Mid-Holocene regional reorganization of climate variability: Analyses of proxy data in the frequency domain

Abstract

Recurrent shifts in Holocene climate define the range of natural variability to which the signatures of human interference with the Earth system should be compared.Characterization of Holocene climate variability at the global scale becomes increasingly accessible due to a growing amount of paleoclimate records for the last 9 000-11 000 years. Here, we integrate 124 proxy time series of different types (e.g., δ 18 O, lithic composition) and apply a modified Lomb-Scargle spectral analysis. After bootstrapping the data in moving time windows we observe an increased probability for generation or loss of periodic modes at the mid-Holocene. Spatial autocorrelation of spectral changes robustly reveals that this (in)activation of modes was organized in regional clusters of subcontinental size. Within these clusters, changes in spectral properties are unexpectedly homogeneous, despite different underlying climatolog- Preprint submitted to Elsevier A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTical variables. Oscillations in the climate system were amplified especially at the upwelling areas and dampened in the North Atlantic. We cross-checked the spectral analysis by counting events in the time series and tested against possible dating errors in individual records or against an overestimation of singular events. A combination of different mechanisms may have affected the coupling intensity between climate subsystems, turning these more or less prone to oscillations.

Neoglacial climate anomalies and the Harappan metamorphosis

“…Such land-cover-and land-use-driven changes were time-transgressive across Asia and Africa (e.g., Lezine et al, 2017;Jung et al, 2004;Prasad and Enzel, 2006;Shanahan et al, 2015;Tierney et al, 2017;Wang et al, 2010;Kaplan et al, 2011) and could have led to a generalized instability of the global climate as it passed from the warmer Holocene thermal maximum state to the cooler Neoglacial state. Therefore the instability seen during ENA may reflect threshold behavior of the global climate system characterized by fluctuations or flickering (Dakos et al, 2008;Thomas, 2016) or a combination of different mechanisms affecting the coupling intensity between climate subsystems (Wirtz et al, 2010).…”

A mid-Holocene climate reconstruction for eastern South America

“…Table 1 displays the classification of the palaeodata and the types of proxy used in our compilation. Our classification was based on Wirtz et al (2010) (see Table 1 in Wirtz et al (2010) for more details). The oxygen and carbon stable isotopic ratios are the fractionation-dependent proxies.…”

On the sensitivity of the simulated European Neolithic transition to climate extremes

“…Previous collections used 18, 50, 60 or 80 records (Wanner et al 2008;Mayewski et al 2004;Holmgren et al 2003;Finné et al 2011, respectively), mostly limited to the last 6000 years. The coverage we use here is sufficient to represent climate variability in almost all land areas of the world (with sparsest regional coverage in central Australia, Saharan Africa, and Northern-Central Eurasia), considering the spatial coherence of climate signals within 1500 km distance found by Wirtz et al (2010) for their similar data set.…”

“…From the global dataset, 26 time series are located in or near our focus area western Eurasia (Table 1). For these time series we analyzed the non-cyclic event frequency according to the procedure in Wirtz et al (2010): time series were detrended with a moving window of 2000 years and smoothed with a moving window of 50 years, then normalized ( Figure 1b). Events were detected whenever a time series signal exceeded a confidence interval with threshold p = 1 − 1/n, where n is the number of data points (Thomson, 1990), and where each event is preceded or followed by a sign change in the time series.…”