The pyramid is a traditional way of visualising and explaining the age structure of a society. If you draw a chart with each age group represented by a bar, and each bar ranged one above the other—youngest at the bottom, oldest at the top, and with the sexes separated—you get a simple shape.
In 1970 that shape was a pyramid because the largest segment of the global population was the youngest (0-5 years old, comprising 14% of the total), followed by the next-youngest (6-10, with 13%), and so on in regular increments until, above 85 years, there were so few people that the shape vanished into a point.
The pyramid was characteristic of human populations since the day organised societies emerged. With lifespans short and mortality rates high, children were always the most numerous group, and old people the least. A population chart of England in 1700 looks likes a pyramid.
But now look at the chart of the global population in 2015. It looks more like the dome of the Capitol building in Washington, DC, than something along the Nile.
Young children are still the largest group, but now make up only 10% of the population and those above them are almost as big, with 9.5%
The angle of the slope changes most markedly after about age 40.
In 1970 the youngest had not only been the largest but also the fastest-growing section of the population. But between 1970 and 2015, the population aged 0-19 grew by only 42%, whereas the population aged 20-39 rose by 128%.
This group added almost twice as many people to the overall numbers than the group aged below 20.
There are now also over 50m people above 85, so the dome of 2015 has a spike.
In 1970-2015 the dominating influence upon the global population was the fertility rate—the number of children a women could expect to bear during her lifetime. It fell dramatically over the period, meaning that the world shifted from having larger to smaller families.
But in 2015-60, the biggest influence upon the population will be ageing. Small families are already becoming the norm, the fall in fertility is slowing down and now everyone is living longer than their parents—dramatically so in developing countries.
So, by 2060, the dome will have come and gone and now the shape of the population looks more like a column (or perhaps an old-fashioned beehive). It is a little fatter near the bottom and curves in at the top.
But up to the age of about 50, the generations are of almost equal size and the shape has vertical sides.
The size of the Earth’s population is still rising, from 7.2 billion in 2015 to 9.5 billion in 2060. But, according to calculations by Emi Suzuki and Wolfgang Fengler of the World Bank, two- thirds of the extra 2.2 billion people will come from the older age groups, those aged 40 to 59 and those between 60 and 79, not from the younger.
The increase in the last, oldest segment is especially marked. Between 2015 and 2060, the number of 60- to 79-year-olds will double to 850m.
That is more than four times the increase in the number of children and teenagers, which will rise by only 200m, or 8%.
The numbers of the oldest people of all (those above 85, here lumped together in one bar) will rise at the fastest rate of all (by 281% in 2015-60), but from a much lower base, so they do not add as many people to the total.
For all of history, humans have lived in societies dominated (in numbers at least) by children. By 2060 children will be no more numerous than any other age group. The year 2015 is, roughly, the half-way point in this astounding transformation.