Were Dansgaard-Oeschger events forced by the Sun?

Abstract

Large-amplitude (10–15 Kelvin), millennial-duration warm events, the Dansgaard-Oeschger (DO) events, repeatedly occurred during ice ages. Several hypotheses were proposed to explain the recurrence pattern of these events. For example, it was suggested that DO events occurred in response to a combination of a random forcing (noise) and century-scale solar forcing. In the first part of this paper, this hypothesis is reviewed in the framework of a minimum-complexity model of DO events, which assumes that the events occurred each time a given forcing exceeds a certain threshold function. In the second part of the paper, measures of multi-modality are used to investigate the recurrence pattern of DO events in the latest part (up to 42.000 years before present) of the NGRIP (North Greenland Ice Core Project) and GISP2 (Greenland Ice Sheet Project 2) stable isotope (δ18O) records from Greenland. It is reported that the onset of DO events in both ice core records shows a maximum degree of multi-modality at a recurrence time of about 1480 years. This pattern, whose statistical significance still needs to be tested in the future, could point to a combination of solar forcing and random variability in triggering DO events.