Although our knowledge of the Earth’s climate becomes less and less detailed the further back we try to probe, two facts are clear. Firstly, the climate has changed on a wide range of timescales, in response to natural process we only partially understand. And secondly, despite this, the Earth’s globally averaged surface temperature has remained within a relatively narrow range for most of its history. Over the past 10 000, as human civilization has arisen, this has been no more than 1°C. On geological timescales the range is perhaps 10 to 20°C, or roughly 5% of the mean when measured on the Kelvin (absolute) temperature scale.

Thus the definition of equilibrium is not straightforward. Nevertheless, it will make sense to firstly examine the Earth’s climate in an equilibrium state before we more closely examine its current disequilibrium. Changes on longer timescales will be briefly discussed in the final section of this chapter.