One Finite Planet

Population Explosion: What happened to get to 8 Billion people?

First Published:

Table of Contents

Humanity has just experienced a population explosion. Whether you believe we are overpopulated or not, the facts are that we have just had a population explosion.

Over two centuries of population peaking at 50x higher than the long term population growth, and peaking at 100x higher during the baby boom.

Yet the explosion occurred with women having less babies than ever before, even during the baby boom, and is ending all by itself.

So what caused the boom, why a population explosion when birth rates are falling? Will our population grow until we are living life like battery hens?

Explosion? It was Population growth far beyond anything before.

A Crescendo Of Growth at over 100x long term growth Rates.

This image has an empty alt attribute; its file name is updated-world-population-growth-1750-2100.png

The long term growth rate of the human population is below 0.02%, but the rate stepped over 50x to 1.0% for the period between 1800 and 2020. For a brief interval from 1965 to 1972, global population growth was even over 2% per year.

The red line in the graph to the left shows the rate of population growth, and there is a clear ‘peak ‘ from 1965 to 1972, when human population growth was above 2% per annum and sufficient to double the global population in just 34 years, compared to the average averages over all human history of population doubling at over 4,500 years at less than 0.015%.

Previous Population Growth: Surprisingly persistent, but never this fast.

While in nature there are population surges that result in brief plagues, population growth at this rate for such a long time is not seen in other species.

In nature, on a planet where the total amount of life has been in gradual decline since a peak 500 million years ago, population growth normally only occurs for short spans following either changes to environment, or evolution of the species. The one exception is humans, who without genetic evolution, have managed long term evolution of their societies. With any other species, their societies of today are not noticeably changed from their societies thousands of years ago.

Humans had previously managed long term population growth as high as 0.066% leading up to 1650 CE through continual evolution of human societies, but never anything close to 1.0% per annum.

An explosion with reduced birth-rates.

Just how did this happen? Did everyone go baby crazy? It turns out, no, people decreased family sizes, yet population still exploded!

The surprise is that during time of 1.0% population growth, birth rates dropped significantly from birth rates prior to 1650. This statistics, as discussed below, reveal that the growth was driven by reduced child mortality rates, and the ‘baby boom’ provides some of the clearest examples.

We live on a finite planet, where for the last few hundred million years there have been no vacancies, so the only way for a species to grow in numbers within any niche, is to outcompete other species already in that niche, and survive by being the new ‘fittest’. Long term growth means an ever-increasing number of other species must be displaced, and the growth must end when problems result from displacing other species.

Fortunately, humans have managed to adjust our reproduction rate to end the explosion. Either our species, like most advanced species, has evolved to limit population growth before it becomes problematic, or some other factor is bringing an end to the explosion. What is not yet clear is whether what comes next is a return to lower growth, population stability, or a population correction.

Analysis of the Explosion.

Historical Rates Of Population Growth In Detail.

  • From 70,000 BCE to 10,000 BCE the population grew at most by 2,500x (3,000 to 4 million)
    • 60,000 years of growth rate < 0.012%, doubling at most every 5,800 years
  • From 10,000 BCE to 1650 the population grew by 125x (4 million to 500 million)
    • 12,000 years (11,650) of growth rate < 0.05%, doubling every 1,700 years
      • 10,000 BCE to year 5,000 BCE, rate 0.004%, doubling every 17,000
      • 4,000 BCE to 1650, rate 0.076%, doubling every 950 years
  • From 1650 to 1800 the population doubled from 500 million to 1 billion.
    • 150 years of growth rate < 0.46%, doubling every 150 years
  • From 1800 to 1923 the population again doubled from 1 billion to 2 billion.
    • 123 years of < 0.6%, doubling in 123 years
  • From 1923 to 1972 the population again doubled from 2 billion to 4 billion.
    • 50 years of 1.4% growth, doubling in 50 years
  • From 1973 to 2023 the population is projected to again double from 4 billion to 8 billion.
    • A second 50 years of 1.4% growth, again doubling in 50 years.
  • Peak growth from 1965 to 1972 when the population grew from 3,339,583,597 to 3,851,650,245
  • A century of doubling every 50 years from 1921 to 2020.

Data Sources.

Genetic tracing provides evidence there were several thousand humans as long ago as 60,000 BCE, and by 60,000 BCE there were humans in Australia, Africa, Europe and China, so even following devastation by the an event such as the Toba eruption over 10,000 in years earlier in 70,000 BCE, there had to be at least 3,000 people worldwide. More likely there were many more, which makes this growth figure a highest possible rate rather than a best estimate.

Using this smallest number of 3,000 back in 70,000 BCE, following the Toba catastrophe as a starting point, enables calculating that the fastest average growth rate for those 60,000 years, which would be a period that even includes the bust of population growth that normally follows catastrophe.

