The most recent third of the Quaternary is most clearly characterized by the alternation between cold glacial periods and warm interglacials. The dominant period of these climate cycles is 100 ka (e.g. J. Imbrie et al. 1993). The terms ‘glacial’ and ‘interglacial’ emphasize the changes in ice volume and sea level associated with the appearance and disappearance of large Northern Hemisphere ice sheets (Lang and Wolff (2010) Clim. Past Discuss. 6, 2223. About eight major glacial–interglacial cycles have occurred during the past 0.8 million years (Williams in D. J. Dunkerley et al. 1993). Ice core records indicate that greenhouse gases co-varied with Antarctic temperature over glacial–interglacial cycles, suggesting a close link between natural atmospheric greenhouse gas variations and temperature (S. Solomons, ed. 2007).