If Atmora once had a green temperate climate before it became a frozen wasteland somehow then why did Skyrim never have a nice temperate environment? When the Atmorans came to modern day Skyrim it was frozen while Atmora wasn't so Skyrim couldn't have changed and also there were Snow elves living in that area at the time.

It's interesting the Snow Elves have this massive Edenic garden right where the rest of the country is frozen wasteland. It's possible that Skyrim used to have very beautiful lush territory before the Snow Elves were killed and it fell to ruin. After all, Saarthal is in the middle a frozen wasteland which is an odd place to put a city when there's plenty of nicer territory nearby.

Maybe but when the Atmorans landed they called them Snow elves right from the start