The Clouds of Saturn

The Clouds of Saturn is a science fiction novel written by Michael McCollum and published in 1991. The Clouds of Saturn is set in the future in which the future Earth becomes uninhabitable due to excessive solar activity and the population needs to be relocated to floating cities in the atmosphere of Saturn to survive. These settlements behave like Greek Polis city-states and often war against each other.

Synopsis
When the sun flared out of control and boiled Earth's oceans, humanity took refuge in a place that few would have predicted. In the greatest migration in history, the entire human race took up residence among the towering clouds and deep clear-air canyons of Saturn's upper atmosphere. Having survived the traitor star, they returned to the all-too-human tradition of internecine strife. The new city-states of Saturn began to resemble those of ancient Greece, with one group of cities taking on the role of militaristic Sparta.

Setting
At the time of the novel, floating cities are possible, suspended in the atmosphere of Saturn from enormous aerostat gas envelopes filled with hydrogen heated by fusion reactors.