The oldest reliable data we have for population for 10,000 BCE, when there were between 1 and 10 million humans, with 4 million now being the best low estimate for 10,000 BCE. The value range from reliable up to date estimates, is between 2.5 and over 4 million. From 3,000 humans in 70,000 BCE to 2.5 million over 60,000 years is a growth rate of 0.0112 and to 4 million is 0.0119 million, so in either case the growth is less than 0.012%.

For the increase from 1 to 2 billion, between 1650 and 1800 and mostly within the time of the industrial revolution, population growth was starting to surge, and was already 10x higher than the previous average.

These figures speak for themselves. From 1650 to present is best documented time in history, and compared to whole of human history, the population growth has been extraordinary.

Because population growth is exponential, a small increase in growth rate over a long time has a huge impact. If the during the time from 70,000 BCE to 10,000 BCE growth was at even 0.1%, which is 1/5th of the rate of growth from 1650 to 1800 and 1/14th of the rate from population grew from 1921 to 2001, there would have been over 1×1026 people, or far in excess of 1 million people for every square metre of the earth surface of both sea and land, by 10,000 BCE.

On Maths Alone: how long can population growth be sustained?

On a finite planet, no growth is sustainable indefinitely, but given it is a large planet, it seems likely it would take a long time to reach the limits of growth. But not as long as might be expected.

Consider an organism that can double its population every 1,000 years.

Since 1 million is 1,000 times 1,000 such an organism could double its population 1,000 times in a million year timeframe, but doubling population even 100 times is more than enough for any fully populate the Earth with that organism. A doubling of population 1,000 times is , and double 63 times in 63,000 years. The ‘wheat and chessboard problem‘ illustrates how large numbers from exponential growth taking 63 steps from the first square. One grain of wheat on the first square (20)as the starting value, leads to 2 grains on the 2nd square (21), 4 on the 3rd (22), 8 on the 4th (23), all the way to 9,223,372,036,854,775,808 on the 64th and last square (263). Given the total land and ocean surface area of the Earth 510,064,472 km2, the 63 steps results in 18,082 organisms per square metre of the entire surface of the Earth. Allowing the 100 doubling steps that would happing within 100,000 years of an organism doubling population every 1,000 years, would result in 2,485,275,234,437,872 organisms per square metre, or 2,485,275,234 organisms per square millimetre of the entire surface of the Earth.

So over 2.5 billion organisms for every square millimetre of the entire surface of the Earth surface if population growth could continue doubling every 1,000 years for 100,000 years.

No organism has ever reached the population level of 2.5 billion per square millimetre, as would result from 100 doublings from an initial population of just 1 organism. This means no organism has ever achieved 1000 population doublings, and clearly, any rate of population growth that would result in 100 doublings is way beyond unsustainable.

For an organism larger than a microbe such as a human, even 18,082 individuals for each square metre of the entire is already beyond sustainable, so just 63 doublings if starting with 2 individuals is also clearly unsustainable. However, we are not starting at 2 people, but over 1 billion people, and it takes 30 doublings to get to one billion. (230=1,073,741,824), so if we started at 1 billion people, clearly 33 doublings, resulting in 18 thousand people for every square metre (every 10 square feet) of the entire surface of the Earth including land and sea, would be unsustainable.

At the peak rate of growth of the population explosion around the 1960s, there was one doubling every 34 years. This means 1,000 years of population growth at that rate clearly impossible, 33*34 = 1,112 so the planet would be totally overrun within 1,000 years.

Is Even A Little Longer Explosion Sustainable?

The maths calculation shows that basically every animal on Earth has had time to reach its maximum possible population.

On this basis, the Earth is ‘fully populated’ to the maximum by almost every species, since they have all had sufficient time to totally carpet the planet. In a fully populated environment, the only way for any existing or new species to increase in population, is to either find unoccupied niches where life does not currently exist or outcompete an existing species in one or more environments.

For humans, this means becoming the new ‘fittest’ for an existing niche, and outcompeting other species. Well before we manage to ‘carpet’ the planet with humans, we are squeezing more and more other species out of every niche. At what point having a planet more crowded with humans, and less populated by other species should stop becomes a value judgement, but any existing species can only increase in population by finding a way to outcompete one or more existing species.

Q: What drove the explosion. A: Not more births, but reduced Infant Mortality.

The explosion was not more about not babies, as birth-rates were falling.

The previous sections summarise what has happened in terms of numbers, and how suddenly population growth took off, but not why it happened. Did humans suddenly start having more babies? It turns out, what happened is also clear from the data, allowing an objective answer. Population growth runs to a simple formulae:

population = initial_population + births - deaths

Fast growth requires either more births, or less deaths. Surprisingly, during the entire population explosion, birth-per-woman decreased. Apart from special cases, which mostly relate to changes in data collection, from 1749 to the current time, birth-rates around the world were stable until around 1880 and then started to fall, with the most significant falls happening from 1950.

