World Population growth rate shows signs of slowing - eventually.

Is the world's population going up or down? Well, both - the total population is going through its fastest phase of growth, but the growth rate is starting to show signs of decline, in fact we may be past the fastest rate of growth already.

The most recent estimate by the United Nations (2015) suggests that we may soon be emerging from the most rapid phase of global population growth. The current 7.4 bn people is likely to reach over 11 bn by the end of the century, but the population will be in a much slower phase of growth by then. As a majority of the world's countries experience the later stages of the Demographic Transition Model (DTM), the global population can be seen exhibiting the characteristics of stage 5 by 2100 as the birth rate starts to decline and an ageing global population predominates.

The rate at which it takes the world's population to add an extra billion people is at an eye-watering 12 years at the moment. But this is due to lengthen as smaller families become the norm in much of Asia and Africa and is likely to be well over 30 years by the second half of the century:

3-4 bn : 16 years (1959-1975)

4-5 bn: 12 years (1975-1987)

5-6 bn: 12 years (1987-1999)

6-7 bn: 12 years (1999-2011)

7-8 bn: 12 years (2011-2023)

8-9 bn: 15 years (2023-2038)

9-10 bn: 17 years (2038-2055)

10-11 bn: 31 years (2055-2086)

The issues of population size on their own are really not the key challenges. The questions will focus on supplying enough food and fresh water, absorbing the waste, providing the resources, maintaining the biosphere, managing migration and generating a sustainable future within an economic context of more elderly people than young - whilst not damaging the planet. That should keep geographers busy for a while yet.