New Ice Age explorers crossing the glaciers between New York and some other city

Along about ten or fifteen years ago I read an intriguing young adult novel with the premise that a second Ice Age had come, and buried the various cities of North America beneath a mile or so of ice. But the larger ones (New York, LA, Chicago…) had survived this and developed a bare sustainability, using nuclear power or something to maintain underground gardens and so forth. They were able to communicate with other cities by radio, but that was about it, and no one had any real idea what was going on outside the continent after hundreds of years of isolation, nor indeed much idea what went on above them on the ice.

The protagonists were a group sent out to discover why the other cities were suddenly suffering radio silence, and along the way their mission morphed into more of a "just what all is even out there, anyway?" exploration. Pretty sure they made it to, or at least heard about, the tropics for the first time, and their unglaciered temperate climes.

I have been entirely unable to search for this book effectively, since I have no idea exactly what it was called, who it was by, or even the names of the characters. Tips on managing despite these handicaps (hypnosis??) would be much appreciated, as would a solid identification of the book. (It was probably written back in the Sixties or so, when global cooling was still a thing respected scientists were worried about.)