The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives1. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa2, 3, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw4, 5, 6. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events7, 8, 9. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision2, 3, 10. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Difference between gas age and ice age (Δage) at WAIS Divide. (802 KB)
a, Comparison of WDC Δage with other Antarctic cores. Ice core abbreviations: EDC, EPICA Dome Concordia; EDML, EPICA Dronning Maud Land; TALDICE, Talos Dome; WDC, WAIS Divide. Δage values are taken from refs 23, 63, 64, 65. The vertical axis is on a logarithmic scale. b, Δage uncertainty bounds obtained from an ensemble of 1,000 alternative Δage scenarios; details are given elsewhere23. A Δage scenario obtained with an alternative densification model (ref. 39 instead of ref. 38) is shown in blue. c–e, Histograms of the 1,000 Δage scenarios at 20 kyr bp (c), 40 kyr bp (d) and 60 kyr bp (e); stated values give the distribution mean ± the 2σ standard deviation.
- Extended Data Figure 2: Determining the timing of the abrupt DO transitions. (625 KB)
a, b, DO 17.2, 17.1, 16.2 and 16.1 (from oldest to youngest41) as recorded in NGRIP δ18O (a) and WDC CH4 (b). Horizontal orange bars denote pre-transition and post-transition levels; the transition midpoint (50% of signal amplitude) is indicated by a red dot; the 25% and 75% signal amplitude markers are indicated with blue dots. c, Comparison of WDC CH4 (grey) with EDML CH4 (orange)3, 50, 66. d, Hypothetical gas-age distribution for WDC due to firn densification and gradual bubble closure, using a truncated log-normal distribution67. e, Shift in transition midpoint induced by filtering of the atmospheric record in the firn column.
- Extended Data Figure 3: Cropping of individual records in the stack to prevent overlap of events. (406 KB)
a, DO/AIM 12, where no cropping is needed. b, DO/AIM 17.1, where the most cropping is needed. Full time series with five-point running average are plotted in grey, and the contributory records are plotted in blue and orange for NGRIP and WDC, respectively. The yellow vertical shading bar in background shows the NH lead time (200 years); the purple rectangle gives the −1,200 to +1,200 time window.
- Extended Data Figure 4: Number of records and fitting procedure. (346 KB)
Number of contributory records to the WDC δ18O stacks for abrupt NH warming (interstadial onset) (a) and for abrupt NH cooling (interstadial termination) (b). Blue rectangles indicate the time window over which the fitting procedure evaluates the fit to the data (−600 to +700 years); shaded vertical yellow bars show NH lead time.
- Extended Data Figure 5: Evaluating the performance of the breakpoint detection algorithm. (704 KB)
a, Breakpoint detection as a function of data window size using both linear (blue) and quadratic (orange) functions, compared with the BREAKFIT algorithm48 (grey dots with 1σ error bars). The data window is applied symmetrically, meaning that equal numbers of years (half the window size) are used before and after the detected breakpoint. Data falling outside this window are ignored in the fitting procedure. b, Root mean square deviation between the WDC δ18O stack and the fitting curve.
- Extended Data Figure 6: Alternative stacking of AIM events. (626 KB)
a, Stack of NGRIP δ18O (blue), WDC CH4 (green) and WDC δ18O (orange with fit) for just the major AIM events (4, 8, 12, 14 and 17), aligned at the abrupt NH warming. b, As in a, but for only the minor AIM events (3, 5.1, 5.2, 6, 7, 9, 10, 11, 13, 15, 16 and 18). c, As in a, but for eight randomly selected DO/AIM events. d, As in c, but aligned at the abrupt NH cooling. Events are averaged with their original amplitudes and normalized after stacking for convenience of visualization. e–h, Histograms of NH lead time associated with a–d, respectively, generated by binning the 4 × 105 solutions from the sensitivity study. The distribution mean and 2σ uncertainty bounds are listed in the panels. Shaded vertical yellow bars (upper panels) show NH lead time.
- Extended Data Figure 7: Timing of sea-salt sodium. (416 KB)
a, Lagged correlation between NGRIP δ18O and the time derivative of WDC δ18O (orange), and between NGRIP δ18O and the time derivative of WDC ssNa (grey). The dots indicate the maximum (anti-)correlation at 167-year and 229-year NH lead for WDC δ18O and ssNa, respectively. A fourth-order Butterworth bandpass filter with a 500–10,000-year window is applied to the time series to isolate the millennial-scale variability. b, DO3–18 stack of NGRIP δ18O (blue), WDC CH4 (green), WDC δ18O (orange) and WDC ssNa (grey), aligned at the midpoint of the DO warming signal. The estimated breakpoint in the stacks (dots) occurs at t = 218 and 195 years for WDC δ18O and ssNa, respectively. c, As in b, but for the abrupt NH cooling events, with the estimated breakpoint at t = 208 and 199 years for WDC δ18O and ssNa, respectively. Shaded vertical yellow bars show NH lead times.
Extended Data Tables
- Supplementary Data 1 (4.5 MB)
This file contains the WAIS Divide d18O, CH4 and ssNa data.
- Supplementary Data 2 (27.5 MB)
This zip file contains the computer code (in Matlab) used in the analyses presented in our paper. It also contains a "readme" file that has some more information. All these files are needed to run the code.