The world's population should be at most 50 million.

[The Grand Nudger]

You're overlooking some rather hard facts.

1. The reason we got better at growing food - is because wee came up with better ways of turning oil into food. Take a look where all that high nitrogen fertilizer comes from - it' s petrochemically based. Consequently you are freeing up more CO2 to grow the food that' s feeding the masses. That means more CO2 into the astmosphere.

Ish, that bump was from the 30's till the 90's.  Since the 90s, biotech is the major contributor (and the trend increase is being maintained by it).  Regardless, can you see why I'm not particularly worried about people starving -because there are too many people-..when the rate of population growth is half the rate of yield increase in staple crops?  That's not counting novel arrangements or more efficient production methodologies.

(we could cut oil out of the loop entirely, but we'd have to change our eating habits and the distribution network, people will continue to starve for the same reason they do now.)

2. The limiting factor on alternative energy is demand. Have you any idea what kind of power the world consumes? Start small - look at one aluminum refining plant. It takes GIGAWATTS of electricity to operate one. You know how big an area of solar cells that would take? I had a rather sharp engineer tell me once that after doing the math - he found one aluminum refinery would take a solar array of roughly the size of Kansas. 

Demand is a consumer metric.  You need consumers.  An aluminum plant with no customers doesn't need alternative energy anymore than a rural coop with noone who will pay the diff does.

There is no downside to a smaller population.

There' s a multitude to a larger one.

Now you're just being ridiculous.