Abstract
Abstract. Different paleoclimate proxy records evidence repeated abrupt climate transitions during previous glacial intervals. These transitions are thought to comprise abrupt warming and increase in local precipitation over Greenland, sudden reorganization of the Northern Hemisphere atmospheric circulation, and retreat of sea ice in the North Atlantic. The physical mechanism underlying these so-called Dansgaard–Oeschger (DO) events remains debated. A recent analysis of Greenland ice core proxy records found that transitions in Na+ concentrations and δ18O values are delayed by about 1 decade with respect to corresponding transitions in Ca2+ concentrations and in the annual layer thickness during DO events. These delays are interpreted as a temporal lag of sea-ice retreat and Greenland warming with respect to a synoptic- and hemispheric-scale atmospheric reorganization at the onset of DO events and may thereby help constrain possible triggering mechanisms for the DO events. However, the explanatory power of these results is limited by the uncertainty of the transition onset detection in noisy proxy records. Here, we extend previous work by testing the significance of the reported lags with respect to the null hypothesis that the proposed transition order is in fact not systematically favored. If the detection uncertainties are averaged out, the temporal delays in the δ18O and Na+ transitions with respect to their counterparts in Ca2+ and the annual layer thickness are indeed pairwise statistically significant. In contrast, under rigorous propagation of uncertainty, three statistical tests cannot provide evidence against the null hypothesis. We thus confirm the previously reported tendency of delayed transitions in the δ18O and Na+ concentration records. Yet, given the uncertainties in the determination of the transition onsets, it cannot be decided whether these tendencies are truly the imprint of a prescribed transition order or whether they are due to chance. The analyzed set of DO transitions can therefore not serve as evidence for systematic lead–lag relationships between the transitions in the different proxies, which in turn limits the power of the observed tendencies to constrain possible physical causes of the DO events.
Highlights
In view of anthropogenic global warming, concerns have been raised that several subsystems of the earth’s climate system may undergo abrupt and fundamental state transitions if temperatures exceed corresponding critical thresholds (Lenton and Schellnhuber, 2007; Lenton et al, 2008, 2019)
In the following we apply the above methodology to the different pairs of proxies that Erhardt et al (2019) found to exhibit a decadal-scale time lag, based on an assessment of the combined estimate, namely (Ca2+, Na+), (λ, Na+), (Ca2+, δ18O), and (λ, δ18O) from the NGRIP ice core and (Ca2+, Na+) from the NEEM ice core
The DO events are likely to be caused by the same physical mechanism, changing boundary conditions and other natural climate fluctuations will lead to deviations in the exact timings of the different processes involved in triggering the individual DO events
Summary
In view of anthropogenic global warming, concerns have been raised that several subsystems of the earth’s climate system may undergo abrupt and fundamental state transitions if temperatures exceed corresponding critical thresholds (Lenton and Schellnhuber, 2007; Lenton et al, 2008, 2019). First discovered in the δ18O records from Greenland ice cores, the so-called Dansgaard–Oeschger (DO) events are considered the archetype of past abrupt climate changes (see Fig. 1) (Johnsen et al, 1992; Dansgaard et al, 1993; Bond et al, 1993; North Greenland Ice Core Project members, 2004). These events constitute a series of abrupt regional warming transitions that punctuated the last and previous glacial intervals at millennial recurrence periods. A global impact of DO events on climate and ecosystems is evident in many proxy records (e.g. Moseley et al, 2020; Buizert et al, 2015; Lynch-Stieglitz, 2017; Kim et al, 2012; Fleitmann et al, 2009; Voelker, 2002; Cheng et al, 2013)
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