As of July 2014, the United Nations (UN) stated that the world population was 7.244 billion, and it was predicted to reach 7.325 billion by July 2015. The U.S. Census Bureau estimates that there is one birth every eight seconds and one death every 12 seconds, which means that there is a net gain in the world population of one person every 14 seconds.

Global population is increasing at about 1.14 percent a year. In the 1960s, this rate reached a peak of over 2 percent and has been in decline ever since. This growth in the world's population has both positive and negative implications. The presence of more people, especially the young, can contribute to economic development and higher living standards for all. At the same time, each human on the planet creates a demand on resources of food, water, and energy and also creates waste. Moreover, demand for natural resources, and particularly for fossil fuels, is creating climate change and other environmental problems.

The largest countries in the world in terms of population are China and India. These are the only countries with over 1 billion people each. The population of the United States is nearly 319 million, as of 2014. According to the World Population Review, over half of the world's current population lives in 10 countries. These are: China, India, the United States, Indonesia, Brazil, Pakistan, Nigeria, Bangladesh, Russia, and Japan. India is set to surpass China as the world's country with the largest population by around 2030. More than 100 million people live in the world's largest cities, most of which are in Asia, and the largest of which is Shanghai, China. A total of just under 300,000 people live in the world's smallest countries, by population, of which the smallest is Vatican City, home to just 500 residents.

As well as absolute numbers, the structure, or demographics, of the world's population is important. Of note, there has been a shift in the age profile of the population. Reduced child mortality, coupled with high fertility, has led to an increase in the number of young people, particularly in Africa. In developed countries, there has also been an increase in the number of older people, thanks to high standards of living, which promote higher life expectancy. This trend is also being seen in some emerging economies like Vietnam and Bangladesh. Moreover, the world's population is increasingly concentrated in cities. These demographic shifts offer both opportunities and challenges.

It should be borne in mind that these population figures are estimates, rather than exact numbers. When the last milestone was passed and world population exceeded 7 billion, on October 31, 2011, according to the UN, it is not known who the seven billionth person actually was or where he or she was born. There is not even agreement on when the milestone actually happened, with the U.S. Census Bureau and the World Bank believing that the date when the human population reached 7 billion was in March or April 2012. Data on population comes from estimates prepared by most national governments, which may be based upon censuses taken at periodic intervals and also on mortality and fertility data. The UN also collects population data, which is published each year. When looking at how the world population will change in the future, researchers make projections based upon various kinds of analyses.

Estimates of the growth of world population over time begin with the start of the agricultural era, around 10,000 years ago. At this time, world population was about 5 million people. Over the next 8,000 years, it grew to 200 million at the most conservative estimate, and to 600 million according to the most generous estimate. The growth rate during this period was likely less than 0.05 percent per year. It took until around 1800 for the human population to reach 1 billion.

The Industrial Revolution was a great landmark in human population history. The global population experienced a sharp increase during this period, doubling by around 1930, to reach 2 billion. It then took only another 30 years to reach 3 billion, in 1959. Thereafter, time to reach the next billion got ever shorter, with 4 billion reached in 1974, 5 billion in 1987, and 6 billion in 1999. This means that during the 20th century, world population grew from 1.65 billion to 6 billion. Looking back over the entire course of human history, from the emergence of modern man around 50,000 years ago, at a rough estimate, around 106 billion people have been born.

In 1798, Thomas Malthus (1766–1834) made a famous prediction that the population would grow more rapidly than its ability to feed itself. Eventually, food would run out and people would starve. He believed that the global population would grow geometrically following the mathematical series: 1, 2, 4, 8, 16, 32, and so on, while food production could only grow arithmetically, following the series: 1, 2, 3, 4, 5. Food supply would not be able to keep up with the expanding population's needs, according to this theory. Although it is true that more than 800 million people in the world do not have enough to eat, the predicted mass starvation has not yet occurred. This is mainly because the food supply has been able to grow faster than Malthus ever envisaged, thanks to the use of fertilizers and modern plant-breeding techniques.

However, more recent thinkers have also proposed that the rapid expansion of the human population could lead to disastrous results. One is the Stanford University population biologist Paul R. Ehrlich (1932–). Like Malthus, he warned of mass starvation in his 1968 book The Population Bomb. Later, he pointed to other problems associated with increased population. These stem from the threefold increase in human activity since the end of World War II (1939–1945) and include expansion of agriculture, deforestation, damming of rivers, and the vast increase in the use of motor vehicles. Added to these is the increase of methane and carbon dioxide levels in the atmosphere that has come from the escalating use of fossil fuels to support the demands of the human population. All these issues came to the forefront in the 1970s, with the growth of the Green politics movement in Europe.

The size of the human population, the numbers of children people have, and how and where people live all have an impact on important aspects of the planet, from air and water quality to wildlife habitats, weather, and climate. In the early 1970s, Ehrlich, together with John Holdren (1944–), later President Barack Obama's (1961–) science adviser, and Barry Commoner (1917–2012), a founding member of the modern environmental movement, put forward a formula to measure the impact of a growing human population on Earth. Page 544  |  Top of ArticleKnown as IPAT, the formula states that Impact equals Population multiplied by Affluence multiplied by Technology. IPAT expresses the notion that it is not just population numbers that matter but the natural resources that each individual requires.

