During the last ice age, the northern and southern polar regions danced between warm and cold periods, but the two were out of step with each other. When the Arctic cooled, the Antarctic warmed and vice versa. To explain this polar offset, paleoclimate researchers have proposed that the answer lies in the oceanic conveyor-belt system of currents that transport heat around the globe.

A new paper by Stephen Barker of Cardiff University and his colleagues (subscription) offers evidence to support this model. That team describes a detailed record of the end of the ice age, as witnessed by plankton and geochemical proxies from a site in the South Atlantic Ocean at 41 degrees S. Those indicate that the changes there happened abruptly, just as they did in the North. Past studies of records from Antarctica had suggested that the transitions were gradual in the far South, which potentially presented a problem for the conveyor-belt model, says Jeffrey Severinghaus in a News and Views this week. The new data, however, suggests that in the mid-latitudes, the southern Atlantic see-sawed from cold to warm and back quite quickly, matching predictions made by the conveyor model.

It also points toward a mechanism for how the ice age ended. The trigger would have been the periodic Milankovitch changes in Earth’s orbit, which increased the amount of sunlight hitting the northern high latitudes during summer starting around 22,000 years ago. The extra energy started melting the northern ice sheets, thus dumping considerable amounts of freshwater into the North Atlantic. That, in turn, slowed the Atlantic Meridional Overturning Circulation (AMOC)—the conveyor belt that carries cold water from the North Atlantic into the deep ocean and eventually to Antarctica. With an anemic AMOC, the Southern Ocean warmed up and released carbon dioxide stored in the deep ocean, thereby turning up the earth’s thermostat enough to melt the northern ice sheets, suggest Barker and colleagues.

Severinghaus goes even farther. If the slow-down in AMOC did cause a massive discharge of carbon dioxide from the southern Ocean, that should give us pause, he says. Models predict that AMOC should weaken in response to global warming, which could release enough carbon dioxide to finish off some of the ice sheets left standing from the last glacial period, he suggests. Moreover, current climate projections do not take into account this positive feedback, which could amplify the warming effects of the pollution we put into the atmosphere.