Hello and welcome to Just Have Another Think - our twice monthly look at the environmental ecological
and social impacts of the 21st century climate emergency. Two of the questions I get asked most
often on this channel are number one - 'Is the human population increasing out of control?' and
number two - 'what are the risks to human health in the coming decades as a result of climate
change?' Both very important questions indeed. So important in fact that far better minds than
mine have been carrying out painstaking research over recent years to establish the answers and
offer as accurate a picture as possible of how the 21st century is likely to play out.
It'd be pretty much impossible to do justice to both of those questions in just a single video,
so this week I'm going to focus in on population growth. Let's start with a very quick look at how
the human population has developed throughout history. The brilliant website Our World In
Data provides a chart based on United Nations population estimates going back 12 000 years.
Obviously we didn't have anything like a census all the way back then, but based on the best
available scientific analysis the UN estimates that the number of humans thawing out after the
last ice age was just under 2.5 million. By the beginning of the rise of ancient Egypt around 3000
BC, the population had risen to about 45 million, and by 1AD we were up to about 190 million or so.
That number rose fairly modestly all the way up to the 14th century when famine and bubonic
plague gave us a bit of a kicking and brought an abrupt halt to growth. Things eventually got
going again though, and by 1750 there were just over 800 million humans on the planet.
And then things went a little bit crazy. Largely as a result of the industrial revolution,
population more or less doubled within 150 years. So we entered the 20th century at 1.65 billion.
And that was just the bottom of an exponential curve. When my Dad was born in 1943 the population
was 2.34 billion, and by the time I came along in 1969 that had shot up to 3.63 billion.
Over the course of just my lifetime world population has more than doubled again,
and today's number is 7.8 billion and counting. Here's the clock at the Worldometer website
showing how the number is currently increasing based on statistics for births versus deaths.
Looks quite alarming doesn't it? But although the absolute population number is still going up,
the global fertility rate - which is the average number of children per woman - has actually been
rapidly declining since the middle of the last century. In 1950 women, on average, had just
over five children each, and that number varied dramatically from country to country of course.
Places like Rwanda, Kenya and the Philippines had an average of seven children per woman.
China and India were just over six, and the United States and most of Europe stood at somewhere
between two and a half and just over three. Only one country had a fertility rate less than the
statistical replacement rate, and that was tiny little Luxembourg. And when I say replacement rate
I just mean the simple arithmetic that says if each man and woman on average have two kids then
those two kids will just replace the parents when the parents die and the population will remain
static. Statisticians more precisely put that number at 2.1 for reasons that are, to be honest,
just too dull to talk about. Anyway, anything above replacement rate results in increasing
population and anything below replacement rate results in a declining population.
Since about 1965 the world has seen a drop in fertility rates never before experienced in
the whole of human history. The global average now stands at just over 2.4
and is projected by the United Nations to be about 2.1 by the end of this century. But the U.N
projections also say the global population will hit 11 billion by the end of this century.
So how can we square that apparent discrepancy? Well the population projections are based on
what the UN assume to be an inevitable fill up of adult human beings based on current demographics.
In 2014 there were seven billion people on the planet and that number was made up of about 2
billion kids below the age of 15 another 2 billion between 15 and 30 a billion people between 30 and
45 another billion between the age of 45 and 60 and then a final billion over 60 years old.
There's clearly 3 billion people missing from these age groups. So where are those people?
Well some of them aren't there because they died early, but most of them aren't there because they
were never born in the first place, because of course in the 20th century there were fewer people
overall and therefore fewer women to have babies. If we run the clock one generation into the future
this older age group dies while this younger age group grows up and has roughly two billion kids.
Move everyone on another generation and now it's time for this older age group, including me,
to depart, and this younger age group to have their own two billion kids. Go another generation
again and the same thing happens and hey presto within three generations you get your inevitable
fill up taking the population to 10 billion people as we enter the second half of the century. And
then the UN adds a billion or so on top of that to account for longer average life expectancy.
So 11 billion by 2100 has been the generally accepted thinking for quite some time.
But in September 2020 the Institute for Health Metrics and Evaluation published a brand new
paper using what they claimed to be more accurate statistical analysis techniques than the UN.
