If we polled climate scientists for the weirdest thing we learned by drilling into Greenland’s ice, Dansgaard-Oeschger cycles would be strong contenders. During the colder parts of Greenland’s ice age history, it has frequently experienced a wicked case of climate whiplash. Parts of Greenland could endure a warming of 10 degrees Celsius in the space of a couple of decades. That would be followed by centuries of cooling and, eventually, another abrupt warming.

Look at ocean sediment cores in the Atlantic and you’ll see something else happening at the same time: sand and stones appearing in the seafloor mud, carried there by dirty, slowly melting icebergs. (In fact, some icebergs may have made it as far as Florida.) These impressive launches of iceberg armadas are called Heinrich events—and there’s clearly some connection between Heinrich and Dansgaard-Oeschger.

We’re pretty sure these wild events relate to the downward flow of salty, dense surface water in the North Atlantic, which completes the conveyor belt that turns northward-flowing surface water into southward-flowing deep water. If the salty surface water loses its greater density, the conveyor belt—and the warmth it carries northward—seizes up. Conversely, switching the conveyor belt back on can rapidly deliver warmth northward, which may explain some of the sudden warmings.

How do you make salty water less dense? Mix in some freshwater—maybe in the form of a sudden armada of salt-free icebergs.

That explanation was tempting, but closer inspection revealed a problem—in at least some instances, the cooling appeared to have come before the icebergs. To learn more, a group of researchers led by Cardiff University’s Stephen Barker looked to an ocean sediment core going farther back, covering the last 440,000 years. The core (which has been used for other research before) was collected about 400 kilometers southwest of Iceland, in an area where the seafloor sediment piles up quickly enough for individual samples from the core to represent less than 200 years.

The researchers used the abundance of one species of foraminifera plankton to track changes in ocean temperature and compared these changes to the arrival and disappearance of iceberg-delivered sand grains. The results confirmed what had been seen in some cores covering shorter spans of time: the iceberg activity followed the start of the cooling by a few centuries, on average. The disappearance of the icebergs, however, lined up closely with the rapid warming events.

The researchers also analyzed a different ocean sediment core originating farther north—about 300 kilometers northwest of Ireland. There, the gap between the start of cooling and the arrival of the icebergs was nonexistent. It looks like the cooling phase started later. (However, it’s also true that the northern core location isn’t in the direct path of icebergs released from Canada’s Hudson Bay, while the southern core site is more likely to have seen some from that particular source. If Hudson Bay icebergs set out earlier than others, it could confuse this picture.)

So what gives? The answer probably has to do with the boundary between colder polar water and warmer water from the south, the researchers say. That boundary currently hugs close to Greenland, but it would have wandered south in colder times, passing over the site of the more northern core before reaching the waters off Ireland.

Rather than a jamming of the Atlantic conveyor belt by a huge delivery of freshwater icebergs, the researchers suggest a different scenario. It starts with a gradual cooling in the North Atlantic—there are several possible reasons for the conveyor belt to slow and do this—which would have allowed the polar water boundary to drift down over the northern core site. Somewhere past that point, a tipping point in the strength of the conveyor belt could have caused it to rapidly drop into an alternate mode, with the downward mixing of water taking place farther south. That would also bring the polar water boundary south to Ireland, and do so much more suddenly.

How would that cooling create icebergs? Counter-intuitively, rapid cooling in the North Atlantic was bad news for ice shelves. A bit below the surface, the water would actually become warmer than it had been due to the changes in ocean circulation. That would include the water at the base of ice shelves, leaving them melting away from beneath. Without ice shelves, glaciers would release lots of dirty icebergs.

It’s possible, the researchers say, that those icebergs provided a feedback that lengthened the cold spell by making it hard for the conveyor belt to resume its prior configuration. But when it did resume, it kicked back in very rapidly, and strongly, too, leading to the whiplash warming felt across the North Atlantic as the polar water boundary retreated northward.

From there, the wild Dansgaard-Oeschger cycle could repeat, with the Atlantic conveyor belt flipping back and forth between two configurations, unable to permanently settle into either.

Studies like this allow scientists to test hypotheses about the causes of past climate changes—even ones as weird as the Dansgaard-Oeschger events. There’s nothing about sailing the North Atlantic today that makes it obvious why those events happened the way they did. Without going after these geologic records, we wouldn’t know they had ever happened in the first place. Heck, we might not even realize they were possible.

Nature, 2015. DOI: 10.1038/nature14330 (About DOIs).