During the last ice age, it sometimes suddenly became warmer. Caves shed light on the precipitation that fell then

They’re called Dansgaard-Oeschger cycles, the abrupt climate transitions from cold to warm (and back to cold) that occurred dozens of times during the last ice age. Within a few decades the climate could change drastically, with local differences in average annual temperature of around 10 degrees Celsius, after which it could take more than a thousand years for the next change.

Although changing ocean currents are often cited as a cause, much is still unclear about the cycles. German climate scientists conclude this week PNASbased on cave research, that the rapid changes were accompanied by large-scale changes in precipitation patterns.

The research into the Dansgaard-Oeschger cycles got off the ground when the Americans brought a 1,390 meter long core from Greenland in the 1960s. In 1958 they had started building a Cold War project in the Greenland ice cap: a base hidden in the ice to launch nuclear missiles. But Project Iceworm failed: the movements of the ice were so strong that the secret headquarters and the tunnels that had been built in the ice sheet were crushed.

Wafer-thin layers

Wafer-thin layers were visible in the ice core. Seasonal patterns, because summer snow melts and therefore has a different texture than winter snow. The Dane Willi Dansgaard studied the ratio of the oxygen isotopes ¹⁶Oh and ¹⁸O in every layer, and on that basis could provide an overview of the air temperature over the last 100,000 years.

In colder periods there is relatively little ¹⁸Oh in the ice. The lighter ¹⁶O evaporates more easily than ¹⁸O and is therefore common in clouds. If those clouds then rain out above land, the water on land contains relatively much ¹⁶Oh and that in the oceans a lot ¹⁸O. Normally the water from the land returns to the oceans by itself via rivers, but in ice ages this does not happen: then the precipitation that falls on land freezes in the form of ice – with the result that the ice is relatively much ¹⁶O contains, and the seawater a lot ¹⁸O.

Together with the Swiss Hans Oeschger, Dansgaard discovered that in the last ice age alone, the Weichsel Ice Age (roughly between 115,000 and 12,000 years ago), abrupt temperature rises took place regularly on twenty-five occasions, lasting on average just under 1,500 years. Those warmer periods within an ice age are also called ‘interstadials’; the cold periods are called stadials.

Within paleoclimate science, there has long been ambiguity about changing atmospheric circulations during the Dansgaard-Oeschger cycles. For that reason, the German authors decided for it PNASarticle investigating the oxygen isotope ratio of 67 caves (across all continents except Antarctica). A slightly different interpretation applies to the ratio in caves than to that in ice cores: the amount ¹⁸O mainly reflects the precipitation patterns. The less ¹⁸Oh, the heavier the fall.

In Asia, especially during the warmer interstadials, there were intense showers, the researchers write. The monsoons may also have lasted longer. In the Caribbean, too, it rained mainly during the interstadials. In South America, the opposite was true: during the interstadials it became drier. In North America, the summers in particular became wetter and the winters drier. The measurements matched a computer model.

For Europe, judging by the model, there was more precipitation during interstadials, but the relative amount ¹⁸O in the caves actually increased during that period. The researchers suspect that this is because the Atlantic Ocean near the European coast warmed up so much that the amount ¹⁸O in the seawater (and the rainwater and the cave water) rose, causing a different measurement series.

Precisely because the consequences of rapid climate change are sometimes difficult to estimate in contemporary models, it is interesting to look at comparable processes in the past, say the authors.