Very intersesting map. I like it a lot, and I'd play on it often. Although...
As many others have suggested, make a bonus for holding all the islands. Also, the 'frozen lands' (Antarctica and perhaps Greenland) should be smaller.
A question about fortifying. I "assume" that no body can cross the uninhabitable zone, yes? That is fair, and logical, however, I think that fortifications should be allowed to cross, because they are not going to stay put ON TOP of the uninhabitable zone, just passing through it (rather quickly to avoid melting their rifles

)