By Javier The existence of a ~ 2400-year climate cycle, discovered in 1968 by Roger Bray, is supported by abundant evidence from vegetation changes, glacier re-advances, atmospheric changes reflected in alterations in wind patterns, oceanic temperature and salinity changes, drift ice abundance, and changes in precipitation and temperature.
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.
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).
In part B, we will go over the arguments that the ~2400-year cycle of the production of cosmogenic isotopes Be represents a cycle in solar activity.
In part C, we will discuss what it is considered the most likely mechanism by which solar variability could affect climate, as proposed by the authors researching the subject. The development of palynology (pollen studies) by Lenart von Post in the 1930’s allowed Knud Jenssen and Johs.
The most significant and regular one is the ~ 2400-year Bray cycle.
Recently, the Bray (Hallstatt) Cycle was reviewed by analyzing the main findings of some of the most significant articles by researchers who have studied it.
Bray’s glaciological and solar studies were reproduced in 1973 by Denton and Karlén who did a more detailed study of world glacial advances and came up with essentially the same periodicity, 2500 years (figure 51 A). Throughout this work both the climatic and solar cycle are referred to as the Bray cycle, and the lows of the cycle, associated with enhanced C production, and climatic changes manifested by cooling, glacier advances, increased drift ice in the North Atlantic, and atmospheric, oceanic, and precipitation changes, are numbered from more recent backwards as B1, B2, …, with B1 the Little Ice Age. Dark grey trace, reconstruction of time coefficient by singular spectrum analysis of detrended and normalized alkenone based SST variance, from a NW Africa marine sediment core, as a proxy for AO/NAO oscillation. The AO often shares phase with the NAO, that reflects differences in the strength of two pressure centers in the North Atlantic: the low pressure near Iceland, and the high pressure over the Azores. The authors show evidence that the increased salinity, temperature, and water stratification, at times of abrupt climate change, are due to an increase in the Atlantic inflow of warmer saline subtropical gyre waters at the expense of the fresher and colder subpolar gyre waters. Holocene Ireland hydrology has been reconstructed from oaks and pines collected from bogs. Holocene variations in subtropical Atlantic SST from marine sediment core 658C. Ice-rafted debris stack (inverted) from four North Atlantic sediment cores. (2013) reconstruction of intermediate water temperatures at the equatorial Indo-Pacific Warm Pool, the warmest oceanic region in the world.
By then Hans Suess had determined the short-term fluctuations in C levels for the past 7000 years from tree rings. Synthesis of Holocene worldwide glacier fluctuations showing three broad intervals of glacier expansion within the last 6000 years and a fourth one recognized in Scandinavia. The atmospheric 2400-year climate cycle The next great advance in the characterization of the 2400-year climatic cycle came from the study of ice cores. Fluctuations in the strength of these pressure centers alter the alignment of the jet stream affecting temperature and precipitation distribution. They interpret it as a negative feedback from the subpolar gyre, that stabilized the AMOC, at times of freshwater inputs, particularly during the early Holocene when the ice sheets were still melting rapidly, and at the 8.2 kyr event when the outbreak of proglacial Lake Agassiz took place (Thornalley et al., 2009; figure 53 b). Irish bog-grown oaks (Quercus spp.) and pines (Pinus sylvestris L.) frequency (inverted scale) during the Holocene as evidence of changes in moisture delivery to Ireland. These trees, accurately dated through dendrochronology (oaks) and carbon-dating (pines), provide a record of dry conditions when the decreased water table levels allowed the colonization of these marginal environments by trees (Turney et al., 2005). The record documents a well-known shift in African monsoonal climate at 5.5 kyr, when changes in the earth’s orbit displaced the African monsoon southward, bringing much drier and warmer conditions to subtropical Africa and ending the African Humid Period. It is proposed that the increase in iceberg activity in the North Atlantic is tied to the increase in cold water advection from the Arctic and Nordic seas. Their reconstruction displays a very similar profile to the global reconstruction of Marcott et al.
They could distinguish the sediment layers into wet/dry, cold/warm, periods, and developed crude dating methods. Analytical pollen zones defined by Knud Jenssen and Johs. Figure 50 summarizes decades of work by botanists to establish vegetation stages in the Northern Hemisphere Holocene.