Figure 23.1 shows a prediction of the world population together with its past history. This figure shows that the world population had already reached 7 billion in 2011 and will be 9.2 billion in 2050. Another prediction indicates that the population of the world would be 11 billion by the end of this century.

It is not easy to predict the future demand for primary energy. Let me estimate it taking an extremely naive way. Figure 23.2 shows how much primary energy per capital is consumed annually in each country in terms of tons of oil equivalent. In 2009, the average of consumption of primary energy was 1.8 t/year and the world population was 7 billion. It is a reasonable assumption that everybody in the world hopes to enjoy the American life using 7 t/year, or at least the average of OECD countries by using 4 t/year. Let us assume that in the near future the average will become 4 t/year and the world population will be 10 billion in 2100. Then, simple arithmetic tells us that the total demand for primary energy will be 3.2 times [=(10 x 4)/(7 x 1.8)] more than the present consumption.

More realistically, the International Energy Agency (IEA) predicted the future demand for primary energy. The demand in countries other than OECD in 2035 will be 1.8 times more than in 2010. The demand for primary energy in the world in 2035 will be 1.35 times more than in 2010. We should be careful because this increase of 35 % will occur only 25 years from now. If this increase continues linearly for the next 100 years, we find a 140 % increase, that is, altogether 2.4 times more than the present consumption. According to IEA, the demand for electricity in the world in 2035 will be 1.73 times more than in 2011, which is an increase 2 times as fast as that for primary energy.