Nevertheless, questions remain. The problem with global warming is that it is largely something that will happen to people not yet born. What is the value of a life ruined by climate change 150, 250 years hence compared with the value of a life (perhaps ruined by Aids or malaria) today? Economists use a technique called discounting to weigh up the relationship between future and present costs. Is a life lost to flooding in 2108 worth as much, or half as much, or a tenth as much as a life lost today for want of better Aids research or new drought-­resistant GM crops for Africa? The likely effects of climate change and the lengths to which we should go to combat it remain complex and largely unknown.

Optimism is unfashionable, but let's say we manage to mitigate the worst effects of climate change. Let's say it will be awful, but not quite the end of the world. What then? If we manage to keep the nukes at bay and adapt to a warmer planet, could our civilisation ­survive?

To answer this we need to define our civilisation. It is a curious thing, unlike any in history. It has no centre, unlike that of Rome, and no single ethos or tradition. Loosely, we may talk of an occidental civilisation that takes the traditions of the ancient Mediterranean and grafts upon them the ideas of the modern European and especially British Enlightenment. It has spread to include almost every­where, even those places that vociferously reject the very symbols of that civilisation's triumphs.

Our civilisation is unique in its size and complexity; Rome, at its height, commanded perhaps 90m people across a land area rather less than that of the United States. Our world, tied together by myriad threads of technology, culture, shared assumptions and ambitions, encompasses billions and touches all continents.