IPCC population projection models are often criticized for having simple assumptions. They generally model predicted population based on the expected rate of world GDP growth and the correlated predicted reduction in birth rates. They don't use a system dynamics based model, that could model the complexities of overshoot and climate change and its impacts on population. For example global warming will have varying levels of cascading non-linear effects on extreme weather events, things like increases in economic inequality, increase in domestic and international conflict, increase in state fragility, increase in pandemics, increase in population displacement and migration, decrease in ecological biodiversity, decrease in food, fuel, and water resources, increase in droughts and desertification of fertile land, increased fragility of global supply chains, ect..... None of these factors are part of the IPCC models, meaning that their predictions shouldn't be taken as realistic predictions of populations. Rather they should be seen as models that predict population numbers in a world where climate change and overshoot will have little to no impact on human civilization in the near future.

It's also no secret within academic climate science circles that the IPCC has long been politically motivated to underestimate the scale of the problem. Which is why very few climate scientists actually believe that the Paris Accord is realistic. We all know there is no chance the world can avoid 1.5 C mean global warming and that we will likely see a potentially disastrous 2 C increase by 2050. Many already assume that there will be no remaining carbon budget even for the 2 C target

The IPCC infamously fails to account for carbon cycle feedbacks and their associated tipping points when setting their own emissions targets. Meaning that even a 2C warming may well trigger irreversible runaway “hothouse Earth” conditions. In coming years, we will see an ice-free Arctic Ocean, more rapidly melting permafrost, methane releases, an increase in wildfires, and other short-term positive feedbacks that will put climate change on steroids.

Even in the best case, the world can expect more and longer heat waves and droughts, more violent tropical storms, extended wild-fire seasons, accelerating desertification, water shortages, crippled agriculture, food shortages, rising sea levels, and broken supply lines. Coastal cities will be flooded and some may eventually be abandoned. Many other cities are likely to be cut off from food-lands, energy, and other essential resources with the breakdown of national highway and marine transportation networks; this alone would make urban life untenable. According to the recent Environmental Risk Outlook 2021 (2021), at least 414 cities with a total 1.4 billion plus inhabitants, are at high or extreme risk from a combination of pollution, dwindling water supplies, extreme heat stress, and other dimensions of climate change.

From this perspective, it's absurd to even entertain the idea that adding 2+ billion more people on the planet in the next 20 years would be a good idea. We should be currently engaging in an international effort to reduce our current consumption, reduce our energy demand, reduce our birth rates, reduce economic inequality, and ultimately start to move away from a growth for the sake of growth economic model towards an ecologically sustainable economic system.

It's simply immoral and reckless to keep chugging along with the business as usual economic model. Doing so is to condemn billions of people to a brutish, painful and short life.

Both can be true. It's true that western countries disproportionally use more energy and resources per capita than developing countries. It's also true that global human civilization is in an advanced state of ecological overshoot mainly because of overconsumption and overpopulation. We are currently consuming more resources than our ecosystems can regenerate naturally, we are operating beyond the earth's carrying capacity. On top of that we are dumping entropic waste back into the ecosphere in excess of nature’s assimilative capacities, which only speeds up the process of system wide ecological collapse that will absolutely lead to dramatic population contraction in the near future. In this sense, overshoot is actually the cause of climate change and numerous co-symptoms including plunging biodiversity, ocean acidification, tropical deforestation, landscape/soil degradation, contamination of food supplies, depleting aquifers, the pollution of basically every life-supporting system. Our contemporary growth obsessed technological civilization is literally consuming and polluting the biophysical basis of our own existence, and somehow we seem to think that there's nothing that can go wrong by adding a couple more billion humans on the planet.