The growth rate of the world's human population decreased during the late 20th century. It should be expected to continue to decrease during the 21st century. Three projections are favored that show the world's population increasing from 6 billion at the end of the 20th century to either 10, 11, or 12 billion by 2100. In all three projections, it is estimated that the percentage of the world's population living in the developing countries will reach about 90% in 2100.

Energy consumption per capita in the 21st century is predicted to decrease in the developed countries, probably to increase moderately in the developing countries, and, as a result, to increase in the world as a whole. Given that the developing countries will contain the great majority of the world's people during the 21st century, the correct estimation of their future energy consumption will be critical in forecasting the total energy consumption in the world. Even a modest increase in the energy consumption per capita in the developing countries will result in their total energy consumption surpassing that of the developed countries during the first half of the 21st century; by 2100, the developing countries could be consuming three times as much energy as the developed countries. Will this be possible?