How does population change

However, this forecast, like all population forecasts, is subject to change. Population growth is difficult to predict because unforeseen events can alter birth rates, death rates, migration, or the resource limits on population growth. Countries may also choose to undertake mitigation measures to reduce population growth. For example, in China, the government has put policies in place that regulate the number of children allowed to each couple.

Other societies have already begun to implement social marketing strategies in order to educate the public on overpopulation effects. Certain government policies are making it easier and more socially acceptable to use contraception and abortion methods. Such policies could have a significant effect on global fertility rates. An estimated million women in the poorest countries of the world either did not want their last child, do not want another child or want to space their pregnancies, but they lack access to information, affordable means and services to determine the size and spacing of their families.

In the United States, in , almost half of pregnancies were unintended. At the same time, other countries may roll back access to contraception, as has happened recently in Afghanistan. Or they may implement pro-natalist policies, like those seen in much of Europe where governments are concerned with sub-replacement fertility.

Any of these changes could affect fertility rates and therefore alter forecasts of population growth. At the same time, other factors could affect mortality rates, which would also alter population forecasts. Death rates could fall unexpectedly due to advances in medicine or innovations that stretch resources so population can continue to grow past what seemed like intractable resource limits.

For example, in the mid th century, the Green Revolution in agriculture dramatically increased available food by spreading farming technology like fertilizer and increasing efficiency in agriculture.

In the future, production might be increased by innovations such as genetically modified crops, more efficiently employing agricultural technology, and aquaculture. At the same time, death rates can also increase unexpectedly due to disease, wars, and other mass catastrophes. Population Growth Forecasts : This video uses commonly cited statistics about population growth predictions to advocate for population control.

The problem with activism surrounding population growth is that forecasts cannot predict unexpected changes in fertility and mortality rates. The Green Revolution : The Green Revolution was a period of rapid technological innovation in agricultural, which made food resources more widely available than expected and thus reduced the global mortality rate.

This type of unanticipated change can reduce the accuracy of population forecasts. Malthus believed that if a population is allowed to grow unchecked, people will begin to starve and will go to war over increasingly scarce resources. According to Malthus, this was a mathematical inevitability. Malthus observed that, while resources tended to grow arithmetically, populations exhibit exponential growth.

Thus, if left unrestricted, human populations would continue to grow until they would become too large to be supported by the food grown on available agricultural land. In other words, humans would outpace their local carrying capacity, the capacity of ecosystems or societies to support the local population. Even in his time, this solution was controversial. According to Malthus, the only alternative to moral restraint was certain disaster: if allowed to grow unchecked, population would outstrip available resources, resulting in what came to be known as Malthusian catastrophes: naturally occurring checks on population growth such as famine, disease, or war.

Proponents of this theory, Neo-Malthusians, state that these famines were examples of Malthusian catastrophes. On a global scale, however, food production has grown faster than population due to transformational advances in agricultural technology. Overpopulated Urban Slums : Malthusians would cite epidemics and starvation in overpopulated urban slums, like this one in Cairo, as natural checks on growing populations that have exceeded the carrying capacities of their local environments. Demographic transition theory outlines five stages of change in birth and death rates to predict the growth of populations.

Whether you believe that we are headed for environmental disaster and the end of human existence as we know it, or you think people will always adapt to changing circumstances, we can see clear patterns in population growth. Societies develop along a predictable continuum as they evolve from unindustrialized to postindustrial.

Demographic transition theory Caldwell and Caldwell suggests that future population growth will develop along a predictable four- or five-stage model. In stage one, pre-industrial society, death rates and birth rates are high and roughly in balance.

An example of this stage is the United States in the s. All human populations are believed to have had this balance until the late 18th century, when this balance ended in Western Europe. In fact, growth rates were less than 0. Population growth is typically very slow in this stage, because the society is constrained by the available food supply; therefore, unless the society develops new technologies to increase food production e. In stage two, that of a developing country, the death rates drop rapidly due to improvements in food supply and sanitation, which increase life spans and reduce disease.

Afghanistan is currently in this stage. The improvements specific to food supply typically include selective breeding and crop rotation and farming techniques. Other improvements generally include access to technology, basic healthcare, and education. For example, numerous improvements in public health reduce mortality, especially childhood mortality. Prior to the midth century, these improvements in public health were primarily in the areas of food handling, water supply, sewage, and personal hygiene.

Another variable often cited is the increase in female literacy combined with public health education programs which emerged in the late 19th and early 20th centuries. In Europe, the death rate decline started in the late 18th century in northwestern Europe and spread to the south and east over approximately the next years. Without a corresponding fall in birth rates this produces an imbalance, and the countries in this stage experience a large increase in population.

