Learn German: Das Meer ist hier wärmer als in Deutschland. - The sea is warmer here than in Germany.

Many short, common German adjectives that contain an a, o, or u take an umlaut when they change into the comparative (the "-er" form). This is why warm becomes wärmer (warmer). You will also see this rule in words like alt becoming älter and groß becoming größer.

Why do we use als here? I thought it meant "when" or "as".

You are right that als can mean "when", but in comparisons it means "than". Whenever you use a comparative adjective to say two things are unequal, you connect them with als. If two things were exactly the same, you would use so ... wie (as ... as) instead.

Could I change the word order and say "Das Meer hier ist wärmer"?

Yes, you could! Saying Das Meer hier groups "the sea here" into one big subject. In our sentence, however, hier acts as an adverb describing where the sea is warmer. In German, it is very common for adverbs to sit right after the conjugated verb (ist).

I sometimes see die See used for the sea. Why does this sentence use das Meer?