Discworld Book 6: Wyrd Sisters

The Sumner family can read the signs: the droughts and floods, the blighted crops, the shortages, the rampant diseases and plagues, and, above all, the increasing sterility all point to one thing. Their isolated farm in the Appalachian Mountains gives them the ideal place to survive the coming breakdown, and their wealth and know-how gives them the means. Men and women must clone themselves for humanity to survive. But what then?