2013-09-29 07:45:01
By Hannah Barnes BBC News
The respected broadcaster and naturalist, Sir David Attenborough, told the BBC
recently that population growth was "out of control" - but one expert says the
number of people on the planet could peak in 40 years. Who should we believe?
"The world's population is increasing out of control," Sir David told the BBC's
Today programme.
"Since I first started making programmes 60 years ago, the human population has
tripled."
Two striking claims.
Let's take the second one first - that the world's population has tripled in 60
years.
In 1950, around the time Sir David began his broadcasting career, there were
2.53 billion people in the world. Sixty-three years later and the latest
estimate of world population is 7.16 billion.
David Attenborough
Born in 1926, younger brother of actor and director Richard Attenborough
Wrote and presented the Life series of wildlife documentaries for BBC TV,
beginning with Life on Earth in 1979
Also narrated The Blue Planet, Planet Earth, and Frozen Planet
Patron of Population Matters, a campaign group that urges couples to have two
or fewer children
That is a little shy of tripling - more like a factor of 2.8 - but it's not far
off.
The "out of control" claim is less easily measurable, but perhaps it could be
interpreted as the idea that the population will continue to grow at the same
rate, roughly tripling in 60 years.
If this happened, the world population would reach almost 40 billion people by
the end of this century.
But the latest United Nations projection puts the figure at little more than a
quarter of that - less than 11 billion.
That's still 50% more than we have today, but it shows the UN expects much
slower population growth in the decades to come than in decades gone by.
Some might consider that an increase in the world population from seven billion
to 11 billion by 2100 still represents out-of-control population growth.
But this UN figure - contained in its World Population Prospects, published
every two years - is considered by one expert, at least, to be much too high.
Graph - How much will the world's population rise by?
"When I looked at them I discovered that they were almost certainly wrong,"
says Sanjeev Sanyal, Global Strategist for Deutsche Bank, of the latest update
of the World Population Prospects, released in June this year.
Population growth projections feed into many other forecasts and models -
projections of energy use, for example, or corporate profits - so people like
Sanyal scrutinise these UN figures carefully.
And he finds the UN projections "difficult to justify" for a number of reasons.
Mother and baby in Philippines
"If you look at fertility rates - the number of babies that a woman has over
the course of her life - in very large parts of the world, those fertility
rates are now below what is needed to replace the population," he says.
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"Much of Europe, Japan, large countries like China, even Brazil, don't produce
[the necessary] 2.2 or 2.3 babies [per woman]. Some of them are way below that
level and as a result it is almost certain that these huge countries are going
to see rapidly declining populations within a few decades from now."
The replacement rate is higher than two, because some women will die before
they reach the end of their child-bearing years.
Also, in developing countries the UN predicts rapidly expanding populations.
In Nigeria, for example, it expects the current figure of roughly 160 million
to increase to almost one billion by the end of the century.
Graphic showing population growth
Sanyal is sceptical.
"Surely Nigerians will recognise at some points that things are getting crowded
and stop having so many babies?" he argues.
He predicts the Nigerian population in 2100 will be 400 million fewer than the
UN suggests.
Growing Africa
Nigerian baby
Taking its population as a whole, this century Africa's story will be one of
incredible growth - beyond that of any other region in the world. It's expected
to account for more than half of the total global population growth between now
and 2050.
Will Africa be the world's engine room?
His forecasts are lower for the world's two largest countries too. He predicts
China's population will be 60 million fewer than the UN forecasts for 2100, and
India's 100 million.
"Even the US is quite suspect," Sanyal says.
Here, the UN predicts a rise from 312 million today to 462 million in 2100.
"That would be extraordinary for a country which already has birth rates below
the replacement rate You will need huge amounts of migration into the US to
reach anywhere near [that]."
It is likely that lots of people will migrate to the US. Sanyal accepts that
the US population will grow.
But to increase at this rate he insists that other countries would have to be
showing falls in population - falls that do not appear in the UN figures.
Overall, Sanyal paints a very different picture from the UN, with world
population peaking around 2050 at 8.7 billion and declining to about 8 billion
by the end of the century. That's about a billion higher than it is now, but
well short of the UN's 11 billion.
Both Sanyal and the UN start with the same data - national censuses from 2010.
The difference arises because they make different assumptions about fertility,
mortality and migration.
"I took into account two or three things which I think are inadequately
reflected in the UN [report]," Sanyal explains.
The world at seven billion
The world's population hit seven billion in 2011. After growing very slowly for
most of human history, the number of people on Earth has more than doubled in
the last 50 years.
Are there really seven billion of us?
What's your number?
"I have probably accounted more aggressively for things like gender bias in
countries like China and India. The fact that they are countries with far fewer
women of childbearing age than their overall population would suggest."
The UN predictions also assume, according to Sanyal, that all fertility rates
will eventually converge towards the replacement rate - an "odd assumption" in
his view.
"We have not seen any country where fertility rates have declined very
dramatically [only] to have seen them drift back up to the replacement rate,"
he says.
And the UN has underestimated the impact of urbanisation on reducing fertility
rates, he argues. Up to now, as he puts it, urbanisation has been "a very
powerful contraceptive" in all countries.
For their part, the UN experts say that Sanyal must have been assuming very
sharp declines in fertility rates, which they do not share, and very small
changes to the global fertility rate can have a huge impact decades down the
line.
The UN's own predictions highlight this.
UN population growth estimates The blue line is the medium variant, red is high
and green low
The 10.9 billion figure in 2100 is what is known as the "medium-variant" - it
represents what the UN sees as the middle road.
But if you assume a fertility rate of half a child below that, the world's
population would have fallen to 6.8 billion by the end of the century. Go up by
half a child in the UN's model and it hits 16.6. billion.
What's more, small changes in fertility rates have a more pronounced effect
over time. Sanyal's forecast and the UN's differ by 800 million at 2050. Yet,
this increases to 2.8 billion by 2100.
There is plenty of room for disagreement. Let's hope the disagreements don't
get "out of control".