Perhaps Robert Silverberg's 1964 novel Time of the Great Freeze? (That was the answer to this other question.) It has its own Wikipedia page. Here is a blurb from the 1980 Ace Books edition, as quoted at Majipoor.com:
New York City
2650 AD
Underground!
Miles beneath the layer of ice that covered Earth in the New Ice Age of 2300 AD, men survive in the subterranean cities they built to save themselves as the ice crept with killing cold over all living things. For three hundred years no one has seen the surface or communicated with any other city. Until now. Now the few scientific instruments that remain seem to indicate that the Ice Age may be ending; outside temperatures are reaching a level that may make life possible — though not easy — on the outside.
But life in the underground cities is comfortable, and those few who are brave enough to be curious about the unknown frozen world above are suspect; troublemakers. A small party of these “troublemakers,” led by Dr. Raymond Barnes, with a few scientists and others who think they might prefer freedom to safety, has been allowed to take the long-unused elevator up through the ice to the outside. But they go more as exiles than as a scientific expedition; they are not expected — and may not be allowed — to return.
The following plot summary is from a review by Joachim Boaz:
The years is 2650 A. D. Three hundred and fifty years earlier a New Ice Age covered large portions of the globe with ice. Most people trekked south or died as food ran out and towns were overcome. Those that remained in New York and the other great cities of America were able to construct underground cities before the ice covered them over. The societies underground have become increasingly isolated and insular with repressive governments. Food comes from hydroponics facilities and nuclear reactors provide the heat and energy. The cities regulate births inorder to control population due to the limited resources at hand. Sadly, Silverberg devotes only a few pages to the life in underground New York — the most interesting concept in the book. And, there’s news that the ice might be slowly melting!
The narrative follows a group of characters, and our young interchangeable hero Jim, who have made radio contact with London, also an underground city. The group is soon aprehended by New York’s authorities and after a shame trial the aged mayor sentences them to expulsion. Fortunately, they were planning an expedition to the surface anyway to make contact with London. Unfortunately, they have only 12 hours to prepare before they are kicked out of the city. Due to the time constant they still manage to deck out their expedition perfectly with powered sleds, weapons, food, everything! Yes, a gigantic plot hole. At NO point in the narrative do the intrepid explorers realize that they forgot something, or need particular tool, etc. After 300+ years underground I suspect most societies would completely forget how to equipped a trans-Antarctic sort of expedition!
