That's a throw to a single notion of possible change that was current at the time the cartoon was drawn up.
The reason for the retreat of the ice sheets remains elusive, however.
Whereas there was a change in the relative strength of the sun roughly 20,000 years ago thanks to variations in the planet's orbit, it was smaller than changes that preceded it and failed to trigger a melt.
From a Scientific American article of 2012 that discusses another paper with an alternative notion of cause:
I agree with you in some sense. That is one of the better illustrations but it isn't very compelling.
In fact, one telling point is that it demonstrates that in 20,000 BCE industrialisation and rapid global warming of 4 degrees would have ushered the world into the unthinkable horrors of ... somewhat better than the present day because they'd have consumed more energy than we have. Change is clearly not automatically a bad thing.
The point is the slope of the graph, not the position. Yes, climate changed in the past, just never so quickly, and by quick, we mean really really quick (in comparison to previous warming and cooling cycles). We really are in unknown territory here, and yes, things might actually be OK, or maybe not so bad as we move from a planet that can sustain 8 billion humans to 2 billion humans, or maybe we all get replaced by transhuman AI in a few generations anyways that have no problem adapting. We know there will be consequences, we just argue about how severe those consequences are, and if lifestyle changes can really turn things around (climate change optimists), or are we powerless to do anything at all (climate change pessimists).
Read the “limits of the graph” section about 1/4 of the way down on the right. The graph is smoothed significantly because we simply don’t have the data resolution that people want to believe. In reality the average temps undoubtedly oscillate with changing seasonal and solar system effects across decades. Forest fires, asteroids, volcanic ash.
> ... as we move from a planet that can sustain 8 billion humans to 2 billion humans ...
Core of the issue though, isn't it? The planet can't support 8 billion humans right now. This living standards of around 7.8, 7.9 billion of them are unacceptably low IMO. So if it can't support 8 billion now and it can't support 8 billion in the future, I'd like some fairly concrete explanations of why we shouldn't be promoting cheap energy and industrialisation in the here and now. It appears that industrialisation drives down population and may even lead to transhumanism, so per capita improvements to prosperity seem to be the most viable solution to our sustainability problems. I'd also bet working on cheap energy will probably drive up the carrying capacity too, that is what happened all the other times securing energy got cheaper.
And as we can see in the Chinese example that we're talking about today, if we had just gone full-greed and kept building coal plants we'd probably have stumbled on a low-carbon equilibrium by accident anyway because that is more or less what seems to be happening in China. Coal and oil aren't that competitive these days. There is a pretty high chance fossil fuels would have been pushed into 2nd class status already if the deplorable fools in the 80s and 90s hadn't succeeded kneecapped western nuclear programs.
The nice thing about sustainable energy is that, at a certain point of economy, no one has a good reason to not prefer it over fossil fuels. So whether you think climate change is a problem or not, well, it wouldn’t matter if it were cheaper anyways. Or if people die off to a more sustainable carry load, who cares if the remaining humans are all slaves to evaporation farms and are riding around on sand worms addicted to some spice drug.
The problem is only if we get the opportunity to make hard choices for better outcomes. Do we have the self control to do that as a species, or are we doomed to act like petulant five year olds who can only think about short term benefits? Can humanity pass the marshmallow test?
But it is more of an abstract problem and requires thinking about long term rather than short term consequences.