The only contact I had with opossums when I was a kid growing up in Maine was in books. Children’s books, Life Nature Library, and novels set in the American South. In recent years they have become increasingly common in southern Maine and make up almost as much roadkill as gray squirrels.
Combine the presence of opossums (including the dead one I found in one of our outbuildings) and milder winters (riding my motorcycle in January and February? in Maine?) and I see evidence of it in my personal experience. Evidence of what? Climate change, coming soon to a wildlife habitat near you.