4-10C warmer on average globally.

The mean arctic circle temp was around 6-7C, so pretty damn warm compared to today.

A little bit of reading suggests the arctic during the winter would've gotten down to around -10C so definitely cold enough to require adaptation, but also warm enough to be fully survivable.

Most of these finds evidencing year-round arctic-dwelling dinosaurs is being found in modern day Alaska and Alberta, but there's also similar finds from the other side of the world too!