How do scientists line up Greenland and Antarctic ice to read abrupt climate jumps?

Ice cores from Greenland and Antarctica don't tell the same story, and the difference is where the interesting science lies. Greenland shows sudden jumps; Antarctica shows slow, much smaller swings. Reading them together shows how the climates of the two hemispheres are linked.

The jumps are called Dansgaard–Oeschger events, and 25 of them occurred in the last glacial period. In one, about 11,500 years ago, Greenland's average annual temperature rose around 8 °C in 40 years. The Vostok core in Antarctica was drilled before the Greenland cores, and D-O events were not widely recognised until the Greenland cores GRIP and GISP2 were done; Vostok was then re-examined to see whether they had been missed. Antarctic cores suggest the events are related to the so-called Antarctic Isotope Maxima through a coupling of the hemispheres called the polar see-saw.

To compare two cores, scientists need a common clock. Within one hemisphere, volcanic layers can synchronise ice records, but connecting hemispheres is harder. A geomagnetic reversal about 40,000 years ago, the Laschamp event, can be identified in cores, and methane measurements can connect the chronology of a Greenland core with an Antarctic one.

Not everything is settled. Some scientists say D-O events recur at multiples of 1,470 years, but this is debated: the older parts of the GISP2 core do not show the regularity, nor do the same events in the GRIP core. The processes behind them are still unclear, though the events appear to reflect changes in North Atlantic circulation.