I may be wrong, but I think the world population is growing more quickly than you portray, and will be for some decades yet, and we currently have far too many people for how we conduct ourselves as a species.
I may also be wrong to think it, but I would expect if you halved the world population, assuming it was done equally across the world, you would indeed halve human carbon emissions; half as many people, half as many homes, cars, power stations, etc.
Based on current trajectories, the world's population is not going to double again (or even increase by 50%), it will level off at about 10 billion and at that point perhaps start to shrink. It's possible this level is not sustainable with an acceptable standard of living, but it's not obvious and it certainly isn't inevitable that humanity will reproduce out of control.
The planet can't cope now - the environment is falling apart right now, already - and the mid-range estimate there's another two or three billion people to come over the next few decades.
There are estimates in excess of this, there are estimates lower. Estimates which have population declines see slow declines only.
I've not seen any real understanding of why the fertility rate is dropping, particularly in first world countries. That's a concern; what happens if the factors causing this to happen are transient?
That's part of why the estimates are only estimates, of course.
Can you provide a source for the mid-range estimate you mention? Global population is on the brink of decline rather than further growth. [1]
> I've not seen any real understanding of why the fertility rate is dropping, particularly in first world countries. That's a concern; what happens if the factors causing this to happen are transient?
The cited article discusses several potential causes and none are going away in the near future. On a global basis, humanity's current demographic profile locks in decades of either very low growth or a decline in most regions. Worth noting that these changes have all come about at a much faster pace than even the most aggressive estimates.
Despite this as our overall numbers will start shrinking, resource consumption will grow and its growth can't be prevented by force without conflict. The only class of realistic solutions to environmental degradation are technological ones. Tapping into new stockpiles (likely out in space) and increasing efficiency slash decreasing resource intensity of economic activity are more viable if one aims to avoid armed confrontation.
I may also be wrong to think it, but I would expect if you halved the world population, assuming it was done equally across the world, you would indeed halve human carbon emissions; half as many people, half as many homes, cars, power stations, etc.