The Baltic isn't all that warm either - half of it freezes over in winter.
The crops are of course not nearly as good as in Germany, but you cant find similar area from the same latitude in Russia or Canada.
https://upload.wikimedia.org/wikipedia/commons/a/a8/Liminka....
The author may be spot on with his cereal theory, but I'm not convinced by the suggested mechanism. The nascent fluid dynamicist in me doesn't see how a sea with only one tight opening can exchange enough water for a significant thermal exchange, one which would have to be pretty sizeable given the freezing. I could be wrong, but I do note that the 'gulf stream' graphic barely gets past Newfoundland, let alone showing the North Sea or the Baltic...
But usually wind and weather comes from west. So as golf current warms up the coast of Norway, some of that warmth comes here. It's a weather pattern, but I don't seem to find a good picture now, sorry. http://www.weatherwise.org/sebin/x/a/russian-inferno-photo6....
I remember from geography class that we are in this climate zone and it's called a Hadley cell. And inside that cell air circulates, but it cannot really break away. So kind of Golf course breaks into that cell from below. http://en.wikipedia.org/wiki/Hadley_cell
EDIT: sorry, lost in translation, I meant a Ferrel cell http://en.wikipedia.org/wiki/Atmospheric_circulation#Ferrel_... Ans that's not very accurate as Finland seems to be in polar front.
http://montessorimuddle.org/wp-content/uploads/2011/08/Annua...
Nowadays only gulfs properly freeze in winter but there are reports from XV century of people travelling over the ice in sleighs and even tales of inns being built on the ice. That was a little anomalous due to the Little Ace Age but even in the XX century ice floe would cover over 90% of the Sea once in a while.
Do you have any sources or links for this? This sounds really interesting, I want to read up on it!