Abstract: What critics and publishers now call climate fiction is a growing genre that captures significant critical attention. This essay theorizes the relation between two of the genre's features, one formal and one political, in the scale frame they address: the planetary scale of climate, or what many scientists call the Earth system. The archive that articulates these features consists of a popular, digital discourse about climate fiction, much of which appeared since Hurricane Sandy. Instead of looking at individual works or narrating their history, I read this popular "metagenre" to draw out its implicit theory of the genre. According to the metagenre, climate fiction's abstract features consist of a didactic purpose and a normative form. Climate fiction's purpose, whatever its social effects, is ultimately to contribute to planetary climate stability. Second, the genre's definitive form is (or should be) extrapolative realism. Extrapolative realist narrative builds on scientific consensus to imagine plausible futures. The striking thing about climate fiction is that its purpose and form exist in a contradictory or inversely proportional relationship. If the genre fulfills its goal by contributing to the equilibrium of the climate, then its verisimilitude will diminish. If the climate continues to destabilize, then the genre's realism will have been vindicated at the expense of its purpose. Climate fiction is unique because it promises extrapolative realism in the content of individual novels and films but does so in the constitutive and paradoxical presence of a goal that would prevent such future climates from materializing. The point of analyzing climate fiction's constitutive paradox between purpose and realism is not to revel in irony, leave the final word to ideology critique, or dismiss the popular aesthetics of climate fiction. Rather, this paradox can be generalized to the worldview or grand narrative of the Anthropocene: the contradiction between climate fiction's purpose and form sheds light on a more general temporal structure bound up with technocracy and scientific legitimation.