Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Figure 4.7. Records showing characteristic climate changes for the interval from 65,000 yearsago to the present. (Top) The Greenland Ice Sheet Project (GISP2) δ 18 O record (Grootes et al.,1993; Stuiver and Grootes, 2000), which is a proxy for air temperature, with more positive valuescorresponding to warmer temperatures (Cuffey and Clow, 1997). Numbers 1–18 correspond toconventional numbering of warm peaks of Dansgaard-Oeschger oscillations. (Bottom) The Byrd δ 18 Orecord (Johnsen et al., 1972; Hammer et al., 1994), with the time scale synchronized to the GISP2time scale by methane correlation (Blunier and Brook, 2001). Antarctic warm events identified asA1, etc. Vertical gray bars correspond to times of Heinrich events, with each Heinrich event labeledby conventional numbering (H6, H5, etc.).1. The Last Glacial Maximum (19,000–23,000 years ago), when ice sheetscovered large parts of North Americaand Eurasia, and the concentration ofatmospheric CO 2 was approximately 30%lower than during pre-industrial times.Although the Last Glacial Maximum(LGM) was characterized by relativelylow climate variability at millennial timescales, it had a different AMOC than themodern AMOC, which provides a goodtarget for the coupled climate models thatare used to predict future changes.2. T he l a s t d eg l a c iat ion (11, 50 0 –19,000 years ago), which was a time ofnatural global warming associated withlarge changes in insolation, rising atmosphericCO 2 , and melting ice sheets, butincluded several abrupt climate changeswhich likely involved changes in theAMOC.3. Marine Isotope Stage (MIS) 3 (30,000–65,000 years ago), which was a time ofpronounced millennial-scale climatevariability characterized by abrupttransitions that occurred over large partsof the globe in spite of relatively smallchanges in insolation, atmospheric CO 2concentration, and ice-sheet size. Justhow these signals originated and weretransmitted and modified around theglobe, and the extent to which they areassociated with changes in the AMOC,remains controversial.136