High-resolution data from the GISP2 ice core, Greenland, and the Byrd ice core, Antarctica, covering the Younger Dryas interval (YD) and adjacent times, modified slightly from ref. 37. (a) Byrd ice-isotopic data (6, 38, 39). (b–e) GISP2 data mostly from ref. 8, with the accumulation and temperature originally from ref. 40. (b) Temperature converted from ice-isotopic ratios (41) using the glacial-interglacial calibration of ref. 12, shown in °C. (c) Accumulation in m ice/year. The estimate of temperature in the Younger Dryas from (8) is plotted as a circle; uncertainties of about ± 3°C are not much larger than the symbol. (d) Methane data (parts per billion by volume) from ref. 23 (□) and ref. 8 (x). (e) δ¹⁵N data in per mil from ref. 8, with ○ from their figure 2 and x from their figure 4. (f and g) GISP2 sodium (f) and calcium (g) data from ref. 17, normalized to their mean concentration in the millennium before the Little Ice Age following ref. 36 by using the accumulation-rate estimates of ref. 9. Most of the ice-core data, and many related data sets, are available on The Greenland Summit Ice Cores CD-ROM, 1997, National Snow and Ice Data Center, University of Colorado at Boulder, and the World Data Center-A for Paleoclimatology, National Geophysical Data Center, Boulder, CO, www.ngdc.noaa.gov/paleo/icecore/greenland/summit/index.html.