In stage three, birth rates fall. Population growth begins to level off. The birth rate decline in developed countries started in the late 19th century in northern Europe. While improvements in contraception do play a role in birth rate decline, it should be noted that contraceptives were not generally available nor widely used in the 19th century and as a result likely did not play a significant role in the decline then. It is important to note that birth rate decline is caused also by a transition in values; not just because of the availability of contraceptives.

During stage four there are both low birth rates and low death rates. Birth rates may drop to well below replacement level as has happened in countries like Germany, Italy, and Japan, leading to a shrinking population, a threat to many industries that rely on population growth.

The number of children and the number of middle-age adults are expected to increase only slightly, and the social and economic effects of aging may be felt more acutely in the future. Even in the past, the rate of growth in the population of seniors ages 65 and older exceeded the rate of growth in the populations of younger cohorts.

In the U. A similar trend characterized changes in the global population by age groups. The global population of those 65 years and older more than quadrupled, from The population of seniors in the U.

Notably, the annual rate of growth in the population of seniors in the future is expected to be much as it has been in the past. A temporary surge in the growth rate of this population will occur from to as the baby boomers turn 65 and older. However, the rate of growth in the population of seniors is expected to slow sharply from to That is only slightly higher than the annual rate of growth of 2.

The key distinction between the past and the future is that the growth in the population of young children virtually grinds to a halt through the middle of the century. The future may also feature steep drop-offs in the rates of growth of to year-olds.

From to , this age cohort grew at an annual rate of 1. The annual rates of growth are projected to be only one-third as high from to —0. Because of the lack of change in the size of younger age cohorts, seniors are expected to account for much higher proportions of overall population growth in the future than they did in the past. Similar to the projections for the U. The aging of the U. Globally, the median age rose from 24 in to 29 in In the past, therefore, the U.

In the future, however, the world is projected to age more rapidly than the U. The median age in the U. The increase in the global median age likely will be sharper, from 29 in to 36 in As the world population turns older at a faster clip, the U. In , the U. That share had slipped to 7. In a reversal of past trends, the share of children younger than 15 who live in the U.

It is projected that the U. By then, the U. More specifically, one-in-five U. In , the youngest age cohort accounted for The age structure of the global population is headed in the same direction as the U.

The share of children younger than 15 in the global population is expected to drop from The share of people 65 and older is expected to double, from 7. Overall, the age structures of the U. The old-age dependency ratio in the U. Most developed countries have reached stage four and have low birth and death rates, while developing countries continue to make their way through the stages. There are two important relationships that help explain how the level of development of a country affects its population growth rates:.

Combining these two relationships, we would expect that as a country develops, population growth rates decline. Generally, this is true. Over the last two decades we have seen declining population growth rates in countries at all stages of development. In the average woman on the planet had 5 children. The first panel in this chart shows this fundamental change.

The total fertility rate at which a population replaces itself from one generation to the next is called the replacement fertility rate. If no children died before they grew up to have children themselves the replacement fertility rate would be 2. Because some children die , the global replacement fertility rate is currently 2. Why then is global population growth not coming to an end yet? The number of births per woman in the reproductive age bracket is only one of two drivers that matter here.

The second one is the number of women in the reproductive age bracket. If there were few women in the reproductive age bracket the number of births will be low even when the fertility rate is high. At times when an increasing share of women enter the reproductive age bracket the population can keep growing even if the fertility rate is falling.

The second chart in this panel shows that the population growth over the last decades resulted in increasingly larger cohorts of women in the reproductive age bracket. As a result, the number of births will stay high even as the number of births per woman is falling. This is what the bottom panel in the chart shows. According to the UN projections, the two drivers will cancel each other out so that the number of births will stay close to the current level for many decades.

The number of births is projected to change little over the course of this century. In the middle of the 21st century the number of births is projected to reach a peak at million and then to decline slowly to million births by The coming decades will be very different from the last. How close we are to peak child we looked at in a more detailed post. Population momentum is one important driver for high population growth.

But it of course also matters that all of us today live much longer than our ancestors just a few generations ago. Life expectancy is now twice as long in all world regions. In all of this it is important to keep in mind that these are projections and how the future will actually play out will depend on what we are doing today.

Population momentum is driven by the increasingly large cohorts of women in the reproductive age bracket. And this is when global population growth will come to an end. Hans Rosling explained it better than anyone , with the help of toilet rolls. At the global level, population changes are determined by the balance of only two variables: the number of people born each year, and the number who die. How large of an impact does migration have on population changes across the world?

In the United States we see that since the early s, migration into the USA has exceeded emigration out of the country. This means net migration has been positive, and resulted in a higher population growth rate than would have occurred in the scenario with zero migration. In , for example, the actual population growth rate was 0. With zero migration, this would have been 0. This is also true for most countries across Europe. In fact, population growth would have been negative i.

