It was repeated and reprinted in various forms, by both professional ecologists and laymen, usually as a rhetorical tool to emphasize the dangers of a global population explosion. Consider, for example, the introductory words of political scientist Lester W.
We currently add over 90 million people to the planet every year. There are more people alive today than have ever died. If other creatures could speak, they would call us an epidemic. Westing, an Amherst College ecologist, if it was true. First, he rejects the estimates provided by Deevey, Keyfitz and Desmond. By disregarding precursor species such as Homo erectus and Homo habilis, he starts the clock at two Homo sapiens in , B.
Demographer Carl Haub would soon set the clock much later, at 50, B. With the United Nations global population estimate for mid being about 6. Nonetheless, measuring the changing ratio between the people living right now and those long gone had become, by the turn of the millennium, something of a touchstone for both overpopulation devotees and skeptics.
One reason the question keeps coming up is that somewhere, at some time back in the s, a writer made the statement that 75 percent of the people who had ever been born were alive at that moment.
Mathematical biologist Joel E. Cohen has argued against these calculations. In other words, because of the dramatic drop in natural-growth rates — from 2. And so, future generations will continue to be but a small and ever-shrinking fraction of the total of humanity that ever lived.
Oded Carmeli Oct. Get email notification for articles from Oded Carmeli Follow. Open gallery view. A bill of mortality, from London in Helped officials know when it was time to evacuate the city. Credit: Heritage Image Partnership Ltd. In there were 2. Now in , there are 7. By the end of the century the UN expects a global population of This visualization of the population pyramid makes it possible to understand this enormous global transformation.
Population pyramids visualize the demographic structure of a population. The width represents the size of the population of a given age; women on the right and men to the left.
The bottom layer represents the number of newborns and above it you find the numbers of older cohorts. Represented in this way the population structure of societies with high mortality rates resembled a pyramid — this is how this famous type of visualization got its name. In the darkest blue you see the pyramid that represents the structure of the world population in Two factors are responsible for the pyramid shape in An increasing number of births broadened the base layer of the population pyramid and a continuously high risk of death throughout life is evident by the pyramid narrowing towards the top.
There were many newborns relative to the number of people at older ages. The narrowing of the pyramid just above the base is testimony to the fact that more than 1-in-5 children born in died before they reached the age of five.
Through shades of blue and green the same visualization shows the population structure over the last decades up to You see that in each subsequent decade the population pyramid was fatter than before — in each decade more people of all ages were added to the world population.
If you look at the green pyramid for you see that the narrowing above the base is much less strong than back in ; the child mortality rate fell from 1-in-5 in to fewer than 1-in today. In comparing and we see that the number of children born has increased — 97 million in to million today — and that the mortality of children decreased at the same time.
If you now compare the base of the pyramid in with the projection for you see that the coming decades will not resemble the past: According to the projections there will be fewer children born at the end of this century than today.
The base of the future population structure is narrower. We are at a turning point in global population history. Between and today, it was a widening of the entire pyramid — an increase of the number of children — that was responsible for the increase of the world population.
As global health is improving and mortality is falling, the people alive today are expected to live longer than any generation before us. This is now happening at a global scale.
For every child younger than 15 there were 1. Richer countries have benefited from this transition in the last decades and are now facing the demographic problem of an increasingly larger share of retired people that are not contributing to the labor market.
In the coming decades it will be the poorer countries that can benefit from this demographic dividend. The change from to today and the projections to show a world population that is becoming healthier.
When the top of the pyramid becomes wider and looks less like a pyramid and instead becomes more box-shaped, the population lives through younger ages with very low risk of death and dies at an old age.
The demographic structure of a healthy population at the final stage of the demographic transition is the box shape that we see for the entire world for The world population has grown rapidly , particularly over the past century: in there were fewer than 2 billion people on the planet; today there are 7.
The change in the world population is determined by two metrics: the number of babies born, and the number of people dying. The line chart shows the same data, but also includes the UN projection until the end of the century.
It is possible to switch this chart to any other country or world region in the world. In around 55 million people died. The world population therefore increased by 84 million in that year that is an increase of 1.
Again it is possible to switch this chart to any other country or world region in the world. How do we expect this to change in the coming decades? What does this mean for population growth? Population projections show that the yearly number of births will remain at around million per year over the coming decades. It is then expected to slowly decline in the second-half of the century. As the world population ages , the annual number of deaths is expected to continue to increase in the coming decades until it reaches a similar annual number as global births towards the end of the century.
As the number of births is expected to slowly fall and the number of deaths to rise the global population growth rate will continue to fall. This is when the world population will stop to increase in the future. Population growth is determined by births and deaths and every country has seen very substantial changes in both: In our overview on how health has changed over the long run you find the data on the dramatic decline of child mortality that has been achieved in all parts of the world.
And in our coverage of fertility you find the data and research on how modern socio-economic changes — most importantly structural changes to the economy and a rise of the status and opportunities for women — contributed to a very substantial reduction of the number of children that couples have. But declining mortality rates and declining fertility rates alone would not explain why the population increases.
If they happened at the same time the growth rate of the population would not change in this transition. What is crucial here is the timing at which mortality and fertility changes. It is shown in the schematic figure. It is a beautifully simple model that describes the observed pattern in countries around the world and is one of the great insights of demography.
