Revealed: Secrets of Abrupt Climate Shifts

This story is the dream of every science writer. It features some of the most dramatic and rapid climate shifts in Earth’s history, as well as tenacious scientists braving the hostile ice and snows of Greenland and Antarctica for years on end to bring home that most precious material: kilometre-long cores of ancient ice, dating back over a hundred thousand years. Back in their labs, these women and men spend many months of seclusion on high-precision measurements, finding ingenious ways to unravel the secrets of abrupt climate change. Quite a bit has already been written on the ice core feat (including Richard Alley’s commendable inside story “The Two Mile Time Machine”), and no doubt much more will be.

It was the early, pioneering ice coring efforts in Greenland in the 1980s and 90s that first revealed the abrupt climate shifts called “Dansgaard-Oeschger events” (or simply DO events), which have fascinated and vexed climatologists ever since. Temperatures in Greenland jumped up by more than 10 ºC within a few decades at the beginning of DO events, typically remaining warm for several centuries after. This happened over twenty times during the last great Ice Age, between about 100,000 and 10,000 years before present.

The latest results of the EPICA team (the European Project for Ice Coring in Antarctica) are published in Nature today (see also the News & Views by RealClimate member Eric Steig). Their data from the other pole, from the Antarctic ice sheet, bring us an important step closer to nailing down the mechanism of the mysterious abrupt climate jumps in Greenland and their reverberations around the world, which can be identified in places as diverse as Chinese caves, Caribbean seafloor sediments and many others. So what are the new data telling us?

These data connect the Antarctic ups and downs of climate to the much greater ones of Greenland. This is hard, as dating an ice core is a difficult art (no pun intended). If one makes an error of only 5% in determining the age of an ice layer, for 40,000-year-old ice that’s an error of 2,000 years. But to understand the mechanisms of climatic changes, one needs to know the sequence of events – for example, one needs to know whether a particular warming in Antarctica happens before, after, or at the same time as a warming in Greenland.

To get around this problem, Thomas Blunier and colleagues nearly ten years ago pioneered an ingenious method to synchronise the ice cores of Greenland and Antarctica by analysing changes in the amount of methane in air bubbles in the ice. Changes in methane are recorded at both poles, and they should occur almost exactly in step as gases are quickly mixed through the whole atmosphere. After the ice cores are synchronised by aligning the methane variations, the relative timing of Greenland and Antarctic temperature changes can be seen.

While Blunier and colleagues were originally able to connect only a handful of large climate events, the results published today take this method to a new level by applying it to the new, high-resolution Dronning Maud Land ice core. The new data confirm with unprecedented precision what Blunier found: Antarctica gradually warms while Greenland is cold. But as soon as Greenland temperatures jump up in a DO event, Antarctic temperatures start to fall (see graph). This happens for every DO event, and it is a peculiar and tell-tale pattern that is also found in model simulations of these events (see graph).

Figure: The top two panels show idealised model DO events on an arbitrary time axis (in years), highlighting the phase relationship between Greenland and Antarctic temperatures: when a DO event hits Greenland, Antarctica switches from warming trend to cooling trend. The bottom panels show the “real thing”, the noisy data from ice cores. Note the expanded scale for Antarctica in both cases. Time here runs from left to right – normal for regular folks, but somewhat unusual for the ice core experts (my apologies to these).

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