Cod, for example, are not found below a latitude running roughly from New England to Spain. South of that line, warmth and low oxygen are just too great for the species.

Dr. Deutsch and Justin Penn, a graduate student, recreated the world at the end of the Permian Period with a large-scale computer simulation, complete with a heat-trapping atmosphere and a circulating ocean.

As the Siberian volcanoes flooded the virtual atmosphere with carbon dioxide, the atmosphere warmed. The ocean warmed, too — and according to the model, it began losing oxygen.

Some parts lost more than others. On the surface, for example, fresh oxygen was produced by photosynthetic algae. But as the ocean warmed, its circulatory currents also slowed, the model demonstrated.

Oxygen-poor water settled to the bottom of the oceans, and before long, the deep was gasping.

Rising temperatures and plunging oxygen must have made huge swaths of the oceans uninhabitable. Some species survived here and there. But most disappeared completely.

“Everything was losing a lot of habitat, creating the risk of extinction,” said Dr. Deutsch. “But the risk was actually higher in places that were cold. That was a bit surprising.”

You might expect that animals near the Equator would be at a greater risk, because the water was warm to begin with. But Dr. Deutsch’s model suggested a very different kind of apocalypse.