The explanation by whom (Scandinavians?/ or non-Scandinavian scholars in the South) can be a bit tricky, but the classical concept of climatic zones in accordance with the difference in latitude had probably been known and applied to Scandinavia for long than generally assumed.

While more can always be said on the topic, I hope that following previous posts of mine on relevant topics can at least partly satisfy OP's curiosity:

• How did the Greek explorer Pytheas describe the Norse people in the 4th century BC?
• What did Vikings think about polar night and polar day?

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The passages cited in the first post is the discussion on the alleged voyages and finds by Greek navigator Pytheas of Massalia (4th century CBE) in the North Atlantic by Greek-Roman geographer Strabo (contemporary of Augustus, around 0 CE). At least Strabo allude to the concept of climate zones in the context of interpreting Pytheas's finds in the cultivation of ancient Scandinavians.

On the other hand, the main source cited in my second linked post above is King's Mirror, an educational treatise composed in the royal court of Norway around the middle of the 13th century, at most a generation after famous Eddas and Heimskringla (the most famous compilation of the royal biographies of Norwegian kings from legendary past by Snorri Sturluson, the author of Prose Edda) were written down on parchment. It means that the extant written evidence might have been affected by the contemporary European-Christian way of thinking or knowledge to greater extant than generally assumed.