Introduction
The Earth’s climate has changed dramatically in the past. It has swung in and out of ice ages, at whose peak great swathes of North America, Europe and northern Asia were covered in sheets of ice three kilometres thick. It has been through periods of extreme heat, where subtropical climates existed in high northern latitudes. The height of the oceans has changed by more than a hundred metres.
But human civilization has seen few of those changes. Over the ten thousand years or so in which our civilization emerged, the Earth’s climate has been unusually stable. Global temperature and sea levels have hardly varied. We have taken advantage of this period of stability to grow crops, build cities, and develop a global economy.
