Humanity has just experienced a population explosion. Whether you believe we are overpopulated or not, the facts are that we *have* had a population explosion. A period of population growth at rate over 100x accelerated relative to the average. Why? Not because of more food, the industrial revolution, or access to fossil fuel, but all due to modern medicine.

For almost the entire existence of humans on Earth, population growth has been at a rate where doubling the population took at least 4,500 years, but at peak of the ‘explosion’ from 1965 to 1972, growth was sufficient to double the global population in just 34 years.

The red line shows a 100 year long mountain on the 400 year graph of population growth. Just how did this happen? Did everyone go baby crazy? It turns out, no, people *decreased* family sizes, yet population *still* exploded!

- The Stats: Yes it was a population ‘explosion’.
- Population growth is not normal.
- The explosion is over!
- Q: What drove the explosion. A: Not more births, but reduced Infant Mortality.
- What About the ‘Baby Boom’?
- Explosion over? How did we return to ‘normal’?
- Conclusion: We may have dodged a ‘bullet’?

## The Stats: Yes it was a population ‘explosion’.

- From 60,000 BCE to 10,000 BCE the population at most by 10,000x (1,000 to 2.5 Million)
- 50,000 years of growth rate < 0.015%, doubling at most every 4,500 years

- From 10,000 BCE to 1650 the population grew by less than 50x (2.5 Million to 500 Million)
- 12,000 years of growth rate < 0.05%, doubling every 1,500 years
- 10,000 BCE to year 1, rate 0.04%, doubling every 1,600
- year 1 (pop to 188million) to 1650, rate 0.06%, doubling every 1,200

- 12,000 years of growth rate < 0.05%, doubling every 1,500 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
- 7 years of over 2% growth, peaking at 2.1% annual growth, doubling in 34 years.

- A century of doubling every 50 years from 1921 to 2020.

The oldest reliable data we have for population is that in 10,000BCE there were between 1 and 10 million humans, with 2.5 million being the best estimate. Genetic tracing provides evidence there were at the very least 1,000 humans as long ago as 60,000 BCE. Using this smallest number possible of 1,000 as a starting point, enables calculating that the fastest average growth rate for those 50,000 years. The world was different, but 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* 1,000 people worldwide. More likely there were many more, which makes this growth figure a highest possible rate rather than a best estimate. This show how low the annual population growth has to be, to only have 10 million humans after even those 50,000 of population growth. There were probably more than 1,000 people in 60,000 BCE, and the more people as a starting value, the slower the growth rate to reach 10 million by 10,000 BCE, so we can be confident the average growth rate over that time was no more than a doubling every 4,500 years.

Then 10,000 years ago, glacial ‘ice-age’ periods took a break, civilization started, and the population growth accelerated to double that previous average, but still below 0.05% so that even over 10,000 + 1650 year, the population only grew another 50 times.

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 60,000 BCE to 10,000 BCE growth was at even 0.1%, which 1/5th of the rate of growth from 1650 to 1800 and ^{1}/_{14}th of the rate from population grew from 1921 to 2001, there would have been over 5×10^{21} people, or 1 million people for every square metre of the earth, by 10,000 BCE.

## Population Growth Despite Earth is normally fully populated?

### A Finite Planet Is Soon Full.

Consider an organism that can double its population level 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 (2^{0})as the starting value, leads to 2 grains on the 2nd square (2^{1}), 4 on the 3rd (2^{2}), 8 on the 4th (2^{3}), all the way to 9,223,372,036,854,775,808 on the 64th and last square (2^{63}). Given the total land and ocean surface area of the Earth 510,064,472 km^{2}, 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.

Since no organism has ever reached the population level of 2.5 billion per square millimetre that would result from 100 doublings of an initial population of just 1 organism, every organism that has existed long enough for 100 population doublings has fully populated the Earth, and has a population constrained by the environment.

### Growing A Population of a Full Planet.

Since the Earth is normally ‘fully populated’ with by almost every species, the only way for any existing or new species to increase in population is to either find unoccupied niches, or outcompete an existing species in one or more environments.

Every new species must find a new niche or become the new ‘fittest’ for an existing niche. Any existing species can only increase in population by finding a way to outcompete one or more existing species.

## The explosion is over?!

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
- 7 years of over 2% growth, peaking at 2.1% annual growth, doubling in 34 years.

- 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.

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

#### It is not births.

The previous sections summarise what has happened, but not why it happened. 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 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.

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.

Mortality in the past – around half died as children

……

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.

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.

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

Hans Rosling

This means that historically, around 6 children per couple was the average, but less than half went on to have their own families. But once almost all children survived, with that many children per couple, we had a population explosion.

## 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 changing the rate of births, but then, rather amazingly, the birth-rate * did* change

*to end*the explosion.

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.

## What About the ‘Baby Boom’?

The ‘baby boom’ sounds like a period when there we many extra babies, and in many countries there were more babies per family than in the preceding few years, but * not more than any historical normal*. It was more like a throwback to birth-rates close to 20 years ago, only this time with far more the babies surviving to become adults.

There were years immediately following the 2nd world war when people who had delayed parenthood for many reasons during the war became parents, but in many countries, those parents just had their children a few years later and did not have *extra* babies.

In the US and other new world countries such as Canada and Australia, the return to birth rates close to those from 1920s and before continued for longer, but overall, as far as birth-rates go, it was a blip on the overall trend of a reduction in birth-rates. In the ‘old world’ typically a very short blip, and in the ‘new world’ a longer blip, but in historic terms, still low birth rates.

The peak time of babies born, and living to become adults, was 1965 to 1972, ironically just after the time the term ‘baby boomers’ was is typically said to include. This the was peak population growth, and create a generation, together with those ‘baby boomers’ created generation where for the first time, almost everyone reached a adulthood. Together, as a result, there represent the most rapid growth in population. However, they were not numerous because more babies were *born*, but rather because all babies *survived*.

However, although the rate of population growth as a percentage of the existing population has been decreasing since 1972, the total number of children born each year has still been increasing until reaching ‘peak child‘ around 2020.

## 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.

The explosion is also coming to an end without planning, and with many people perhaps not even realising. It feels like ending the explosion at least with some hope of finding a way to live with the resultant number of people on the planet is just luck.

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?