The Earth system has responded and will continue to respond to various external forcings (solar, volcanic and orbital) and to changes in atmospheric composition. Paleoclimate data and modelling provide quantitative information on the Earth-system response to these forcings. Paleoclimate information facilitates understanding of Earth-system feedbacks on timescales longer than a few centuries, which cannot be evaluated from short instrumental records. Past climate changes also document transitions between different climate states, including abrupt events, which occurred on timescales of decades to a few centuries. They inform about multi-centennial to millennial baseline variability, against which the recent changes can be compared to assess whether or not they are unusual.

I will present an overview of Chapter 5 of the IPCC’s AR5 which assesses the understanding of past climate variations, prior to the instrumental period, using paleoclimate reconstructions as well as climate models of varying complexity. The content of this chapter is largely restricted to topics for which substantial new information has emerged since AR4. Broad areas of focus include:

1. Greenhouse-Gas Variations and Past Climate Responses

2. Global Sea Level Changes During Past Warm Periods

3. Observed Recent Climate Change in the Context of Interglacial Climate Variability

4. Past Changes in Climate Modes

5. Abrupt Climate Change and Irreversibility