Glacial abrupt transitions in ocean circulation were much slower than ice-core records imply

Dansgaard-Oeschger (DO) events during the last ice age are thought to reflect rapid shifts in the Atlantic Meridional Overturning Circulation (AMOC). Greenland ice cores suggest warming transitions within only a few decades—or even just over 10 years. Using precisely dated speleothem oxygen-isotope records from the Asian monsoon region, we have shown that the same DO warmings unfolded over more than a century. Isotope modeling revealed that Greenland records primarily capture a rapid atmospheric reorganization triggered by the initial sea-ice retreat in the northeastern North Atlantic, whereas the parallel AMOC recovery and hemispheric thermal adjustment proceeded much more slowly. These findings reveal that natural AMOC transitions evolved more slowly than the rapid North Greenland oxygen-isotope shifts would imply, thereby refining our understanding of “abruptness” during DO events.