The world has changed - in the desperate try to stop the global warming, by destroying carbon dioxide, scientists released a self-replicating biological compound in the atmosphere. But the release got out of hand and soon Earth's whole supply of CO² was destroyed. Without its natural warming, Earth started to cool down within a few decades.
Without any other place to go, Humanity settles the last habitable place on Earth, the oceans. Because of the limited space in the domes deep under the ocean, billions die on the surface of a freezing world. And soon a thick layer of ice and snow covers the earth. Down in the oceans the refugees managed to adapt to their new circumstances. A lot of knowledge is lost in the first years of exile, and so only a few remember that mankind used to live on the surface beneath a deep blue sky.