Direct test for critical slowing down before Dansgaard-Oeschger events via the volcanic climate response

TL;DR

This study tests for critical slowing down before Dansgaard-Oeschger events by analyzing climate responses to volcanic eruptions, finding evidence of CSD mainly before cooling transitions, and highlighting limitations of statistical early-warning signals.

Contribution

It provides a direct test for critical slowing down before abrupt climate changes using volcanic eruption data, advancing understanding of climate tipping point precursors.

Findings

No evidence of CSD before warming transitions.

Evidence of CSD before cooling transitions.

Statistical EWS are more frequently observed than direct CSD evidence.

Abstract

It is tested whether past abrupt climate changes support the validity of statistical early-warning signals (EWS) as predictor of future climate tipping points. EWS are expected increases in amplitude and correlation of fluctuations driven by noise. This is a symptom of critical slowing down (CSD), where a system's recovery from an external perturbation becomes slower as a tipping point (represented by a bifurcation) is approached. EWS are a simple, indirect measure of CSD, but subject to assumptions on the noise process and measurement stationarity that are hard to verify. In this work the existence of CSD before the Dansgaard-Oeschger (DO) events of the last glacial period is directly tested by inferring the climate's recovery from large volcanic eruptions. By averaging over hundreds of eruptions, a well-defined, stationary perturbation is constructed and the average climate response is measured by eight ice core proxies. As the abrupt DO warming transitions are approached, the climate response to eruptions remains the same, indicating no CSD. For the abrupt DO cooling transitions, however, some key proxies show evidence of larger climate response and slower recovery as the transitions are approached. By comparison, almost all proxies show statistical EWS before cooling and warming transitions, but with only weak confidence for the warming transitions. There is thus qualitative agreement of CSD and EWS, in that the evidence for bifurcation precursors is larger for the cooling transitions. However, the discrepancy that many proxies show EWS but no direct CSD (and vice versa) highlights that statistical EWS in individual observables need to be interpreted with care.