With the world population rapidly increasing, this was an interesting exercise to kickstart a very informative topic. I found the population of the year of my birth in the 1960s to be 3.2 billion with an urban population of 35%. To find yours, check here.

As a mature student I was interested in the difference between that year & the birth year of my fellow students, approximately 1999. The world population then was 6.1 billion, having almost doubled with an urban population estimated at 46%.

In subsequent lectures I encountered the Population Doubling Time, which is the amount of time it would take for a population to double in size, assuming growth remained constant which is:

70 divided by average annual rate of population growth (in % per year) so the larger the rate of growth, the faster the doubling time. 

Fast forward to the current day and we have a population of 7.6 billion and an urban population of 55%. Luckily world population is not still growing at the same rate as it would be 12 billion by 2050 whereas the current estimate according to the UN World Population Prospects: 2017 Revision is 9.8 billion. To see how we got to this starting point, we go back to the beginning...

World population has changed over time from the era of the hunter-gatherer with a low resource, low impact, low population of 5 - 10 million through the Agricultural and Neolithic revolutions which increased the natural resource base and food security. In the mid-1600s population growth became more rapid with increased life expectancy due to commerce, food production and nutrition. During the time of the Industrial Revolution in the nineteenth century world population had reached 750 million with advances in medicine and sanitation enhancing survival rates. 

These advances  also increased carrying capacity which is defined as: the number of people supportable within a given area by the technology at their disposal or the maximum number of organisms in one species that can be supported in a particular environmental setting.

World population was transformed in the 20th century as technological and social changes brought steep declines in birth rates and death rates around the world. The century began with 1.6 billion people and ended with 6.1 billion, mainly because of unprecedented growth after 1960. John Wilmouth of 'The Guardian' states that " we're not adding to the population through births, we're mostly adding to the population because people are living longer". This can be seen with some of parents' generation living into their 90s whereas many of the generation before didn't survive past 60s and early 70s.

Population momentum can explain continued growth after fertility rates reduce. I found this website useful to explain the concept as it explains how a country's population continues to grow once replacement fertility is reached.

Replacement fertility level is defined as the number of children needed to exactly replace their children's generation, so accounting for premature death, this is 2.1.