General population increase in the world was negligible until the Industrial Revolution. From the time of the Roman Empire to the colonization of America, the world population grew from about a quarter billion to a half billion persons. By the early 19th cent., however, it had grown to one billion, and subsequently rose to more than 2 billion by 1930, 3 billion by 1960, 4 billion by 1975, more than 5 billion by 1990, and more than 6 billion by 2000; the United Nations estimates the world population will peak at about 10 billion around 2100. In world terms, the population is growing at about 1.2% annually (compared with 0.1% in ancient times and a rate of 1.75% as recently as the 1990s) in population. Although a 1.2% growth rate may appear small, it annually adds some 77 million persons to the world's population, with nearly all of this growth taking place in less developed nations (as it has since the 1950s).
During the Industrial Revolution, advancements in sanitation, technology, and the means of food distribution made possible a drop in the death rate so significant that between 1650 and 1900 the population of Europe almost quadrupled (from about 100 million to about 400 million) in spite of considerable emigration. As the rate of population growth increased, so did concern that the earth might not be able to sustain future populations. The phenomenal increase in numbers led Thomas Robert Malthus to predict that the population would eventually outstrip the food supply. Karl Marx emphatically rejected this view and argued that the problem was not one of overpopulation but of unequal distribution of goods, a problem that even a declining population would not solve.
In the late 20th cent., a major population difference arose in the comparative growth rates of the developed (0.6%) and developing (2.1%) nations. Africa's annual growth rate is about 3%, compared to 1.7% for Asia, 0.7% in Latin America, and 0.3% in Europe. If current rates hold steady, many developing countries will double their populations in 25 years or less, compared to 50 years or more for industrialized nations. Great Britain, for example, has a present doubling rate of 140 years, while Costa Rica has one of 19 years.
Great Britain has accomplished what is known as demographic transition, i.e., it has moved from a condition of high birthrate and high death rate (before the Industrial Revolution), to one of high birthrate and low death rate (during industrialization), and finally to one of low birthrate and low death rate (as a postindustrial society). Most of the countries in the Third World are in a condition of high birthrate and declining death rate, contributing to what is known as the population explosion.