Decreasing degree of geologic assurance Cumulative Production IDENTIFIED UNDISCOVERED Demonstrated Inferred Probability Range Measured Indicated Hypothetical Speculative ECONOMIC Reserves MARGINALLY Marginal Reserves Marginal SUB- subeconomic resources subeconomic resources Decreasing degree of feasibility Decreasing degree of geologic assurance Figure 3.1. Classification of mineral resources (U.S. Bureau of Mines and U.S. Geological Survey, Circular 831, 1980). Based on McKelvey’s classification system (McKelvey, V.E., “Mineral Resource Estimates and Public Policy”. American Scientist, 60, 1 (1972): 32-40.

Figure 3.2. Biomass production as a function of time. Time Figure 3.2. Biomass production as a function of time.

Figure 3.3. Recruitment as a function of biomass. Recruitment (R) Biomass (S) Figure 3.3. Recruitment as a function of biomass.

Source: United States Geological Survey (USGS) Figure 3.4. Select raw materials consumption in the U.S.

Figure 3.5. U.S. energy consumption by fuel source. Source: U.S. Energy Information Administration

Source. U.S. National Park Public Use Statistics Office Figure 3.6. Recreation visits to U.S. national parks.

Table 3.1. Mineral reserves to extraction ratios MINERAL YEAR 1995 2000 2007 Copper 2 3 Gypsum 42 36 39 Phosphate Rock 28 26 40 Platinum-Group Metals 78 127 54 Crude Oil 10 12 13 Source: United States Geological Survey (USGS)

Source: U.S. Energy Information Administration Figure 3.7. U.S. crude oil reserves.

Source: U.S. Energy Information Administration Figure 3.8. Nominal and real retail gasoline price trends in the U.S.