Fig. 2: Permafrost evolution over the last 3 million years.

A Data shown are the Siberian⁶⁷ and Canadian⁶⁸ speleothem records of thaw from the continuous (dark pink and blue, respectively), discontinuous (medium purple and blue, respectively), and sporadic (light pink and blue, respectively), with gray bars indicating likely aggradation of northern permafrost ~1.3 Ma. Blue triangles indicate the timing of ice wedge casts from Alaska and Canada that suggest previous permafrost presence, and black arrows indicate the oldest extant permafrost from Batagay megaslump, Siberia (~650 ka)⁷⁵ and Canada (~750 ka)⁷¹. B Antarctic ice core carbon dioxide and methane measurements^156,157 C LR04 deep-sea oxygen isotope stack¹⁵⁸.