Cold Faith - Webb Shaune Lafferty

A protracted volcanic winter has devastated Earth, leaving only isolated pockets of survivors. With food becoming scarcer and the air growing toxic, Rab sees just one chance for survival: a perilous journey north in search of a fabled city where ships were rumoured to have been launched, ferrying the people of Earth to a salvation planet. But Rab didn’t count on being forced into taking the last three children from his dying village, or on coming across the hostile and sceptical Sunny living with her grandfather in comparative luxury among a large band of refugees in an underground sanctuary

When her grandfather begs to join Rab’s party, Sunny, to Rab’s dismay, insists on coming with them. Years ago, Sunny had been the lone survivor of a similar quest. The reason behind the failure of that quest is something Sunny has never revealed, not even to her grandfather. And Rab is now to discover why.