A couple of things. That was when I started seeing reports of melting exceeding accumulation. The average air temperatures were out of normal. It was also the sea water temperature anomalies of the time. They were colder than normal when everything else was hotter - demonstrating influx of cold meltwater into the Northern Atlantic. Seeing reports of the slowing of deep oceanic currents about the same time.

It was also a guess. My only expertise in glaciology is a graduate level class with an expert. But he taught me to watch for anomalies like these.

The scenario that haunts me is a sudden collapse of an ice sheet. We have evidence it has happened before. I've mentioned before the story of Doggerland. (https://en.wikipedia.org/wiki/Doggerland) It's a cautionary tale.