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Graph of human population from 10,000 BCE–2000 CE showing the immense population growth, since the 19th century.
Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth. Steve Jones, head of the biology department at University College London, has said, "Humans are 10,000 times more common than we should be, according to the rules of the animal kingdom, and we have agriculture to thank for that. Without farming, the world population would probably have reached half a million by now." The world’s population has significantly increased in the last 50 years, mainly due to medical advancements and substantial increases in agricultural productivity.
The recent rapid increase in human population over the past two centuries has raised concerns that humans are beginning to overpopulate the Earth, and that the planet may not be able to sustain present or larger numbers of inhabitants. The population has been growing continuously since the end of the Black Death, around the year 1400; at the beginning of the 19th century, it had reached roughly 1,000,000,000 (1 billion). Increases in medical technology have led to rapid population growth on a worldwide level. Current projections show a steady decline in the population growth rate, with the population expected to reach between 8 and 10.5 billion between the year 2040 and 2050. Map of countries by population density, per square kilometer. (See List of countries by population density.) Areas of high population densities, calculated in 1994.
Map of countries and territories by fertility rate (See List of countries and territories by fertility rate.) Human population growth rate in percent, with the variables of births, deaths, immigration, and emigration - 2006
The scientific consensus is that the current population expansion and accompanying increase in usage of resources is linked to threats to the ecosystem. The InterAcademy Panel Statement on Population Growth, which was ratified by 58 member national academies in 1994, called the growth in human numbers "unprecedented", and stated that many environmental problems, such as rising levels of atmospheric carbon dioxide, global warming, and pollution, were aggravated by the population expansion. At the time, the world population stood at 5.5 billion, and low-bound scenarios predicted a peak of 7.8 billion by 2050, a number that current estimates show will be reached around 2022. Definition
Overpopulation does not depend only on the size or density of the population, but on the ratio of population to available sustainable resources. It also depends on the way resources are used and distributed throughout the population. Overpopulation can result from an increase in births, a decline in mortality rates due to medical advances, from an increase in immigration, or from an unsustainable biome and depletion of resources. It is possible for very sparsely populated areas to be overpopulated, as the area in question may have a meager or non-existent capability to sustain human life (e.g. a desert).
The resources to be considered when evaluating whether an ecological niche is overpopulated include clean water, clean air, food, shelter, warmth, and other resources necessary to sustain life. If the quality of human life is addressed, there may be additional resources considered, such as medical care, education, proper sewage treatment and waste disposal. Overpopulation places competitive stress on the basic life sustaining resources, leading to a diminished quality of life.  History of concern
Concern about overpopulation is relatively recent in origin. Throughout history, populations have grown slowly despite high birth rates, due to the population-reducing effects of war,...