Fertility rates refer to the rates of birth per 1,000 women of reproductive age in a given population. When the fertility rate is at the replacement level, a population will remain stable, neither growing nor shrinking. However, when the fertility rate deviates from the replacement level, the size of the population will change. Fertility rates above the replacement level will cause the population to grow; fertility rates below the replacement level will cause the population to shrink.
The population reached 6 billion people around 1999. By 2015, if fertility rates stay the same, at least another billion people will be added for a total of more than 7 billion people worldwide. However, in some countries the birth rate is falling while the death rate is not, leading to a decline in the population growth rate. The population growth rate has been decreasing in higher income countries, but the number of people added to the population each year is increasing because the population base has become larger, due to increasing rates in lower income countries.
High fertility rates lead to population growth, which, under certain circumstances, can cause a condition known as "overpopulation." Overpopulation is not a function of the number or density of individuals, but rather the number of individuals compared to the resources they need to survive. In other words, it is a ratio: population to resources. Humans are not unique in their capacity for overpopulation; in general terms, overpopulation indicates a scenario in which the population of a living species exceeds the carrying capacity of its ecological niche.
Thus, in order to judge whether a given area is overpopulated, one must take into account the resources available in that area. For example, if a given environment has a population of 10, but there is food and drinking water enough for only nine people, then that environment is overpopulated, while if the population is 100 individuals but there is enough food and water for 200, then it is not overpopulated. When estimating whether an area is overpopulated, resources to be taken into account include clean water, food, shelter, etc. In the case of human beings, additional important resources include arable land and social services, such as jobs, money, education, fuel, electricity, medicine, proper sewage and garbage management, and transportation.
Overpopulation can have deleterious effects. When population outstrips available resources, calamity can result, including famine, shortages of energy sources and other natural resources, rapid and uncontrolled spread of communicable diseases in dense populations, and war over scarce resources, such as land. Dense populations may also settle available land and crowd out other land uses, such as agriculture.
Overpopulation can be addressed with a combination of population control and help to promote self-sufficiency. One of the most important measures is the empowerment of women educationally, economically, politically, and in the family. Where women's status has improved, there has generally been a drastic reduction in the birthrate to more sustainable levels. Other measures include effective family planning programs, local renewable energy systems, sustainable agriculture methods and supplies, reforestation, and measures to protect the local environment.
Presently, every year the world's human population grows by approximately 80 million. However, that population growth is not distributed evenly across all countries. Most population growth comes from developing countries, where birthrates remain high. Meanwhile, about half the world lives in nations with sub-replacement fertility. In some of these countries, the population has actually begun to shrink (e.g., Russia). All the nations of East Asia, with the exceptions of Mongolia, the Philippines, and Laos, have fertility rates below replacement level. Russia and Eastern Europe are dramatically below replacement fertility. Western Europe also is below replacement. In the Middle East Iran, Tunisia, Algeria, Turkey, and Lebanon are below replacement. The United Nations projects that the world's human population will stabilize in 2075 at 9 billion due to declining fertility rates. Some countries still have growing populations due to high rates of immigration, but have native fertility rates below replacement: Canada, Australia, and New Zealand are similar to Western Europe, while the United States is just barely below replacement with about 2.0 births per woman.
A new fear for many governments, particularly those in countries with very low fertility rates, is that a declining population will lead to underpopulation and will reduce the GDP and economic growth of the country, as population growth is often a driving force of economic expansion. To combat extremely low fertility rates, some of these governments have introduced pro-family policies that include incentives, such as payments to parents for having children and extensive parental leave for parents.