This raises some interesting questions :-
- What's the maximum global population for a technological society that is sustainable indefinitely?
- What's the minimum viable population size? One that can still build a sufficiently large hierarchy to support things like silicon chip foundries or an electric grid.
- How do we get from where we are now with 7.4B people to somewhere between these two limits?

- How do we spread the process out over a long enough timescale that it doesn't involve mass excess deaths and is a soft landing rather than the full crash and burn.
- In the relatively short term, where would you tell your children and grandchildren to move to, to have the best hope of riding out the changes and with a reasonably happy and productive life?

That's not just physical location, but social and skills locations as well. I've seen suggestions of each continent's Great Lakes plus a few outliers like Chile. But I suspect something naturally defensible might also be a factor.
- What does "mitigate the likely effects" actually mean? More trees or solar sunshades in low earth orbit? Or perhaps more flood defences.
- What does "adapt to global warming" actually mean? Does it just mean migrating away from the Persian Gulf and Florida?

There's a LOT of detail wrapped up in that. Not least because we're starting from where we are now in late 2015. And because the questions imply we're talking about trying to direct a hive mind of >7B individual nodes. And come up with answers from first principles. Neither of which may actually be possible.

Much more likely is "Humans will strive to expand their global civilization until it becomes physically impossible to do so." and "Since we can't fix it as individuals and we can't get off the planet we might as well point out the interesting bits to each other as the ship sinks." If 4-5C rise and 10B people by 2100 is a given, then what?

@jbond Almost all developed countries have birth rates below the replacement rate, because adults living in a safe and comfortable society with equal rights for both sexes and good opportunities tend to only have 1 or 2 kids. Even chinas population is in sharp decline. If we can help boost the economies of developing nations to the point where they become safe and stable, we will probably see declining populations most places and we will need to actively encourage procreation just to maintain a steady population! There will be some challenges, like the point we are coming to in many nations where there are disproportionately large numbers of elders compared to working adults who will need to be supported but otherwise we should be fine.

@Clementulus Modelling. UN Demographics and Worldometers
https://population.un.org/wpp/
https://www.worldometers.info/world-population

Last year's revision says yearly increment begins to fall in the next 10 years. 10b around 2060, peak some time after that.

Contracting workforce is a problem for "Business As Usual". The question is whether we can have a managed contraction of workforce and GDP. Or keep increasing productivity. Or have financial collapse as the debt borrowed from future growth unwinds.

@Clementulus However. Beware models that focus too much on just one thing, like fertility. Resource constraints, food production, economic stability, pollution, climate change, black and grey swans, pandemics, supply chain instability, local and global wars, water shortages and floods. All these things can radically change the forecast. Some of them are predictable to some extent.

@jbond Sorry but I'm not sure the point you are trying to make. The very sites you posted to me illustrate the point I made earlier; that the total global population growth rate is declining, with the sharpest declines in the most advanced economies. All that other stuff about wars and climate change is just word salad. The reason the human population has been able to grow is precisely because we learned how to overcome floods, poor harvests, wars and other challenges and there is no particularly convincing argument to be made that we will somehow just suddenly start failing at the very thing that we have been doing for the past 20 000 odd years. Will there be more challenges to come? Of course. Will people overcome them? All signs point to yes. And in the meantime population growth rate will continue to fall until we reach an equilibrium population.

@Clementulus Population growth rate in absolute terms has been almost constant for 5 decades at ~80m/year, 12-14 years for each +1b. It's the linear middle part of the S-Curve. It's only just beginning to drop.

The point about long term forecasting is that it's difficult and very dependent on models. The UN demographic models can be criticised because they don't pay much attention to socio-economic, resource and pollution constraints. Social collapse could have a *huge* effect on population.