Ok, there's this 70s/80s movie where earth's orbit has changed and it's moved closer to the sun. I don't remember much else but I do remember it getting hotter and hotter and near the end people start melting and then become talking puddles on the ground.

I also think there's a group of survivors in the sewers and they are protected for a while because it's cooler down there. Not 100% sure about the last bit though, could be from a different movie. Thanks for your help in advance