 
World Population from 1950 to 2010, with Estimated Population Projections from 2010 to 2100 SOURCE: Adapted from “Figure 1. Population of the world, 1950–2100, according to different projections and variants,” in World Population Prospects: The 2012 Revision, Volume 1, p. xv, United Nations Department of Economic and Social Affairs, Population Division, 2013. SOURCE: Adapted from “Figure 1. Population of the world, 1950–2100, according to different projections and variants,” in World Population Prospects: The 2012 Revision, Volume 1, p. xv, United Nations Department of Economic and Social Affairs, Population Division, 2013.

The prime needs of the global population, if it is to survive, are for food, clean water, and shelter. With a substantial proportion of the world's more than 7 billion people already going hungry and not having access to clean water, significant advances in sustainable agriculture will be necessary for there to be enough food and water to support a UN projected population of 9.6 billion by 2050. Equally important is access to health care, if an increasing human population is to have a reasonable life expectancy and quality of life. Demand for food will increase by 50 percent by 2030 and by 70 p ercent by 2050. Food production requires water. There is water enough to go around, worldwide, although there are scarcities in some areas. The limiting factor may not be growing enough food—indeed, it is estimated that around 30 percent of food is wasted—but making it available to everyone who needs it.

To thrive and develop, humans need more than their basic survival needs met. Education and energy are both vital requirements for building a dynamic community, whether in a rural or community setting. Improved education, particularly for girls, is important in helping provide them with reproductive choice. This, in turn, can help reduce family size and stabilize global population at a sustainable level. Around 20 percent of people around the world do not have reliable access to electricity. This means no power for lighting, cooking, or the Internet, which has a profound impact upon nutrition, health care, and education. It also severely limits transportation possibilities.

Unfortunately, the demands of an increasing human population are having a deleterious effect upon the environment. This, in turn, can make it harder to meet the demands of this population. For instance, human activity related to a growing population and its needs has resulted in the emission of greenhouse gases into the atmosphere. The resulting climate change has lowered crop yields and caused extreme weather events, such as flooding, which destroy homes and cost lives.

Demand for energy for transportation and electricity has not only increased the use of fossil fuels, driving climate change, but has also had a negative impact upon air quality. Growth in traffic has led to emissions of nitrogen oxides, particulate matter, and ozone into the air, which has been linked to increased heart and lung disease, and premature mortality. In China, for instance, cities regularly exceed the safe particulate matter levels set by the World Health Organization, owing to the traffic increase that has accompanied the country's economic development. This is putting the health of China's urban population at risk.

As the global population grows, so does the amount of solid waste it produces, particularly in cities. The amount of solid waste produced by the human population has increased 10-fold since the beginning of the 20th century. An analysis by Canadian scientists, published in the journal Nature, estimates that 6.61 tons (6 billion metric tons) of solid waste will be produced by 2025. Landfill sites in emerging cities like Seoul, Shanghai, Mexico City, and Rio de Janeiro are fast filling up. Their presence can cause an environmental threat as leakage may contaminate water and land. Burning waste in incinerators can release pollutants to the air. Worst of all, in some places such as India, waste is merely dumped waste on vast, unmanaged open mountains, attracting local residents who scavenge and put their health at risk. It is likely that solid waste production will begin to slow Page 546  |  Top of Articledown as living standards rise and people become more aware of waste reduction and recycling efforts. But waste production is likely to remain an environmental problem well into the next century.

 
Birds fly over the garbage at the Ghazipur landfill site overlooking housing east of New Delhi, India. The population of New Delhi, which is predicted to reach close to 21 million by 2015, generates 8,000 tons of garbage per day. The trash is not separated between organic and inorganic materials—everything from leftover food to batteries and beverage cans goes into Indian bins—causing massive pollution and raising toxic emissions. With 1.26 billion people, India was the world's second-most-populous nation after China (1.37 billion) in 2014, but was expected to surpass China by 2025. © Chandan Khanna/AFP/Getty Images. © Chandan Khanna/AFP/Getty Images.

The three dimensions of population structure used by demographers to analyze the impact of populations are fertility, mortality, and migration. Information on these dimensions generates patterns and trends that policy makers can use to formulate sustainable plans for human development in the future. Fertility is the number of children a woman bears during her lifetime. Mortality is how long an individual lives, while migration looks at the movements of people across the globe, both within and between countries.

The current global fertility rate is 2.5 children per woman, with marked variation between countries. The replacement level, beyond which a population will shrink, is currently 2.1 children per woman. There has been a global decline in fertility from around five children per woman to 2.5, which can be attributed to increased female education and better access to family planning. Fertility continues to be high in sub-Saharan Africa, at an average of 4.6 children per woman. According to the World Bank, fertility rates are highest in Niger, Mali, and Somalia, at 7.6, 6.9, and 6.7 children per woman respectively. Several other African nations have fertility rates in excess of 6 children per woman. In many countries, however, fertility has decreased, sometimes below replacement level, as is so in the United States. In Brazil, average family size has decreased from 6.3 children to 1.9 children per woman over just two generations.

