Inconclusive Early warning signals for Dansgaard-Oeschger events across Greenland ice cores

Abstract. The Dansgaard-Oeschger (DO) events of past glacial episodes provide an archetypical example of abrupt climate shifts and are discernible, for example, in oxygen isotope ratios from Greenland ice core records. The physical causes and mechanisms underlying these events are still subjects of ongoing debate. It has previously been hypothesised that DO events may be triggered by bifurcations of physical mechanisms operating at decadal time scales, as indicated by a significant number of early warning signals (EWS) in the high-frequency variability of records from the North Greenland Ice Core Project (NGRIP). Here, we re-evaluate the presence of EWS by employing indicators based on critical slowing down (CSD) and wavelet analysis and conduct a systematic methodological robustness test. Our findings reveal fewer significant EWS than previous studies, yet their numbers are significant for some of the indicators estimating changes in variability. Additionally, a comparison of different Greenland ice core records also shows significant numbers and consistency for these same EWS estimators preceding a small selection of events in records with high temporal resolution. While those indicators might represent a common climate background, we cannot rule out that signals specific to the different ice core locations are captured. Estimators of correlation times were found to be less consistent and did not provide significant numbers of EWS when considered on their own. Based on these inconclusive results it is not possible to constrain the physical mechanisms underlying the DO events. Instead, our results highlight the complexities and limitations of applying early warning signals to paleoclimate proxy data.