The glacial cycle was longer duration, lower fluctuation.

Glaciers don't melt instantly the moment the temperature flips from "form glaciers" to "melt glaciers". They operate on the integral of temperature -- heat. The temperature determines how fast they melt or form.

These melted really, really fast.

Without taking a position I will point out that your stated reasoning doesn't hold due to how much momentum these systems have and how slowly they respond. It seems entirely plausible that this outcome would still have happened if temperatures had plateaued several years ago.

Do you also wait until the inside of the meat is the temperature you want before taking it off the grill, then are later surprised when it is overcooked?