No one really knows why the Hittite empire fell. But climate change is a constant in human history. The old Romans would probably be a little surprised to see their former grain fields in Northern Africa. And I don’t think it was the coal they were burning in their power plants.
Before the Middle Ages Warming Period, there was another warming period centered around the year 1 AD. This means the Romans were able to conquer their largest land area in the Empire during this warm period when the Alps were passable and the winters mild!
Interesting to note, it got a little colder in years 400-800, and this natural climate change possibly contributed to the decline, (along with a list of 200 other things, including plague.)