Bore hole thermometry indicates that the estimated temperature shift at Summit (Greenland) from the Last Glacial to Holocene times was ~50% higher than the value obtained using the traditional isotopic Δ(T) thermometer calibrated with today's data. A mechanism offered by Boyle [1997> to explain the anomalous ice core Δ shift is examined. The mechanism, called the Boyle Effect, is theoretically possible but if it operated at Summit, it appears to be only capable of accounting for slightly more than half of the Summit Δ 18O anomaly of ~4.3%. A new semi-quantitative mechanism is proposed that could account for the remaining part of the anomaly. Other mechanisms have been advanced but they are not so amenable to quantitative application and testing. ¿ 2001 American Geophysical Union