Falling child mortality, coupled with continuing high fertility, has meant that there is a high population of young people in Africa. However, this growth has not been matched by a corresponding increase in education and employment. These are investments that need to be made if the potential of these young people is to be harnessed for development and improved standards of living. Nongovernmental organizations like Young People We Care are setting up projects to equip young people in Africa with useful skills. More should also be done to ensure that policy makers connect with young people.

Developed countries and so-called advanced developing countries have a different demographic issue in their shift toward an increasingly aging population. The number of people aged over 60 is rising dramatically, with a corresponding increase in the incidence of noncommunicable diseases, such as cancer, diabetes, and dementia. This means that the cost to health and social care budgets will soar, unless more is done to ensure that a long life is also a healthy one. For instance, more monitoring of blood pressure could make a huge contribution, as undiagnosed hypertension is responsible for more preventable deaths and disability through heart disease, kidney disease, and stroke than any other risk factor.

People have always migrated in search of a better life. Migration can benefit the recipient country by adding to the numbers of people with skills who can enhance economic productivity. However, it may also create social tensions if native residents feel that immigrants are depriving them of employment opportunities or failing to integrate into the nation's culture and values. The export of those with vital skills, such as health-care workers and researchers, may also have a detrimental effect upon the emigrants' country of origin.

According to a 2013 report from the Pew Research Center, the number of international migrants rose from 154 million in 1990 to 232 million in 2013. Because of global population growth, the overall proportion of migrants has remained constant at 3 percent of the population. The most notable trend is an increase of migration from middle-income countries, like India and China, to high-income regions, such as Europe and North America. In 1990, 57 percent of migrants went to high-income countries; by 2013, this had increased to 69 percent. Meanwhile, 48 percent of international migrants originated in middle-income countries in 1990; by 2013, this had increased to 58 percent. The United States remains the country of choice for international migrants, with one migrant in five living there. European countries are also attractive destinations for migration.

Providing global access to sexual and reproductive health as well as contraception is key to ensuring that population growth is stable and sustainable. A report from the Copenhagen Consensus, a group of economists specializing in development, suggests that each dollar spent on providing universal sexual and reproductive health by 2030 and making modern contraception available to all women by 2040 gives a return of $120. Fewer children born to a woman means reduced infant and maternal mortality. Overall, a lower population means less pressure on resources locally, nationally, and internationally, and more opportunities for education and economic productivity.

Successful family planning technology, allowing women to limit the size of their families or space out their children more, already exists. The issue now is to make it available to women who want to benefit from it. This means overcoming prejudice and taboo that exists still in some regions and also improving health-care infrastructures to ensure fulfillment of contraceptive needs.

The most recent figures from the UN, presenting world mortality data for 2005–2010, show that half of all deaths now occur in the over-65 age group. In the period 1950–1955, 22 percent of deaths occurred in old age. This increase in the percentage of old-age deaths applies to both developed and less-developed countries. Overall, the increase in the percentage of deaths in old age is due to reduction in child mortality, which refers to deaths between birth and four years of age. The percentage of mid-age deaths, occurring between the ages of 5 and 64, has remained relatively stable since the mid-1960s.

Within this broad statistic, there are some important variations. In less-developed regions, overall, the main reason for the shift to an older-age demographic is because of a decline in childhood mortality. However, in the least-developed countries within this region, there has been an important influence of an increase in mid-age deaths, owing to high prevalence of human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS). In 1950–1955, the percentage of mid-age deaths in these countries was 34 percent; in 2005–2010 it had risen to 40 percent. In more developed countries, the percentage of mid-age deaths declined from 34 percent in 1950–1955 to 25 percent in 2005–2010. The main cause of this decrease is a reduction of deaths from noncommunicable disease, such as heart disease, which can be attributed to improvements in health care. In 2005–2010, the countries with the highest percentage of old-age deaths were Italy (86 percent), Sweden (86 percent), and Greece (85 percent). The three nations with the lowest percentage of old-age deaths were the Democratic Republic of the Congo (13 percent), Chad (12 percent), and Angola (11 percent).

Experts disagree on both the maximum future size of the total human population and when this maximum figure will be reached. A 2013 study from the UN and University of Washington researchers suggests that population growth will not level off without a rapid decrease in birthrates among women in sub-Saharan Africa. World population could reach between 9.6 billion and 12.3 billion by 2100. The study used government data and expert forecasts for mortality rates, fertility rates, and international migration. Previously, however, population experts had said that world population would peak at 9 billion in 2045 and then start to level off. This estimate is based on the assumption that as living standards rise around the world, average family size would start to Page 548  |  Top of Articledecrease and people will live longer. Therefore, growth would slow and eventually stop.

According to the 2013 study, it is likely that the population of Africa will be between 3.5 billion and 5.1 billion people at the end of the 21st century. The population of Asia, at 4.4 billion in 2014, would likely peak at 5 billion around the middle of the 21st century, and then start to decline. The populations of North America, Europe, Latin America, and the Caribbean were likely to remain at below 1 billion each.