3 ideas
When automation and ecological contraction make most labour unnecessary, society splits into a small employed caste, the Sweet, and a majority of permanently unemployed dependents, the Swill. The Swill are kept quiet on state subsistence in flood-threatened high-rise towers.
The novel is framed by scholars in a later, colder age who read the Greenhouse era's records and argue about it. Seen this way, the present is a period that future people will judge. The key question becomes what those living through it knew and chose to ignore, not whether catastrophe was foreseeable. Individual lives then read as testimony in that judgment, which produces both condemnation of the era and sympathy for the people trapped in it.
The book argues that rising seas do their real damage slowly, through unemployment, overpopulation, rationing and class hardening, not through a single catastrophic event. Because each loss is incremental, people normalize every new stage of decline and adapt instead of resisting. The disaster then shows up as a permanent change in how society is ordered, with no single moment of crisis to react to.