|Human Population Growth|
Figure 2. Human population growth over time. Image source: http://www.globalchange.umich.edu/globalchange2/current/lectures/human_pop/human_pop.html
Figure 3. Human population and dispersal through history. Dark patches indicate human inhabitance. Image source: http://desip.igc.org/populationmaps.html
Figures 2 and 3 show the trend of human population growth over time. Growth was comparably slow and checked by outbreaks such as the plague in mid 14th and 16th centuries. However, by the start of the 18th century the Industrial Revolution enabled population growth to increase at a high rate. The increase was due to reduced mortality rather than increased birth which was made possible by increased food production due to enclosure acts and development of agricultural machinery , development of vaccines such as cowpox, pasteurisation of food, and improvement of anaesthetics , and improved diet, housing, sanitation and clothing . At the time, Thomas Malthus noticed the startling population boom and published in his Essay on the Principles of Population (1798) that further human expansion could not be sustained and starvation, disease or war would check the population . The Malthusian predictions were never realised though, as he had not foreseen the advances in human ability to produce more resources or the development of birth control.
The world population now is growing at a high rate, doubling since 1960 to over 6 billion people and future projections by the United Nations Population Fund are for 8.9 billion by 2050  (reduced from a prediction of 9.4 billion in 1996). Predicted population growth is portrayed in Figure 4.
Figure 4. Projected world population growth. Image source: http://www.globalchange.umich.edu/globalchange2/current/lectures/human_pop/human_pop.html
The revised estimated figure by the U.N. Population Fund and the appearance of population growth reaching a peak in Figure 4 is a result of a concept called Demographic Transition (Figure 5).
Figure 5. The demographic transition. Image source: http://www.globalchange.umich.edu/globalchange2/current/lectures/human_pop/human_pop.html
The idea of demographic transition is split into four phases.
Phase one: Pre-modern times, births and death rate were high, but balanced. This was the case with all human populations until the 18th century in Europe .
Phase two: Revolutions in agriculture, technology, healthcare, urbanisation etc reduce death rates and maintained high birth rates cause very fast population growth. The growth of the population was due to reduction in deaths rather than increase in births. Reduced mortality was more prolific in children so the average age of the population begins to reduce .
Phase three: Birth rates begin to fall several years after death rates. Speculative reasons for this include realisation of parents that they do not need as large a family due to reduced infant mortality; children became more expensive to keep due to urbanisation and education and child labour acts of the late 1800s ; women become valued beyond childbearing due to male literacy and work taking the woman from the home .
The change in age structure of the population results in a large proportion of the population being young adults and fewer elderly and infants. A population pyramid expresses this with a defined bulge in the centre (Figure 6).
Figure 6. Phase 3 of the demographic transition, age pyramid dominated by working age groups (sown in orange). Image source: http://www.population-growth-migration.info/essays/DemographicDividend.html
Phase four: Birth and death rates stabilise again but at a reduced lever to Phase one. The population age structure will become older and there will be a lower number of working age group to support the older community (Figure 7). In developed countries, by 2050, it is predicted there will be 2 elderly persons for every 1 child .
Figure 7. Phase 4 of the demographic transition. Image source: http://www.population-growth-migration.info/essays/DemographicDividend.html
Developed nations such as the EU and the US have reached phase four of the transition. The population of these countries is estimated to remain unchanged in the foreseen future due to the low and declining rate of population growth. It is the developing countries that are now beginning to enter the transition whose population is predicted to double between 2005 and 2050 .
Slowing down of population growth in the developed nations is not solely responsible the reason for lower projections as it does not take into account the high mortality rate still going on in the developing countries, notably sub-Saharan Africa and areas of the Indian subcontinent where AIDS and HIV are wiping out huge numbers  (a Malthusian check on the population?), and is expected to stall population growth in these countries between 2005 and 2020 .
Despite the effect of demographic transition, the world population is still predicted to increase as the developing nations go through their own revolutions, so what will be the issues faced by the human population in the future? RESOURCES.