In , the European population increased by 0. The opposite is of course true for countries where emigration out of the country is higher than immigration. Take Nepal as an example: in the mids its actual population growth rate has been lower than it would have been in the absence of migration.

In , its growth rate was 0. With zero migration it would have been 1. This article previously covered aspects of population age structure; you now find this material in our entry on Age Structure.

We evaluate the track record of the UN projections in the entry on future population growth. But historic and current population estimates between sources are also not identical. How do these sources compare? In the chart we see the comparison between the UN shown in red and US Census Bureau in blue estimates globally and by region.

Global estimates have varied by around 0. The largest variation comes from estimates of Asia, Africa and Latin America — where census data and underlying data sources will be less complete and lower quality.

This means some interpretation and judgement is necessary from expert demographers within each organization. Quoting them to more gives a false sense of precision. Across the sources, we can say that there were 7.

The most discussed estimates of world population from the last century are those from the UN Population Division. These estimates are revised periodically and aim to be consistent and comparable within and across countries and time. In short, estimates of the population in the past i.

The estimates of these components are taken directly from national statistical sources or—where only partial or poor-quality data exists—are estimated by the Population Division staff. Population counts from periodic censuses are used as benchmarks. One of the main implications of using the cohort-component method is that it sometimes leads to marked inconsistencies with official country statistics.

The standard methodology used for producing population estimates relies on the so-called cohort-model. Providing high-quality estimates requires reliable and up-to-date census data. Crucial to population estimates are birth and mortality rates: this census data therefore relies birth registration and death reporting. The two maps show the completeness of birth and death reporting across the world. Many countries, particularly those in the least developed regions of the world, have limited census data.

For countries with no data in one or two decades before each revision, the UN relies on other methodologies. One is to derive estimates by extrapolating trends from countries in the same region with a socio-economic profile considered close to the country in question. As discussed in the previous section, there are a number of studies providing historic population data. The most commonly cited source is McEvedy and Jones This above source is an input used in producing the HYDE project data, as well as other datasets.

Further references to this source are available in Goldewijk, K. Long-term dynamic modelling of global population and built-up area in a spatially explicit way: HYDE 3. The Holocene. Historical population data on a sub-national level — including their administrative divisions and principal towns — is collected by Jan Lahmeyer and published at his website www.

The Minnesota Population Center publishes various high-quality datasets based on census data beginning in At the time of writing this source was online at www.

It focuses on North America and Europe. The International Database published by the U. Census Bureau provides data for the time The Atlas of the Biosphere publishes data on Population Density. The world population increased from 1 billion in to 7. The world population growth rate declined from 2.

Other relevant research: Future population growth — This article focuses on the future of population growth. All our charts on World Population Growth Absolute increase in global population per year Age dependency ratio projections Annual number of births by world region Annual number of deaths by world region Birth rate vs.

Population of the world today. In the map we see the number of people per square kilometer km 2 across the world. Singapore has nearly 8, people per km 2 — more than times as dense as the US, and times that of Australia. Of the larger countries 1 , Bangladesh is the most densely-populated with 1, people per square kilometer; this is almost three times as dense as its neighbour, India.

Greenland is the least dense, with less than 0. In our population cartogram these are the countries that take up much less space than on a standard geographical map. Click to open interactive version. Population Growth.

World population from 10, BC to today. How has the world population growth rate changed? The absolute annual change of the population. Population growth over the long run. How long did it take for the world population to double?

How long did it take for the world population to increase by one billion? What are the most populous countries in the world? Here we see that the top five most populous countries are: 1 China 1. Population centers have stayed remarkably stable over this long period. Future population growth. Where do we go from here? The Demography of the World Population from to 6. What are the causes of population growth?

How many are born each year? The stacked area chart shows the number of births by world region from to In , there were approximately million births — 43 million more than back in The line chart shows the same data, but also includes the UN projection until the end of the century. How many die each year? The first chart shows the annual number of deaths over the same period. As the number of deaths approaches the number of births global population growth will come to an end.

The Demographic Transition. In the long time before rapid population growth the birth rate in a population is high, but since the death rate is also high we observe no or only very small population growth. This describes the reality through most of our history. Societies around the world remained in stage 1 for many millennia as the long-run perspective on extremely slow population growth highlighted. At this stage the population pyramid is broad at the base but since the mortality rate is high across all ages — and the risk of death is particularly high for children — the pyramid gets much narrower towards the top.

Stage 2: mortality falls but birth rates still high. In the second phase the health of the population slowly starts to improve and the death rate starts to fall.