Here's how they came about. The initial step was Voyager asking me for a map about 6 months before publication. I'd finished Prince of Thorns long before this. Which answers another popular question: did you start with the map then write the story? No, it was an afterthought.
First I went to a now-defunct website similar to this one and checked what the coastline of Europe would look like if the sea level were to rise 100m (a melting of the ice-caps would raise the levels in this ballpark).
Then I drew a close approximation myself:
Then an artist at Voyager (can't remember who, sorry) made it pretty.
For King of Thorns they took the prettying-up a step further and got colour!
And Emperor of Thorns push the boundaries out.
Because the Voyager artist pushed the boundary out to sea in the top left (and more so in the Prince of Fools map) where I hadn't drawn anything this lead to the unceremonious drowning of all of Ireland - whereas in truth a 100m sea-level rise would leave a collection of small islands.