Climate changes naturally all the time, partly in predictable cycles, and partly in unpredictable shorter rhythms and rapid episodic shifts, some of the causes of which remain unknown. We are fortunate that our modern societies have developed during the last 10,000 years of benignly warm, interglacial climate. But for more than 90% of the last two million years, the climate has been colder, and generally much colder than today.