Heresy!
According to the Algore Cult's Doctrine of Global Warming, if the climate is warmer, the sea levels must be higher. Greenland's glaciers melting, all that fol-de-rol.
How can it be? How CAN it be? The sea level went the wrong way.
The land was higher in that place; decline in elevation was (last I knew) continuing in the Netherlands. The climate was warmer in medieval times than it is now, leading to agriculture at higher latitudes and altitudes than is possible today — but the sealevel is known to have been higher relative to some ports in eastern England, and in some other places here and there, probably meaning that there was less ice in the ‘caps and more water in the oceans.