The decline of the Chinese population is quite striking with the projection showing a 800-900m drop between its 2021 1.4b peak and the 2100 population. Just makes you wonder what the population drop will look like if you add the doom factor to this. Many advanced economies have a growing elderly population many of which are dependent on modern medicine to keep alive. You would think, if things go south they would die in mass. Many countries are already struggling to keep their medical services afloat today just think what the situation would be like in a poorer energy constrained world 50 years from now.

The thing I do wonder about is that many places obesity is the most common form of malnutrition. A reduction in food supply, even a fairly large one may not lead to a significant number of deaths through starvation in a lot of places. The obvious exceptions would be the countries that are already experiencing food insecurity so vulnerable areas such as the Indian sub-continent, the middle east and sub Sahara Africa will suffer a lack of affordable food to feeds its populace. Ironically most of those regions are marked in this graph as having the largest population increases. 

Other things to consider is how the birth rates will change in the future. There has been a strong correlation between women's education levels and the number of babies she has. As people become less educated it stands to reason women could have more babies. This would be further exacerbated if infant mortality increases and modern contraception becomes less prevalent. Also with the way the right are pushing for more traditional family structures women maybe reduced to following their historic roles i.e. becoming housewives and bearing children.

I think what is also something worth considering is which demographic, young or old, rich or poor, men or women will gain the most once the dust of doom settles. The closest comparison I can think of is the Black Death and in that scenario the normal peasant benefited with increased living standards as the elites had less labour they could exploit. Now I am not sure how women's rights will feature in all this but my feeling is there will be a regression to the historic norm. As for the oldies, there will simply be less of them in the new world so there will be a shift in power were the younger (relatively speaking) will be the ones taking on the roles of powers. My guess is heads of states will be people (men) around their 40s.

Granted everything in the last paragraph is highly speculative and I openly admit I could be dead wrong. But sometimes it is fun to stick the neck out in front of the guillotine. Worst that could happen is you are wrong and look like a plonker.