Reducing the global population won’t stop global warming and, after temporarily overshooting 1.5oC, the world won’t return to what it was but we can still act to save our flora.

There can be little doubt that the combination of a historically very high global population and high average consumption per person is causing major environmental problems, including climate change. This is popularly expressed in terms such as humanity currently uses more resources than one Earth can sustainably provide. But unlike the British Royal Family, we don’t have a spare sitting in the wings. One way to reduce humanity’s environmental footprint, as Julian Cribb has concluded in Pearls and Irritations, is to reduce the global population.

It seems likely that, if the global fertility rate eventually stabilises at the rate required simply to maintain a stable population, the present population of around 8 billion will grow to around 12 billion by 2100 before plateauing at about 13.5 billion in 2200 (if humanity survives that long).

On the other hand, depopulation (the effective policy at present given declining fertility rates in many countries) might see the population peak at 10 billion in 2100 and fall to 7 billion by 2200. These two different, hypothetical, population trajectories would result in a 17 per cent difference in population size in 2100 and 90 per cent in 2200.

Would the lower populations make a significant difference to greenhouse gas (GHG) emissions and global warming? Emphatically not, according to US National Bureau of Economic Research. Despite the large differences in population size, the global temperatures are almost identical in 2100 and will differ by less than one tenth of one degree C. in 2200.

The authors add that if net-negative emissions technology (removal of CO2 from the atmosphere) becomes a reality, it is even possible that a smaller population will lead to marginally higher temperatures.

There are two main reasons for these superficially counterintuitive findings. First, fertility shifts take generations to substantially change population size and during that time per capita emissions of GHGs will decline significantly even under pessimistic policy assumptions. Second, it is the total amount of GHGs in the atmosphere that determines the level of global warming,........

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