Mid-Holocene regional reorganization of climate variability

Abstract

Abstract. We integrate 130 globally distributed proxy time series to refine the understanding of climate variability during the Holocene. Cyclic anomalies and temporal trends in periodicity from the Lower to the Upper Holocene are extracted by combining Lomb-Scargle Fourier-transformed spectra with bootstrapping. Results were cross-checked by counting events in the time series. Main outcomes are: First, the propensity of the climate system to fluctuations is a region specific property. Many records of adjacent sites reveal a similar change in variability although they belong to different proxy types (e.g., δ18O, lithic composition). Secondly, at most sites, irreversible change occured in the Mid-Holocene. We suggest that altered ocean circulation together with slightly modified coupling intensity between regional climate subsystems around the 5.5 kyr BP event (termination of the African Humid Period) were responsible for the shift. Fluctuations especially intensified along a pan-American corridor. This may have led to an unequal crisis probability for early human civilizations in the Old and New World. Our study did not produce evidence for millennial scale cyclicity in some solar activity proxies for the Upper Holocene, nor for a privileged role of the prominent 250, 550, 900 and 1450 yr cycles. This lack of global periodicities corroborates the regional character of climate variability.

Holocene land-cover reconstructions for studies on land cover-climate feedbacks

“…The ten PFTs in the left column and the three land-surface types in the right column are used in the dynamic vegetation model LPJ-GUESS and regional climate model RCA3, respectively. The PFTs are a simplification of the PFTs described in Wolf et al (2008). The corresponding 24 plant taxa for which REVEALS reconstructions are performed in the project are indicated in the middle column.…”

“…The latter produces major differences in land cover in south western, south eastern and eastern Europe where landscape openness becomes significantly lower than in the "standard scenario", whereas it is higher in western Europe. Lemmen (2009) developed an independent estimate of human population density, technological change and agricultural activity during the period 9500-2000 BC based on dynamical hindcasts of socio-economic development (GLUES [Global Land Use and technological Evolution Simulator], Wirtz and Lemmen, 2003). The population density estimate was combined with per capita crop intensity from HYDE (version 3.1) to infer areal demand for cropping at an annual resolution in 685 world regions.…”

“…However, most studies have focused on forested vegetation. For instance, Williams et al (2008) used a modern-analogue approach to estimate the past Leaf Area Index (LAI) in Northern America. They tested their approach using a modern training dataset and showed that it performed satisfactorily for a majority of the high number of records used.…”

“…Such descriptions of past landcover would likewise help us to test theories on climateecosystem-human interactions and strengthen the knowledge basis of human-environment interactions (e.g. Anderson et al, 2006;Dearing, 2006;Denman and Brasseur, 2007;Wirtz et al, 2009).…”

“…Palaeoecological data, particularly fossil pollen records, have been used to describe vegetation changes regionally and globally (e.g. Prentice and Jolly, 2000;Williams et al, 2008), but unfortunately they have been of little use for the assessment of human impacts on vegetation and land cover (Anderson et al, 2006;Gaillard et al, 2008). The development of databases of humaninduced changes in land cover based on historical records, remotely-sensed images, land census and modelling (Klein Goldewijk, 2001Ramankutty and Foley, 1999;Olofsson and Hickler, 2008) has been useful to evaluate the effects of anthropogenic land-cover changes on the past climate (e.g.…”