Thanks for writing in, we’ve put these to Casey Briggs.
It’s not like there’s a specific variable in the population models for “climate change”, no. But indirectly these factors would play into the UN’s forecasts. And there’s quite a bit of uncertainty in the forecasts for the second half of the century, including for some of the reasons you’ve suggested.
The Director of the UN’s Population Division John Willmoth told me that the forecasts use information from every country’s national census, as well as detailed information about deaths and births from nations that have it, plus surveys in some countries.
Even he concedes it’s not perfect data: “Ideally, we would have a census every 10 years for every country, we don’t have that. And ideally, we would have complete registration of births and deaths all over the world. We don’t have that either.”
Fertility rates are the single biggest driver of population growth, and different forecasts have a major impact on when the world population peaks. For example, a different model from the IHME suggested fertility rates would fall earlier than the UN had estimated, bringing forward the population peak by a couple of decades.
The difficulty is that there’s so many different ways the next century could play out, and so much of it is in our hands. On energy – can we successfully shift to renewables that can replace fossil fuels? How fast can we do it? Will we see increased conflict around the world?
The UN’s chief António Guterres put the warning starkly this week at global COP27 climate talks:
“Our planet is fast approaching tipping points that will make climate chaos irreversible. We are on a highway to climate hell with our foot on the accelerator.”
Will humans get their act together and slam on the brakes?