Was the 4.2 ka event unusual in context of global Holocene climate variability?

Abrupt climate changes are commonly observed between 4500 and 4000 years ago, and particular attention has been paid to the “4.2 ka event”, which now serves as a stratigraphic marker to subdivide the mid and late Holocene globally. However, proxy climate records are commonly marked by large, and often abrupt, changes in temperature and moisture throughout the Holocene, and it remains unclear how abrupt change in the mid-Holocene compares to changes throughout the epoch. Here, we assess how regional and global temperature and moisture changes between 4.5 and 4.0 ka compare with other major climate events across the Holocene, in particular the 8.2 ka event. To conduct this analysis objectively, we assess more than a 1000 previously published paleoclimate datasets that span all continents and oceans and include a wide variety of archive and proxy types. All of the data are open access, and the analyses were conducted using the open-source “Abrupt Change Toolkit in R (actR)” software package to determine the timing and significance of multiple types of abrupt change. These include excursion events (significant short-term deviations from the mean state), regime change events (significant rapid shifts in millennial-scale means) and trend change events (significant changes in the long-term trend). We detect multiple significant abrupt change events throughout the Holocene, and therefore evaluate the spatiotemporal significance of events against a null hypothesis of observed background variability. Events at 8.2 ka stand out as large spatiotemporally coherent excursions of temperature and moisture centered in the North Atlantic and globally significant. In contrast, although we detect multiple types of abrupt change in moisture and temperature during the between 4.5 and 4.0 ka, the event does not significantly exceed the expectation of occurrence from our robust null model nor stand out as a regionally coherent anomaly. These results suggest that local abrupt changes are common throughout the Holocene; many of these are regionally coherent, but few are hemispheric or global in extent.