The most significant question with regard to the fossil fuels is that of how much larger their rates of consumption may become, and about how

long these sources of energy can be depended upon to supply a major frac­tion of the world’s industrial energy needs. Since the present supplies of coal and oil represent the remains of organic debris of the geologic past, and about 600 million years were required for this accumulation, it should be evident that any additional accumulation likely to occur during the next thousand years will be negligible. Hence, our present consumption amounts to a progressive depletion of an initial stockpile of fixed and finite magnitude. When this is gone, there will be no further accumula­tions of fossil fuels within a time span of interest to man.

The manner in which the consumption of fossil fuels has increased with time can best be shown by means of graphs of annual rates of produc­tion. Statistical data for world production of coal and oil are available since 1860, for coal production in the United States since its beginning around 1820, and for oil production in the world since 1860. For the world, the production of coal since 1860 is shown in Figure 1, and the production of crude oil since 1880 in Figure 2. In Figure 3, the world pro­duction of both coal and lignite and world production of crude oil are

shown where the production rates are expressed in a common unit of ener­gy, the kilowatt-year.

Corresponding data for the United States are given in Figures 4-7. The production of coal in the United States is shown in Figure 4, produc­tion of crude oil in Figure 5, and that of natural gas in Figure 6. Finally, the total annual production of energy in the United States from the com­bined sources of coal, oil, gas, water power, and nuclear energy, expressed in British thermal units, is shown in Figure 7.

Among all of these curves there is a strong family resemblance. In each case the production rate either started from zero during the nine­teenth century, or, as in the case of the world production of coal, had only an insignificant magnitude at the beginning of the century. In each case, the rate of production for roughly a century exhibited an exponential, or compound-interest, growth, before eventually showing signs of a slow­down.

The world production of coal, for example, increased during most of the nineteenth century and up to the beginning of World War I at an an­nual rate of 4.4 per cent per year, or at a rate that would double the pro­duction rate every 16 years. Then after a slowdown until the end of World War II, exponential growth resumed at a rate of 3.6 per cent per year. Coal production in the United States until World War I increased at about 6.6 per cent per year, with a doubling period of about 10.5 years. Subse­quently, owing principally to the replacement of coal by oil and gas, the production of coal in the United States has fluctuated about a mean rate of about 475 million short tons per year.

For the case of petroleum, world production of crude oil up to the present has grown at an average rate of about 6.9 per cent per year, with a doubling period of 10 years. In the United States, from 1875 to 1929, crude-oil production increased at an average rate of 8.3 per cent per year with a doubling period of 8.4 years. Since 1929 the rate of increase of the production rate has progressively declined to a present figure of near zero.

The production of natural gas in the United States since 1900 has in­creased at an average rate of about 6.6 per cent per year. Finally, the pro­duction of total energy in the United States, as shown in Figure 7, has in­creased from 1850 to 1965 at an average rate of 6.9 per cent per year, with a doubling period of 10 years. In about 1950 this rate dropped to an average of 1.8 per cent per year with a doubling period of 39 years, which has prevailed to the present.

Another result obtainable from the study of these curves is an appre­ciation of the extreme brevity of the time during which most of this pro­duction has taken place. Coal, for example, has been mined continuously for about 800 years, and by the end of 1969 the cumulative production will amount to approximately 135 million metric tons. To produce the first half of this production required the 800 years up to 1938; the second half has required only the subsequent 31 years. The second half of the world’s cumulative production of crude oil has required only the 12-year period since 1957. Similarly, for the United States, the second half of the cumulative production of coal has occurred during the 38-year period since 1931, and the second half of the crude oil production during the 16- year period since 1953. In brief, most of the world’s production and con­sumption of energy during its entire history has occurred during the last 20 years.