Fire Time
Poul Anderson
 Fire Time takes place on the planet Ishtar, whose peculiar orbit around three stars (Bel, Ea, and Anu) results in the "Fire Time", a dramatic increase in heat every thousand years. As the northern hemisphere heats up, large numbers of Ishtarians flee south, leading to a collapse of civilization. The presence of visitors from Earth (also engaged in their own war off-planet) raises the prospect of changing the dynamics of history.