The scale of prehistoric climate change is almost incomprehensible. The broad features of the last ice age are well known. For much of the past 100 000 years (100 kyr), tens of millions of cubic kilometres of ice were piled up on the northern continents. Where Chicago, Glasgow and Stockholm now stand there was ice over a kilometre thick. Sea levels were as much as 130 metres lower since huge amounts of water were locked up in the ice sheets, and vast areas of what is now continental shelf were exposed for tens of thousands of years. Then suddenly some 15 000 years ago (15 kya) things warmed up, briefly reaching levels comparable with more recent times, and the ice sheets began to disappear. Even then the climate took one last icy descent into the freezer. For several hundred years, conditions in the northern hemisphere suddenly plunged back to something close to the greatest glacial severity of the ice age. Finally, the climate truly changed for the better so that for about the past 10 kyr the Earth has experienced relatively benign conditions. Perhaps more than anything else, this change has enabled humankind to develop the social structures we see around us today.