evidence: Prior to that, the Earth experienced an ice age lasting for tens of thousands of years. The past million years of the Earth’s history has been characterised by a series of ice ages broken up by relatively short periods of warmer temperatures. [...] The Earth has experienced a number of periods over the past million years in which large continental ice sheets have covered much of the northern hemisphere. These ice ages are associated with a large drop in global temperatures – 4C or more below today’s levels – with much larger changes over land and in the high latitudes. [...] The fact that relatively small changes in external forcings can drive such a large planetary response during ice ages should serve as a cautionary example. This is because human emissions of CO2 and other greenhouse gases push the Earth further out of the range of climate conditions that have characterised the past few million years.