In fact ‘births per woman’ even fell slightly, during the population explosion. This fall in birth rates rules out theories such as ‘greater prosperity, or more available food let to people having larger families’. People actually had smaller families during the explosion than before.

Birth rates were higher on average during the 12,000 years from 10,000 BCE until 1650 CE, yet average population growth over the entire time averaged under 0.05%, growing from 10 million to 500 million.

In summary, as there is no increase in births to contribute to the population explosion, this leaves a reduction in deaths as the the only possible source of this explosion. People did not have more children, the change was to less deaths of children, greatly increasing the number who went on to have their own children.

The answer: Reduced Infant Mortality.

People in the past never lived in ecological balance with nature, they died in ecological balance with nature. It was utterly tragic!

Hans Rosling

Prior to 1800, over 40% of all people born, died before reaching the age of 5. This is without even factoring in deaths between the age of 5, and growing old enough to become parents. When people die before having their own children, this reduces the population.

If a couple has 4 children, and they all become adults and have their own children, then the population has doubled. But it a couple has 4 children and only two become adults and have their own children, then the population is stable.

Across the entire historical sample, the authors found that on average, 26.9% of newborns died in their first year of life and 46.2% died before they reached adulthood [and even starting their own families]. Two estimates that are easy to remember: Around a quarter died in the first year of life. Around half died as children.

What is striking about the historical estimates is how similar the mortality rates for children were across this very wide range of 43 historical cultures. Whether in Ancient Rome; Ancient Greece; the pre-Columbian Americas; Medieval Japan or Medieval England; the European Renaissance; or Imperial China: Every fourth newborn died in the first year of life. One out of two died in childhood.
……
The chances of survival for a newborn today are around 10-times higher than the past. But some in countries mortality rates are still much higher than the world average. The country with the highest infant mortality rate is the Central African Republic where close to 9% of all infants die.

Mortality in the past – around half died as children

Historically, there was also deaths in maternity, further reducing the effective reproduction rate. A woman who reaches adulthood, but dies during maternity, has less children. Plus adults still died for other reasons during their parenting years, at higher rates prior to the population explosion.

Reviewing that historical birth-rate data in detail, historically the level was an average of 6 births for each woman who lived through their fertile years. But around half never reached the start of their fertile years, and another significant percentage died during those fertile years, reducing their number of children from that ‘6’. Of course, that six was an average, not a maximum, and there were families who had 12 or even 18 children, but there were also always people who had no children, so overall, the historic 6 was never widely exceeded as an average, despite that we can all recollect families that were even larger.

This means that historically, around 6 children per couple was the average, but only almost exactly 2 children went on to have their own families. But once mortality rates decreases and almost all children then survived, even as birth rates fell, we had a population explosion.

‘Baby Boom’? Or more a boom in babies survive to become adults?

There was a ‘boom’ in babies being born from around 1946 to 1964 with, as can be seen on the birth rates graph, more births in the US and some other countries than in the previous 20 years.

However, looking at the data, there had been more births per woman every year from when records began up until the depression in the 1920s, than at any time during that post war baby boom.

Then there is the fact that the years of fastest population growth all came from 1965 to 1972 and after the years of the peak in births. This missing piece of the puzzle is infant mortality.

Prior to the 1920s, 20% of all children died even before their 5th birthday. Prior to 1850, it was over 40%. By 1960, almost all lived.

The US Baby Boom: a boom in babies that survive to become adults.

This means that if we take the chart for USA births, and then adjust for the child mortality rate, the result would better reflect the children from each year.

On the graph corrected graph corrected for child mortality, it becomes clear that with mortality rates factored in, there was a genuine population boom, with far more people making their way through childhood. It is also clear that it was the new survival rate than that converted a lower birthrate than in 1910 into a ‘boom’ in 1960.

The orange line on the corrected graph reflects the level required for population stability. The line lowers over time, because the data is not ‘births per woman’ and is ‘per 1,000 people’ instead, as births per woman is not available for prior to 1950. A problem with ‘per 1,000 people’ is the value changes just because people live longer, and 24 births per 1,000 people in a population living to 50, is the same number of children per woman as 36 births per 1,000 in a population living to 75.

Of course, the USA has significant migration rates, which means population growth is always larger than that due to births alone, and immigration keeps the population growing even when birth rates alone do not.

The ‘baby boom’ globally: A Boom but without many more babies.

Comparing the fertility rate graphs of the with the global average, two things are clear:

  • the peak of births in the US was more significant and occurred 5 to 10 years earlier than the global average.
  • the fall in births per woman occurred 10 years earlier and more rapidly in the US than the global average.

