Early-Warning Signals for the onsets of Greenland Interstadials and the Younger Dryas-Preboreal transition

Abstract

The high-frequency band of the $δ^{18}\mbox{O}$ variations in the North Greenland Ice Core Project displays fluctuation levels that increase as one approaches the onset of an interstadial (warm) period. For some of the events it is possible to establish statistical significance using Monte-Carlo simulations with a non-parametric null model with random phases and the same spectral density as the $δ^{18}\mbox{O}$ record during the stadial periods. Similar results are found for the locally estimated Hurst exponent for the high-frequency fluctuations, and it is therefore natural to interpret these findings as so-called "critical slowing down" signatures, i.e. early-warning signs of tipping points. The observed "slowing down" is found to be similar (and perhaps even stronger) in the Younger Dryas, suggesting that there are some similarities between mechanisms of the Younger Dryas-Preboreal transition and the onsets of the Greenland interstadials. It is also verified that the temperature fluctuations during the last glacial period are characterized by long-range dependence, where the stadial periods can be approximately modeled as a $1/f$-noise. Persistent processes can take shape if the physical signal is an aggregation of several different processes, where each process responds to perturbations on a certain characteristic time scale. The results are consistent with the hypothesis that both the onsets of the Greenland interstadials and the Younger Dryas-Preboreal transition are caused by tipping points in dynamical processes with characteristic time scale of the order of decades, and that the variability of other processes on longer time scales mask the early-warning signatures in the $δ^{18}\mbox{O}$ signal.