A good post but not everyone who considers "overpopulation" is racist. I don't recall the Swedish eugenicists using the term "untermenschen" at any point in the 40 years they practiced it but I suppose it's possible.Ageing? What lunacy is this? The planet is billions of years old, and I don't think that human activity is going to account for much in geological timescales. In another billion years, the only residue of human existence will be some weird chemistry in a thin layer of rock, some artificially cut diamonds, and possibly some eroded deep space junk.
The global climate is a changeable, and surprisingly volatile thing. The fossil record suggests that it is prone to chaotic behaviour, and tipping into new states at short notice.
The next point is that the triggers for these tipping points are not well understood. Sometimes catastrophic events, like huge volcanic eruptions, (Deccan Traps) sometimes slower things like changing oceanic currents.
Next, humans. Humans have been having landscape level environmental impacts almost from the point we mastered fire. Populations were tiny, and remained so until recently.
Industrial human activity has put so much GHG in the atmosphere in a geological eyeblink, that the effects are only starting to feed through now. We could (and probably have) hit one of those climate tipping points, and just haven't noticed yet.
The human population is not so much huge, as just really badly distributed, and very badly managed. It is concentrated in coastal and river valley areas, where there is access to ocean resources and fresh water. These are also the areas most vulnerable to drought, flash flooding, and rising sea levels.
If you took the whole global land mass, and spread the population evenly, they would have trouble seeing each other.
The overpopulation issue is, as I have said, usually a fig leaf for the usual tired old racism. If you were really worried about overpopulation, then you should be clamouring for the introduction of the global Chinese one child policy and enforced sterilisation of the untermenschen. How very 1930's eugeneticist.
What you SHOULD be worried about is the the enforced migration of millions of people because their local environment is too degraded and resource poor to support them.
Eugenics scandal reveals silence of Swedish scientists