If fertility fell in lockstep with mortality we would not have seen an increase in the population at all. The demographic transition works through the asynchronous timing of the two fundamental demographic changes: The decline of the death rate is followed by the decline of birth rates. This decline of the death rate followed by a decline of the birth rate is something we observe with great regularity and independent of the culture or religion of the population.
The chart presents the empirical evidence for the demographic transition for five very different countries in Europe, Latin America, Africa, and Asia. In all countries we observed the pattern of the demographic transition, first a decline of mortality that starts the population boom and then a decline of fertility which brings the population boom to an end.
The population boom is a temporary event. In the past the size of the population was stagnant because of high mortality, now country after country is moving into a world in which the population is stagnant because of low fertility. Perhaps the longest available view of the demographic transition comes from data for England and Wales.
In , Anthony Wrigley and Roger Schofield 11 published a major research project analyzing English parish registers—a unique source that allowed them to trace demographic changes for the three centuries prior to state records. As far as we know, there is no comparable data for any other country up until the mid-eighteenth century see the following section for Sweden , where recordkeeping began in The chart shows the birth and death rates in England and Wales over the span of nearly years.
As we can see, a growing gap opens up between the birth and death rate after , creating a population explosion. Statistics Sweden, the successor of the Tabellverket, publishes data on both deaths and births since recordkeeping began more than years ago.
These records suggest that around the year , the Swedish death rate started falling, mainly due to improvements in health and living standards, especially for children. Yet while death rates were falling, birth rates remained at a constant pre-modern level until the s. During this period and up until the first half of the 20th century, there was a sustained gap between the frequency of deaths and the frequency of births. It was because of this gap that the Swedish population increased. The following visualization supports these observations.
The visualization presents the birth and death rate for all countries of the world over the last 5 decades. Countries per continent can also be highlighted by hovering and clicking on them in the legend on the right side of the chart. By visualising this change we see how in country after country the death rate fell and the birth rate followed — countries moved to left-hand-side first and then fell to the bottom left corner. Today, different countries straddle different stages of the model.
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. We surpassed seven billion dead way back between BC and AD1.
Fans of science fiction may be reaching for their copies of Arthur C Clarke's classic, A Space Odyssey, at this point. In that book, he makes the assertion: "Behind every man now alive stand 30 ghosts, for that is the ratio by which the dead outnumber the living.
There were maybe 3. And will we ever reach a point where there are more alive than dead? This would imply a very high rate of population growth. I find that quite unimaginable. This means experts have to make an educated guess. Attempts to answer that question go back to bc , when the Babylonians used a census to work out how much food they needed to feed their people.
Ancient Egyptian, Roman and Chinese societies all carried out regular counts. Earlier this year, both China and the United States reported results from censuses carried out in Many more countries were scheduled to release results this year but have been delayed by the ongoing COVID pandemic.
Both the United States and China reported that they are experiencing record low levels of population growth. Those results made headlines, but they are in line with what demographers expected, says Gerland. With census results or other population counts as a baseline, demographers forecast the various ways in which the number of people will change in coming years.
Beyond births and deaths, researchers also predict how many people will enter or leave a country over time. Like all simulations of future events — from climate change to the course of an epidemic — population predictions get less reliable as they are projected over longer time periods. And birth, death and migration rates are fairly easy to extrapolate over that period from recent trends.
These short- to medium-term predictions do remain vulnerable to shocks, however. In some of the worst-hit countries, the large number of deaths in a relatively short period has already had an effect in lowering life expectancy.
With migration suspended between most countries, the biggest other factor in these post-pandemic calculations of population is the impact on births. Called the fertility rate, the number of children each woman has on average is a totemic figure in demography. When Singapore, for example, first urged each family to have only two children at most in , the fertility rate in the country was estimated at 3.
By , just ahead of its policy U-turn and plea for more babies, fertility had plummeted to 1. It dipped as far as 1. Predictions of population in both the near and long-term future typically come down to estimates of how quickly fertility will change.
And that means demographers have to make some educated guesses about how people will behave as their circumstances alter. In high-income countries, these behavioural changes are usually driven by economic factors.
As opportunities emerge, women prioritize careers, and couples delay having children during a recession. How many people has the coronavirus killed?
In less-wealthy nations, other factors dominate. As more girls are educated, they tend to have fewer children and to have them later. And more people have access to contraception as health systems and distribution networks improve. In that sense, falling fertility rates reflect economic development. Demographers expect that the pandemic will cause a short-term dip in fertility, in richer countries at least, because of the associated economic uncertainty.
By contrast, poorer countries could see a surge in pandemic births because of the disruption to contraception supplies. In a preprint 1 , Sobotka and colleagues report on data for 17 countries across Europe, Asia and the United States showing that the number of births did fall — on average by 5.
Some experts predict that births will rebound. How fertility rates and population numbers will change in the longer term, over several decades or more, is more difficult to predict. And this is where the serious controversy starts. For decades, the UN Population Division had the field largely to itself, and churned out routine updates every couple of years.
Its most recent report, published in , forecasts that global population will continue to rise from its current 7.
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