Standing in front of the partial ruins of Rome’s Colosseum, Boris Johnson explained that a motive to tackle the climate crisis could be found in the fall of the Roman empire. Then, as now, he argued, the collapse of civilization hinged on the weakness of its borders.

“When the Roman empire fell, it was largely as a result of uncontrolled immigration – the empire could no longer control its borders, people came in from the east and all over the place,” the British prime minister said in an interviewon the eve of crucial UN climate talks in Scotland. Civilization can go into reverse as well as forwards, as Johnson told it, with Rome’s fate offering grave warning as to what could happen if global heating is not restrained.

This wrapping of ecological disaster with fears of rampant immigration is a narrative that has flourished in far-right fringe movements in Europe and the US and is now […]