That paper found that global population could peak at about 9.7 billion by 2064 and then decline
to as little as 8.8 billion by the end of the century. So how do they work that out then? Well,
unsurprisingly, their models also accounted for fertility and mortality rates as well as projected
migration patterns, but to analyse fertility the IHME developed statistical models for what they
referred to as 'Completed Cohort Fertility at age 50' or CCF50... and that's the sort of scientific
jargon that regular viewers of this channel will know I just can't get enough of! Apparently
completed cohort fertility modelling is much more stable over time than the usual statistical model
known as total fertility rate or TFR. So I'll do my very best to translate why they think that's
the case. Previous studies have generally forecast a total fertility rate in a given country's female
population and then estimated specific fertility rates at different ages using assumed numbers
that they think are statistically about right. In a country where the total fertility rate is
higher than the replacement number of 2.1 then the strong relationship between declines in fertility
and increases in female education and access to birth control show up quite well
in the long term numbers. But when the total fertility rate goes below replacement level
some countries show declines followed by upturns and other countries, like Singapore for example,
show declines followed by stagnation at very low levels. Because better educated women tend to go
to work and delay childbirth until later on in life, the total fertility rate over a period
of time often goes down and then up again even though the overall fertility over the course
of a reproductive lifespan for any sample group of women may still be declining or stagnant. If those
timeline fluctuations are not properly accounted for then uncertainties can creep into the total
fertility rate models that have a tendency to skew the fertility rate numbers up slightly.
The IHME method of completed cohort fertility at age 50 is defined as the average number of
children born to an individual woman from an observed sample population if that woman
lived to the end of their reproductive lifespan which is defined as being from age 15 to age 49.
the statistical formulae for working out all the specific numbers include fancy things like time
series random walk functions which quite frankly are well beyond the scope of this short video, but
if you really feel like blowing your mind then as always I'll leave links in the description section
to the full paper and all the appendices. But to illustrate the effect of the variation between
the two modelling techniques, the good folks at IHME provide us with these charts comparing
five countries with fertility rates above or below 2.1. The point they're making is that the further
above replacement levels you go, the closer the two modelling methods seem to be in agreement.
But you can see that in the three countries where those fertility rates move towards or
even below replacement then the line for the total fertility rate model does indeed get
a bit wobbly compared to the CCF50 model, and in all three cases the line seems to be ticking back
up a bit as the graph hits 2020, even though it's known that the fertility rates in those countries
are either static or falling. By contrast the CCF50 line shows a pretty stable progression
that can be more accurately projected into the future. So that's fertility cleared up then...
nice and easy! When it comes to mortality rates the IHME employed similarly complex statistical
calculation methods including something called an autoregressive integrated moving average or ARIMA,
which I'm not even going to try to explain, plus some other slightly more understandable variables
like the socio-demographic index, crude population growth rates and deaths from
wars and natural disasters. They also factored in economic factors like gross domestic product.
Once they'd crunched all the numbers they found that the midpoint of the model predictions
was a global average fertility rate of just 1.66 by 2100 compared to the UN's projection of 2.1.
And there are some pretty dramatic swings in the populations of the top 10 countries too.
China's population is predicted to fall by almost a half from 1.4 billion
to 732 million which means they'll be overtaken, believe it or not, by Nigeria which is set to rock
it up from its current 206 million to almost 800 million by 2100. In the United States the
fertility rate continues to fall sufficiently quickly to cause an overall drop in population
but once immigration is factored in the number will actually creep up slightly over the coming
decades keeping them in fourth place. Brazil, Russia, Bangladesh, and Japan all show rapidly
declining populations taking each of them outside the top 10 by the end of the century.
Those four spots will be taken by Ethiopia, Egypt, The Democratic Republic of Congo and Tanzania...
'Tanzaynia'... Tanzania ...'Tanzaynia' TANZANIA!! It's worth reminding ourselves that the IMHE the
UN and every other relevant organization all agree that the two biggest factors in reducing fertility
rates are better education and equality for women and girls plus better access to birth control.
Nowhere are those two challenges more acutely demonstrated than the vast continent of Africa.
If all women and girls were allowed those basic human rights in every African nation tomorrow
then the overall 2100 population would be far smaller. Perhaps as much as several
hundred million fewer people than even the IHME model is predicting.
And of course population isn't the only metric to assess how our species will fare in the future.
Levels of consumption vary massively between rich industrialized nations and poorer countries, with
most African nations showing the lowest levels of all. So although their populations may be rising,
their overall environmental and climate impact will most likely be disproportionately lower. And
then there's all the other risk factors like sea level rise, droughts, floods, famines and diseases
that will all have a massive bearing not just on our own species survival
but also on all the other species that we share this planet with. Which brings me all the way
back to the second of the two questions I posed at the start of the programme...
'What are the risks to human health in the coming decades as a result of climate change?'
That question has been comprehensively assessed by an international collaboration of academic
institutions and United Nations agencies called the Lancet countdown, who published this paper
at the start of 2021. And over the course of the next few weeks I'll be taking a detailed look
at the somewhat daunting predictions outlined in that report. So there we are then folks. Another
slightly mind-boggling set of data for you to chew over at your leisure. I'll be back here to
Just Have Another Think in two weeks time, and of course I'll be over in my regular slot looking at
the latest developments in sustainable technology on the Just Have a Think channel this Sunday.
So thanks very much for watching and I'll see you soon