And to your question - much better to crush dreams earlier like they do it in Europe.
On a more country wide scale, we can expect foreign migration to the U.S. to increase. Sure, some folks can migrate inwards from the coastal U.S., but what will people in coastal El Salvador or Guatemala do?
Even ignoring the absurd reduction to a small proportion of 20 somethings, the original comment was about 20 somethings in general, not just a hypothetocal mobile workforce. The majority of 20 somethings will not be able to load up their $40k sprinter van and head for greener pastures.
It’s literally the most flexible and already mobile portion of the population, and the segment most trying to establish itself (while having some useful skills). They are the ones who are most capable and who will benefit the most from disruption.
It’s the folks too young to have useful (job) skills or capital, or who are already established in a place, or who are physically frail/getting more so, that will be the most put out and hurt by these changes.
And the human race will survive, as terrible as it will be. Compared to the world wars, the expected impacts are likely to be relatively mild - albeit more widely spread.
It's extremely hard to determine cause but 100s of people just died in the heatwave in Oregon, Washington and Canada. Here in California, unbearalbe heat and noxious smoke seem like they'll be with us a third of the years (it's started now where I am and went to October last year). Doyle, Ca was destroyed a couple days ago. It's a pretty small place I hear and towns destroyed last year were also are small.
All this may be coincidence but it's coincidence that makes people think (well, the jet stream slowing is what creates extreme heat and cold aside from average temperature increase, so well, telling people "no problem isn't credible to me, an oldster even less likely to see further futures).
It’s not that these things aren’t happening, it’s just unlikely to concretely cause problems for this demographic.
If Miami get flooded it won't be the only place flooded, the housing crisis alone will bankrupt the country.