The Arctic is very sensitive to changes in the Earth system's climate. During the Cenozoic era, 65 million years ago (Ma) to the present, the Arctic has undergone a transition in extremes from the ice-free greenhouse conditions of the late Paleocene and early Eocene epochs, approximately 55–48 Ma, to the icehouse conditions of the Quaternary, culminating in the extensive continental ice sheets and Arctic Ocean sea ice of the Last Glacial Maximum, approximately 24–20 ka (Moran et al. 2006). (To review relevant climate terminology, see Box 1; to review the geological time scale, see material available online.)5 During the past 55 million years of global cooling, since the Palaeocene-Eocene Thermal Maximum (Zachos et al. 2001, Sluijs et al. 2006),
