All at once or by dribs and drabs. It makes a difference right?

In the 13th century when the climate in Europe went bad, the population declined 50% in 100 years. Those were hard years but society pulled through.

Call that "managed retreat".

Infinitely preferable to 1.5 Billion people starving by 2030. Possibly taking out billions more if they go down fighting. Causing a rapid collapse that leaves a few hundred million by 2100.

Living in the shores of the Arctic Ocean as hunter gatherers for the next 5,000 years.