Human overpopulation occurs if the number of people in a group exceeds the carrying capacity of the region occupied by the group. The term often refers to the relationship between the entire human population and its environment, the Earth, or to smaller geographical areas such as countries. Overpopulation can result from an increase in births, a decline in mortality rates, an increase in immigration, or an unsustainable biome and depletion of resources. It is possible for very sparsely populated areas to be overpopulated if the area has a meager or non-existent capability to sustain life (e.g. a desert).
The human population has been growing continuously since the end of the Black Death, around the year 1400, although the most significant increase has been in the last 50 years, mainly due to medical advancements and increases in agricultural productivity. Although the rate of population growth has been declining since the 1980s, the United Nations has expressed concern on continued excessive population growth in sub-Saharan Africa. As of December 12, 2013 the world's human population is estimated to be 7.13 billion by the United States Census Bureau, and over 7 billion by the United Nations. Most contemporary estimates for the carrying capacity of the Earth under existing conditions are between 4 billion and 16 billion. Depending on which estimate is used, human overpopulation may or may not have already occurred. Nevertheless, the rapid recent increase in human population is causing some concern. The population is expected to reach between 8 and 10.5 billion between the year 2040 and 2050. In May 2011, the United Nations increased the medium variant projections to 9.3 billion for 2050 and 10.1 billion for 2100.
The recent rapid increase in human population over the past three centuries has raised concerns that the planet may not be able to sustain present or larger numbers of inhabitants. The InterAcademy Panel Statement on Population Growth, circa 1994, has stated that many environmental problems, such as rising levels of atmospheric carbon dioxide, global warming, and pollution, are aggravated by the population expansion. Other problems associated with overpopulation include the increased demand for resources such as fresh water and food, starvation and malnutrition, consumption of natural resources (such as fossil fuels) faster than the rate of regeneration, and a deterioration in living conditions. However, some believe that waste and over-consumption, especially by wealthy nations, is putting more strain on the environment than overpopulation.
Attempts to mitigate adverse effects associated with overpopulation have historically included eugenic efforts in the early 19th century. This focused on forcefully sterilizing people thought to have undesirable traits. Almost all developed countries developed laws and regulations around this theme of reducing the reproduction of undesirables. Besides sterilization, the methods included forced abortions, birth control, marriage restrictions according to race, limited genetic testing, racial segregation and segregation of the mentally disabled. The eugenics concept was expanded in Nazi Germany during WWII to forcibly exterminating anyone thought to be undesirable, most notably the Jews. Genocide is the process of reducing the population of a race or ethnic group by murder. Most countries have no direct policy of limiting their birth rates, but the rates have still fallen due to educating people about family planning, increasing access to birth control and contraception. Only China has imposed legal restrictions on having more than one child. Extraterrestrial settlement and other technical solutions have been proposed as ways to mitigate overpopulation in the future.