The Greenland ice cores are a proxy for average temps of the Earth. If average temps in Greenland rise 18 C it’s pretty much a certainty they’ve risen quite a bit everywhere else though not necessarily the same amount.
Clearly, you know more about this subject than I do.
However...
The total change in average temperature in the last glacial was 11 degrees, not 18 degrees.
Also, the Bering Strait was iced over, which meant a much larger volume of warm tropical water was rushing straight at Greenland and the north Atlantic.