What is population growth?
The human population of the Earth has been growing since prehistoric times, but the past 200 years have seen human numbers grow exponentially.
In 1AD, the world's population was believed to have been around 150 million. By the year 1000, it was still below 300 million, and it was not until around 1830 that the global population topped 1 billion.
The world's population in 2004 stood at an estimated 6.35 billion; more than twice what it was just 40 years before. While this is expected by many to rise to around 8 billion by 2020, United Nations demographic studies point to a declining rate of growth, with the peak occurring between around 1985 and 1990, when the population grew by around 87 million per year.
There are widespread concerns about the implications of population growth for the world, in terms of the availability of resources, damage to the environment and the sustainability of social and economic development.
Today, many parts of the developed world have a low, zero or even negative population growth rate. In the postwar era, the population of the developing world, by contrast, has boomed, and between 2000 and 2030, many demographers anticipate virtually all of world population growth to occur in poorer countries, exacerbating these concerns.
Having grown steadily but slowly in previous centuries, the human population's growth rate was transformed for ever by Europe's agricultural revolution of the 1700s and the industrial revolution of the 1800s. As well as giving birth to the "modern" world, the dawning of the "scientific" worldview in this era permitted the production of more food than had ever been possible, the manufacturing of the necessities of life for less than ever before, and significant breakthroughs in medicine, resulting in a plummeting death rate.
Although there had been population spurts in previous centuries, the industrial revolution was the first time this was followed by economic growth, not mass starvation or disease as occurred during the Black Death in the 14th century.
Although Asia had always been, and remains today, home to more than half of the world's population, by 1800, Europeans comprised over a fifth of the world's population, and by 1900, that had risen to almost one quarter.
In the postwar era, however, booming fertility rates - even in the face of high mortality - have seen the developing world become the driving force behind population growth. By 2000, Europe contributed only around 12 per cent of the world's population.
Demographers point to a phenomenon called the "demographic transition", whereby fertility tends to decline as socio-economic development progresses. Particularly important elements of development in this regard are reproductive health and child health and educational improvements. This tends to bring the average number of children per women towards the "replacement level", which is calculated at around 2.1. Some countries today are not reaching this level; a number of former Soviet republics, including Russia, have seen a declining population as a result of low fertility and rising mortality.
Much of the developed world has gone through this demographic transition, and those parts of the developing world that have undergone the most development have also experienced substantial drops in fertility (e.g. South Korea, Singapore, Bangladesh, Colombia etc). Since 1969, the work of the UN Population Fund (UNFPA) has promoted reproductive health in the developing world, having a significant effect on both fertility and mortality.
Nonetheless, the average fertility rate in the developing world remains higher than 3, and as the absolute number of people of reproductive age increases, a slowing in the population growth rate need not actually signal a falling in absolute numbers by which the population grows.
The dangers of local overpopulation have long been understood. In a world where geographical mobility was limited and the majority lived at subsistence level, the result of food production falling below the demands put upon it by the population - whether by the failure of the crops or by an excessive birth rate - was starvation. With a soaring population and migration off the land caused by industrialisation, fears that Britain was approaching this were expressed famously in 1798 by Thomas Malthus, whose "Essay on the Principle of Population as it Affects the Future Improvement of Society" warned that food production, which relied on finite factors of production could not keep up with exponential human reproduction.
Nonetheless, since that time the rate of growth in food productivity has continued to outstrip population growth in absolute terms. Optimistic theorists have argued that technology growth will permit this trend to continue at least until the population reaches 10 billion - although this claim is heavily contested.
It is nonetheless the case that famine and poverty continue to exist in a world whose resources, if divided equally amongst all, could support the current population level. Large-scale global redistribution of wealth and resources, however, is a proposition that would pose an enormous challenge to the prevailing world order, and is naturally unattractive to the "haves".
Population growth is nonetheless widely regarded as a contributing factor to a number of other problems besides the threat of overpopulation. 70 per cent of deforestation worldwide is directly caused by population growth. As human numbers increase, natural habitats are destroyed. Population growth can therefore cause a deficiency in food, water and forested areas that are important for the world's survival. As population grows, consumption needs increase which causes greater poverty. More people use more energy, release more greenhouse gases and increase global warming.
Furthermore, the poor reproductive health that has accompanied population growth in many parts of the developing world is producing an HIV/AIDS epidemic of massive proportions. Religious and ideological barriers are frequently accused of standing in the way of improving reproductive health and stemming population growth, particularly the opposition of the Roman Catholic Church to birth control and family planning.
Most countries treat reproduction as something that cannot be regulated, but a few do not. China has a well-known "one-child rule", introduced in 1979, explicitly as a means of controlling population (although it only applies to ethnic Han Chinese living in urban areas). Its effectiveness is widely questioned, and its human rights implications are frequently condemned. Following a devastating earthquake in the country in May 2008, in which thousands of children were killed, Chinese officials announced that parents whose child had been killed, severely injured or disabled, would be allowed to have another child.
The world population is expected to hit 10.1 billion by 2100, reaching 9.3 billion by the middle of this century. Essentially all of the growth will take place in less developed countries and will be predominately among the poorest populations in urban areas.
Between 2011 and 2100, the population of high-fertility countries, which includes most of sub-Saharan Africa, is projected to triple, passing from 1.2 billion to 4.2 billion.
During the same period, the population of intermediate-fertility countries, such as the United States, Mexico and India, will increase by just 26 per cent, while that of low-fertility countries, which includes most of Europe, China and Australia, will decline by about 20 per cent.
Current world population: (estimated) 6,991,318,084.
Source: UNFPA - 2012
"Family planning is a human right. Yet today some 222 million women in developing countries are unable to exercise that right because they lack access to contraceptives, information and quality services or because social and economic forces prevent them from taking advantage of services even where they are available."
UNFPA - 2012