EXAMPLE 3 U.S. Coal Reserves

Coal accounts for 30% of U.S. energy use, including 40% of U.S. electricity. Proved reserves of U.S. coal would last about 250 years at the current rate of use, so the static reserve is 250 years. How long would the supply last if the rate of use increases 0.77% per year indefinitely, the current rate of growth of the U.S. population?

The corresponding exponential reserve is

That’s quite a difference!