Raper, Chapter 9, TAR WGI); note that these ranges differ from those presented by TAR WGI (see Footnote c of Table 3-9 for an explanation). b Sulphur dioxide concentration. Reference: Global values are background levels (Rovinsky and Yegerov, 1986; Ryaboshapko et al., 1998); European values are annual means at sites in western Europe during the early 1980s (Saunders, 1985). Experiments: Typical purified or ambient (control) and elevated (future) concentrations for assessing long-term SO2 effects on plants (Kropff, 1989). c Deposition of sulphur/nitrogen compounds. Reference: Global values are mean deposition over land areas in 1992, based on the STOCHEM model (Collins et al., 1997; Bouwman and van Vuuren, 1999); European values are based on EMEP model results (EMEP, 1998) and show 5th and 95th percentiles of grid box (150 km) values for 1990 emissions, assuming 10-year average meteorology (maximum in parentheses). Future: Global values for 2015 are from the STOCHEM model, assuming current reduction policies; European values are based on EMEP results for 2010, assuming a "current legislation" scenario under the Convention on Long-Range Transboundary Air Pollution (UN/ECE, 1998) and, for 2050 and 2100, assuming a modification of the preliminary SRES B1marker emissions scenario (B1-SR scenario—Mayerhofer et al., 2000). d Ground-level ozone concentration. Reference: Global/hemispheric values are model estimates for industrialized continents of the northern hemisphere, assuming 2000 emissions (Chapter 4, TAR WGI); European values are based on EMEP model results (Simpson et al., 1997) and show 5th and 95th percentiles of mean monthly grid box (150 km) ground-level values for May-July during 1992-1996 (maximum in parentheses). Experiments: Typical range of purified or seasonal background values (control) and daily or subdaily concentrations (future) for assessing O3 effects on agricultural crops (Unsworth and Hogsett, 1996; Krupa and Jäger, 1996). Future: Model estimates for 2060 and 2100 assuming the A1FI and A2 illustrative SRES emissions scenarios (Chapter 4, TAR WGI).

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