When were global temperatures as high as today?

The geological record contains numerous intervals in which climates were at least as warm as the present day. These intervals can only ever act as partial analogs for current and future warming scenarios as the cause of 20th/21st century climate forcing are unknown in Earth history. In this chapter those warm intervals that are most commonly referenced by the Intergovernmental Panel on Climate Change (IPCC) are discussed. These include: the Palaeocene-Eocene thermal maximum, the Eocene climatic optimum and interglacial stages of both the mid-Pliocene and the Pleistocene. Warmer intervals that occurred in the current interglacial, such as the Medieval Climate Anomaly, are also considered. A review of these intervals highlights the uniqueness of the current climate crisis whilst also identifying ways in which their study can inform our understanding of the response of the Earth system to enhanced warming.