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He observed in the data a possible 2300-2700-year cycle, that he projected into the past from the Little Ice Age, finding that a 2600-year period closely matched both vegetation transitions like the Atlantic/Sub-Boreal, or the Sub-Boreal/Sub-Atlantic transitions, and significant glacier re-advances from the past after the Younger Dryas (Bray, 1968). In this and following figures, blue bars mark the position of the lows of the ~ 2400-year Bray cycle. By the mid-70’s the scientific community was aware of the existence of a 2500-year climatic cycle that caused glacier advances and recessions, and that separated significantly different vegetation stages and cultural phases (figure 51B). In the negative phase, the polar low-pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). Data is missing around the 8.2 kyr event when the basin entered a bioturbated non-varved interval similar to glacial stadials. The last 1300 years register a large increase in the frequency of floods in Spanish rivers.

Since he was the first to correctly identify and describe the ~ 2400 year climatic and solar cycles they should carry his name as this is the tradition. Due to its coincidence with C fluctuations, it was inferred that its cause was solar variability. Therefore, cold Arctic air and storm tracks move farther south, causing a drop in northern hemisphere temperatures and changes in precipitation patterns. Temperature and salinity analysis of the Atlantic Meridional Overturning Circulation (AMOC) using a sediment core south of Iceland, where the Faroe and Irmingen currents branch out of the North Atlantic current, shows that episodes of warm saline sub-thermocline conditions are centered at 0.3 (B1), 1.0, 2.7 (B2) and 5.0 (B3) kyr ago, coinciding with known climatic perturbations in the North Atlantic region (Thornalley et al., 2009; figure 53 b). The authors propose an increased preservation potential and/or increased human impact on the landscape as likely cause. That the global temperature reconstruction truly reflects global temperature changes and is not dominated by northern hemisphere records is confirmed by the Rosenthal et al.

Afterwards we recommend that the interested reader read the post “Impact of the ~ 2400 yr solar cycle on climate and human societies.” The post explores, in detail, the climatic effects and their impact on human civilization in each of the Bray cycle lows during the Holocene. Iversen to improve the postglacial period zonation (figure 50 B), and develop a summer vegetation-based temperature scale for the Scandinavian Holocene by the 1940’s.

The biological 2400-year climate cycle Over a century ago Scandinavian botanists started to reconstruct the climate of the Holocene from peat bog stratigraphy. This temperature scale allowed reconstructions of the Holocene climate very similar to our current understanding by 1950 (figure 50 A, lower diagram).

Their efforts resulted in an understanding that the Holocene climate could be subdivided into periods of different climatic conditions, like in a diagram by Rutger Sernander from 1912 (figure 50 A, upper diagram). Postglacial vegetation and climate periods as understood during the first half of the 20th century. Upper diagram, Rutger Sernander’s view of postglacial warm climate periods in southern and central Sweden, showing his proposed abrupt climate degradation at the Sub-Boreal/Sub-Atlantic transition, termed “fimbulvintern.” The dashed line indicates G. Iversen for southern and central Sweden confirming Sernander’s climatic reconstruction. These stages allow us to distinguish a 2500-year vegetation and faunal cycle.

They discovered a strong association between expansions of northern hemisphere polar atmospheric circulation systems and the 2500-year cycle previously described by his former teacher (O’Brien et al., 1995; figure 46 F & G; figure 52 a & b). Windy periods, indicated by the transport and deposition of coarse sediments, are coeval with cool, stormy periods recorded in GISP2 ice and North Atlantic sediment cores. The Holocene NAO patterns have been reconstructed from a marine sediment core whose alkenone content has been shown to depend on trade winds intensity-dependent upwelling near the coast of NW Africa (Kim et al., 2007; figure 52 e). The low abundance during the LIA (B1) might be due to Atlantic waters being too cold during summers for this warm-loving species. (2003; figure 53 a) may have limited the reduction, or helped restart a stronger AMOC. The same pattern can be found in the Santa Barbara Basin (California), reflected in varve thickness variability, that is known to depend on annual precipitation, and inversely correlates with wind strength (Nederbragt & Thurow, 2005). Dark grey band corresponds to the 2000–3000 years band-pass filter of the data, with the light grey area the 90% confidence level. Temperature proxies at the West African sea indicate that SST were over 2° C lower during the African Humid Period (de Menocal et al., 2000; figures 40 & 55 e), after which the lack of precipitation due to the southward displacement of the African monsoon produced an abrupt warming of the sea surface before joining the global cooling trend of the Neoglacial.

An increase in salt deposition is associated with winter atmospheric conditions today. For the last millennia, the NAO intensity has also been reconstructed from lake sediments in Greenland, showing the very low NAO values that characterized the LIA (Olsen et al., 2012; figure 52 e). Andrews (2009) analyzed the distribution of foreign mineral species by drift ice in Icelandic shelf waters. The described ~ 2750-year cycle in varve thickness correlates very well with the Bray climate cycle (figure 54 e), with periods of higher varve thickness (increased precipitation) at the Bray lows. Within this complex general pattern, the lows of the Bray cycle are once more associated with a significant temporal reduction in SST (figure 55 e).

This is when the north polar vortex expands and meridional circulation increases, and thus represents an increase in cold and windy conditions. The evidence indicates a 2400-year periodic variation in SST and upwelling intensity off NW Africa that is associated with a climatic cycle in oceanic circulation that reflects periodic NAO conditions. While drift ice has been increasing in the past 6,000 years of Neoglacial conditions off Northern Iceland, the detrended data supports the existence of a 2400-year climatic periodicity. A high-resolution record of the strength of the Asian monsoon was obtained from oxygen isotopic analysis of stalagmite “DA” in Dongge Cave (China; Wang et al., 2005). Earth’s axis obliquity is shown to display a similar trend to Holocene temperatures. Holocene reconstruction of intermediate-water temperatures at 500 m depth from a suite of sediment cores in the Makassar Strait and Flores Sea in Indonesia, at the Indo-Pacific Warm Pool. A more complete analysis of SST temperatures in the tropical oceans and the North Atlantic region, the Mediterranean, and Red Sea, was performed by Rimbu et al. The principal mode of variability reflects Milankovitch forcing, delayed in the case of the North Atlantic by the melting of the ice sheets. The Bond record of drift-ice petrological deposition in the North Atlantic is also generally considered to correlate to colder conditions in the North Atlantic region that favor more frequent southward moving icebergs (Bond et al., 2001).

The periodicity found by Mayewski and colleagues (O’Brien et al., 1995) in GISP2 salts is close to 2600 years (figure 52 b). The lows of this NAO cycle are characterized by NAO negative dominant conditions that produce northern hemisphere cooling and precipitation changes. (2004), have argued that during the Holocene, the AO/NAO atmospheric circulation was the dominant climate mode at millennial time scales. Periods of high drift ice coincide with the lows of the Bray cycle (Andrews, 2009; figure 53 c). The record supports episodes of monsoon weakness (dryness) at every one of the Bray lows, most of them highlighted by the authors of the work (figure 54 f). Temperatures expressed as anomaly relative to the temperature at 1850-1880 CE. The secondary mode of variability (principal temporal component from the second empirical orthogonal function) shows in both regions as a ~ 2300-year cycle that agrees well with the Bray cycle (Rimbu et al., 2004; figure 56). Most, if not all, Bond events have been linked to cooling and abrupt climate change outside the North Atlantic area.

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