In late 1999 the world's population reached 6 billion and was growing at an estimated 1.3% per year. This growth rate is characteristic of the last 100 years or so. World population reached 1 billion around 1790, 2 billion around 1927, and 4 billion in 1974.
Many people have expressed concerns about this rate of growth as well as the size of the world's population. If this growth rate remains constant, then world population will double every 50 years, giving a population of 12 billion by 2050, 24 billion by 2100, and so on. (Such growth is called geometric increase.) They say that current growth is unsustainable in that the natural resources to feed and house such populations will soon be depleted. In fact, some state that the world's current population is unsustainable. In other words, even if the world's population remained at 6 billion various natural resources would be in such short supply within a century that wars and mass starvation would result.
Additionally, these people claim that this growing world population will cause devastation to the environment. Various predicted disasters include pollution, species extinction, sea level rise, etc.
Can such predictions be true? A good scientific hypothesis makes predictions which can be verified by later observations. In the 1960s it was predicted that by the year 2000 the world's population would be 7.5 billion and that the world would be running out of oil, causing a global economic crisis. These predictions were false: by late 2000 the world's population will be 6.1 billion, and increased oil production has helped improve the global economy.
Where did these "experts" go wrong? First, population growth does not really continue geometrically. Population growth for individual societies goes through a "demographic transistion." In short, societies over time transition from low population growth to high growth rates, then back to low growth rates again. There are three phases:
What this means is that economic development will inevitably shift a society (and the world) to low population growth. In fact, the current 1.3% annual growth of world population is down from a peak of 2.0% in the 1960s and early 1970s. Currently the world's population is projected to peak before 2100 at perhaps 11 billion.
But is even this population unsustainable? The pessimists point to famine and disease in the third world as evidence of "overpopulation." However, as Nicholas Eberstadt points out,
"Overpopulation" is a problem that has been misidentified and misdefined. The term has no scientific definition or clear meaning. The problems typically associated with overpopulation (hungry families, squalid and overcrowded living conditions) are more properly understood as issues of poverty.
The point is: famine and disease are consequences of poverty (limited access to resources), not high population density. Japan has a far higher population density that Ethiopia, but the greater wealth of Japan grants it a far higher standard of living.
Moreover, the world has enough resources to feed and shelter any foreseeable world population. For example, current agricultural technology is capable of feeding a population of 80 billion using less than 10% of the Earth's land for crops.
The solution to improving the living conditions for the present and future world population is increased access to wealth and resources--in other words, a free market economy. Taiwan has five times the population density of mainland China, but has a standard of living ten times higher. This is because Taiwan has a free market economy, whereas mainland China's communist government still controls both the economy and individual human ingenuity.
Opponents of free markets have variously suggested or imposed various methods to "control" population growth. The most extreme case is mainland China, with severe penalties for families who have more than one child and (despite official claims) mandatory abortions against the will of the mothers, sometimes in the ninth month of pregnancy. Those who prefer to limit human freedom as a solution often praise China's policies in general.