The sharp peak of the rate of global population growth in 1965 to 1972, is in stark contrast with the almost constant global birth-rate though that time as shown above.

It is only when combing the birth-rates with the rapid fall in infant mortality that the population graphs can be explained.

In reality, a global boom in birth rates makes less sense anyway, as the impact of WWII on many countries with huge populations such as India, China and Brazil, was very different from in that in North America and Europe. In the US and Europe the war ending, for many people, allowed people who had delayed parenthood for many reasons during the war became parents, which would concentrate births into a specific window, but not necessarily result in larger families. The biggest shift in family sizes came for reduced mortality of children.

The explosion is Ending?!

Birth-rates Are lower And Population Growth Has Slowed.

Now consider the latest data:

  • From 1921 to 2020 the population doubled twice in a century, from just under 2 billion to almost 8 billion.
    • 100 years of near 1.4% growth, doubling in 50 years
  • Peak growth from 1965 to 1972 when the population grew from 3,339,583,597 to 3,851,650,245
  • From 1972 to 2024 the population will again double from 4 billion to 8 billion.
    • 50 years of near 1.4% growth, doubling in 50 years

The first takeaway is that the trend of increasing rate of population growth has ended, the 50 years after 1972 shows almost the same growth as the 50 years leading up to 1972. The next takeaway given the 50 years leading up 1972 was about rising to the peak in 1972, then the next 50 years the rate fell from that peak. When graphed, this becomes even clearer.

The rate of population growth (magenta line) shows all going back under control. However the green shaded area, shows in absolute terms, population growth is still frightening as, while the rate of growth is now smaller, a small percentage of larger number is still a large number.

The graph clearly shows the ‘explosion’ from around 1925, peaking around 1972, and ending around 2025-2030.

If we had continued at the rate of over > 2% per annum growth since reaching 4 billion in 1972, where would we be now in 2021, and where would the population be by to have a 50 year span, in 2022?

Two percent growth per annum means every year multiplying the population number by 1.02 to get the number next year. Then repeat for each year.

pop2021 = pop1972 x 1.021(2022-1972) = 4 billion x 1.021(50) = 4 billion x 2.69 = 11 billion

We would have added over 7 billion extra people to the 4 billion of 1972, instead of the additional less than 4 billion we have added. Note that we started adding the same extra per year, so most of the rapid growth was before the year 2000, when we had already reached those extra people added before the year 2000. Almost halved the number of extra people. This reflect

In fact, the return of population growth to normal levels is even more advanced than this graph shows, as actual population growth is a ‘trailing indicator’, trailing by as much as a lifetime from the events that cause the growth. In fact the drivers of population growth are already set for almost zero growth, once those drivers take full effect.

So, for now at least, the explosion is basically over. The huge population as a result of the explosion is the problematic legacy, and even a small percentage increase, in that huge population is a lot of additional people to potentially further impact the environment.

Explosion over? How did we return to ‘normal’?

The next question is, why did the population explosion end? Humanity did not stop saving the lives of children or young adults! While the explosion started without increasing the rate of births, but then, rather amazingly, is ending because of a reduction in the birth-rate.

There are other pages here exploring the fall in births per woman, however in terms of ending the population explosion, a change to the other factor in the population equation (births), has almost precisely balanced the change in deaths that initially triggered the explosion. The fall in births has now almost exactly balanced the fall in deaths.

At peak 20th century growth rates of 2.1%, or 42 times that 12,000 year average, the same increase in population as occurred over 12,000 years, would take place in just 200 years. This is not sustainable, but now births per woman has startlingly fallen to way below what was seen during those previous 12,000 years, potentially bringing population growth back to the historic normal.

Conclusion: We May Have Dodged a ‘Bullet’?

The population explosion was not triggered by more babies, but by enabling all babies to become adults, and then parents. Somehow, we have now adapted to this increased survival rate by having less children, but are left with the legacy of now having so many people that the environment is in risk of collapse.

The explosion started without our planning for, or perhaps even contemplating, the consequences of solving infant mortality in terms of population.

The explosion is also ending not as a result of planning, and with many people perhaps not even realising. It feels like ending the explosion early enough that we may avoid a population collapse is just luck. However, an explosion due to reduced child mortalities, would potentially have put the global population far ahead of where it would have been through the historically normal more gradual population increase in response to technical advances, and some population reduction, or at least pause of growth until a return to sustainability, may be necessary.

So the explosion started without us realising, is coming to an end that many of us cannot see. This is a ‘bullet’ most did not see coming, most do not realise has become less threatening, and many chose to deny ever existed.

How long can we continue to survive without a plan?

Updates.

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