As deserts encroach on fertile land, as it has near Dunhuang, China, people will be forced to move towards the poles (Image: George Steinmetz/Corbis)

Explore an interactive map of the world warmed by 4 °C

ALLIGATORS basking off the English coast; a vast Brazilian desert; the mythical lost cities of Saigon, New Orleans, Venice and Mumbai; and 90 per cent of humanity vanished. Welcome to the world warmed by 4 °C.

Clearly this is a vision of the future that no one wants, but it might happen. Fearing that the best efforts to curb greenhouse gas emissions may fail, or that planetary climate feedback mechanisms will accelerate warming, some scientists and economists are considering not only what this world of the future might be like, but how it could sustain a growing human population. They argue that surviving in the kinds of numbers that exist today, or even more, will be possible, but only if we use our uniquely human ingenuity to cooperate as a species to radically reorganise our world.

The good news is that the survival of humankind itself is not at stake: the species could continue if only a couple of hundred individuals remained. But maintaining the current global population of nearly 7 billion, or more, is going to require serious planning.

Four degrees may not sound like much – after all, it is less than a typical temperature change between night and day. It might sound quite pleasant, like moving to Florida from Boston, say, or retiring from the UK to southern Spain. An average warming of the entire globe by 4 °C is a very …