Seminar talk by Johannes Lohmann: “Dynamics and prediction of Dansgaard-Oeschger events from Greenland ice cores”

The Dansgaard-Oeschger (DO) events of the last glacial period provide a unique example of large-scale climate change on centennial time scales. These abrupt climate changes have been convincingly connected to changes in the Atlantic overturning circulation, possibly amplified by changes in sea ice cover over the North Atlantic. Despite significant progress in modeling DO-like transitions with realistic climate models, it is still unclear what ultimately drives these changes. It is an outstanding problem whether they are driven by a self-sustained oscillation of the earth system, or by stochastic perturbations in terms of freshwater discharges into the North Atlantic or extremes in atmospheric dynamics. We address this problem with a data-driven approach focusing on ice core data. A statistical analysis of the complex temporal pattern of DO events suggests that the transitions in between cold (stadial) and warm (interstadial) phases are purely noise-induced and thus unpredictable. However, we present evidence that trends in proxy records of Greenland ice cores within the stadial and interstadial phases pre-determine the impending abrupt transitions. As a result, they cannot be purely noise-induced and are predictable to a certain degree. While we do not identify their physical cause, the observed proxy trends are likely a manifestation of the climate system reorganizing at a specific time scale. With few exceptions, each DO event forms a consistent cycle with two independent time scales seen in proxy trends, and rapid transitions in between. The complex temporal pattern, i.e., what sets the time scale of the individual DO excursions remains to be explained. Nevertheless, the identification of the dynamics underlying these abrupt climate changes are an important step in the determination of their causes.