Research on past climates encompasses climate variability on tectonic, orbital, millennial, centennial and historical timescales. We use a wide range of geoscientific data to reconstruct the evolution of the climate system, including its landscapes and ecosystems, from local to global scales. Climate models are used to simulate extreme climate states in the past, in order to study impacts, mechanisms and feedbacks operating in the climate system under different climate forcings. Past climates provide a thorough test of models to project future changes in a confident way.