A look at the history of our planet‘s climate reveals an interesting phenomenon: as it gets warmer in the region around the North Pole, it tends to get colder around the South Pole. It is almost as if the Arctic and Antarctic are sitting on a seesaw that swings back and forth. Correspondingly, climate researchers call this opposing temperature behavior the „bipolar seesaw“. However, an international study, in which the Alfred Wegener Institute took part, now shows that in the past glaciers in the north and south retreated at the same time challenging the bipolar seesaw concept. The results also show that these phases are characterized by extreme climate changes, as we are experiencing today. The study was published in the scientific journal Nature Geoscience.
The bipolar seesaw is a well-established concept, set in motion by the Atlantic Meridional Overturning Circulation (AMOC), which is also known as the oceanic conveyor belt as it transports warm water from the tropics to the north at the surface and cold water in the depths to the south. “During the last cold period, large parts of the North American ice sheets broke off abruptly, causing icebergs to drift into the North Atlantic and melt there. This cooled the seawater and made it less salty,” says Dr Vincent Rigalleau from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). As a result, the AMOC became weaker and could no longer distribute the heat in the ocean evenly: While the northern hemisphere cooled as a result, the heat accumulated in the southern latitudes. These extremes are known as Heinrich events. “During these events, the bipolar seesaw concept explains the difference of temperatures between the hemispheres with important shifts in both atmospheric and oceanic circulation,” says Vincent Rigalleau.
The AWI geologist was part of an international research team led by the French marine research institute Ifremer that investigated how glaciers in the northern and southern hemispheres developed during these past Heinrich events. For this, the researchers reconstructed 70,000 years of glacial history in the Southern Hemisphere using a four-meter-long sediment core that they excavated off the coast of New Zealand.
Geochemical fingerprints reveal the origin of New Zealand's glaciers
New Zealand is located in the transition area to Antarctica and shows particularly well how the changes there spread northwards. The researchers therefore took geochemical fingerprints of the sediment core produced by erosion and glacial transport of New Zealand glaciers and analyzed where the material came from, under what conditions it was deposited and what climatic conditions prevailed, and accurately constrained the exact timing of glacier retreat. The team also analyzed coastal and river sediments in order to narrow down the signatures of the source regions. This enabled them to trace New Zealand's last glaciation from the source to the depression.
The researchers compared the results on the development of New Zealand's glaciers with studies on their European and North American counterparts and were surprised to discover that there were periods in which glaciers in both hemispheres retreated at the same time contrary to what was thought before, having implications for global sea-level, albedo and carbon budget. Especially during Heinrich events, that is, when the northern hemisphere was suddenly cooling. What the researchers also discovered was that although there was localized cooling in the North Atlantic, each Heinrich episodes were preceded by global warming. According to the study, this global warming is a prerequisite for the AMOC slowdown to override the seesaw effect.
The fact that the New Zealand, European and North American glaciers melted simultaneously contradicts the idea of the bipolar seesaw, a concept based mainly on ice analyses from Greenland and Antarctica. “Our results show how complex, sensitive and interconnected the Earth's climate system is,” explains Samuel Toucanne, lead author of the study and researcher in marine geosciences at Ifremer. A better understanding of these past climate mechanisms is important in order to refine current prediction models and assess the effects of global climate change in connection with human activities.
In addition to Ifremer and the AWI, the New Zealand National Institute of Water and Atmospheric Research, the University of Queensland, the University of New South Wales and the French Centre national de la recherche scientifique are also involved in the study.
Original Publication
Toucanne, S., Vázquez Riveiros, N., Soulet, G. et al. Synchronous bipolar retreat of mid-latitude ice masses during Heinrich Stadials. Nat. Geosci. (2026). https://doi.org/10.1038/s41561-025-01887-x