We have the examples of the U.S. border, Australia's camps on Naru, the various permeant refugee camps spread across the Eastern Mediterranean and many more as examples.
As the situation becomes more intense, migrants will become more desperate and bold. The reaction from the wealthy enclaves will become more and more repressive. (Who themselves will be seeing a diminished standard of living).
In my opinion, the only thing that could spare us this future is a global commitment to universal rights and more equal resource distribution. But this would require countries like the United States to write rights of non-citizens deeply into their constitutions. And that is likely unattainable at this time.
Before we had international borders, plenty of humans lived migratory lives as well.
Edit:
In case anyone thinks this is just snark, no. I'm in the PNW and I grew up in Georgia and lived in California for a lot of years, including the High Desert where I was south of Death Valley and 115 in July was not unusual. I'm dirt poor and have been for years. I spent years homeless.
The heat wave per se is not a big deal to me. But it's late in the month and I'm nearly broke.
If the heat kills me, it will be because I'm out of funds, not because 107 is a big damn deal per se. The world has spent a lot of years making it clear to me that it doesn't care about my welfare at all and no amount of virtue, hard work, yadda is ever enough to adequately remedy my problems.