Time of the Great Freeze

Time of the Great Freeze by Robert Silverberg. mass_market edition. ISBN: 9780441811908.

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.