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 established with proxy records from many parts of the world.
Introduction
In our attempt to better understand the nature of our planet’s abrupt climate changes I have already reviewed the glacial-interglacial cycle, and the Dansgaard-Oeschger cycle’s that take place during glacial periods. I now start reviewing the millennial climate cycles that abruptly impact the slowly changing Holocene climate. 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. That article summarizes the current scientific understanding of the ~ 2400-year cycle. In part A of this article, we are going to review, in detail, the evidence for the existence of the ~ 2400-year climate cycle. In part B, we will go over the arguments that the ~2400-year cycle of the production of cosmogenic isotopes 14C and 10Be 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. 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.
The biological 2400-year climate cycle
Over a century ago Scandinavian botanists started to reconstruct the climate of the Holocene from peat bog stratigraphy. They could distinguish the sediment layers into wet/dry, cold/warm, periods, and developed crude dating methods. 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).
Figure 50. Postglacial vegetation and climate periods as understood during the first half of the 20th century. A). 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. Andersson’s opposite view of continuous temperature evolution. Lower diagram, Late Glacial/Postglacial temperature evolution in southern and central Sweden based on biological evidence, after Magnus Fries, showing the temporal disposition of the nine pollen zones in Roman numbers. The thin line represents a near-millennial oscillation in humidity. Dates in calendar years. Source: T. Bergeron, 1956. Fornvännen, 51, 1-18. B). Analytical pollen zones defined by Knud Jenssen and Johs. Iversen for southern and central Sweden confirming Sernander’s climatic reconstruction. Dates in calendar years. Source: O.K. Davis, 2009. Introduction to Quaternary ecology.
The development of palynology (pollen studies) by Lenart von Post in the 1930’s allowed Knud Jenssen and Johs. 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. This temperature scale allowed reconstructions of the Holocene climate very similar to our current understanding by 1950 (figure 50 A, lower diagram).
Figure 50 summarizes decades of work by botanists to establish vegetation stages in the Northern Hemisphere Holocene. These stages allow us to distinguish a 2500-year vegetation and faunal cycle. Some botanists, like Rutger Sernander, proposed that these transitions were abrupt and not gradual. In particular, he proposed that the last transition between the Sub-Boreal and the Sub-Atlantic, at around 650 BC corresponded to the “Fimbulvintern” or Great Winter of the Sagas that marks the end of the Nordic Bronze Age (figure 50 A), and made the Nordic countries a colder place.
The glaciological 2400-year climate cycle
In the early 1950’s, researchers noticed a correlation between glacier movements in North America and sunspots for the previous 300 years. In the 1960’s James Roger Bray constructed a solar index starting in 527 BC by combining telescopic sunspot observations with naked-eye sunspot and auroral observations. He also constructed an index for postglacial major ice re-advances from glaciers all over the world. He compared these two observations and found a high degree of correlation, and good agreement with Icelandic sea-ice, and 14C production variations. 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). 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.
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). By then Hans Suess had determined the short-term fluctuations in 14C levels for the past 7000 years from tree rings. Even then, they were thought to represent variations in solar activity. As Bray had done previously, Denton & Karlén (1973) correlated periods of major glacier advances to periods of high 14C production (low solar activity).
Figure 51. Holocene glacier fluctuations. A). 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. Source: G. Denton & W. Karlén, 1973. Quat. Res., 3, 155-205. B). Holocene subdivisions and glacier fluctuations in the European Alps showing the complex pattern of advances and retreats that do not always correspond between Austrian and Swiss Alps. The uncalibrated radiocarbon dates scale is shown together with the corresponding calibrated scale in calendar years BP. In this and following figures, blue bars mark the position of the lows of the ~ 2400-year Bray cycle. Source: D. Bressan, 2011. Scientific American.
Since then glaciologists have reconstructed Holocene glacier movements from hundreds of glaciers all over the planet, and glacier variability has become more complex (figure 51 B). Today we still recognize the major global advances that define the 2400-year cycle (Mayewski et al., 2004; figure 47), but there is hardly a century, especially during the Neoglacial, when glaciers were not advancing somewhere.
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). Due to its coincidence with 14C fluctuations, it was inferred that its cause was solar variability. 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 14C 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.
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. Paul Mayewski, one of George Denton’s students, was the scientist in charge of coordinating the effort of over 200 scientists in the American Ice Core Program that in 1993 completed the Greenland Ice Sheet Project II (GISP2). He described this effort and its fruits in his 2002 book “The Ice Chronicles: The Quest to Understand Global Climate Change.” While other researchers took on studying gases, isotopes, or dust in the GISP2 ice core, Mayewski and colleagues studied the chemical composition of major ions brought to the ice by the wind, using them as tracers for atmospheric circulation. 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). An increase in salt deposition is associated with winter atmospheric conditions today. This is when the north polar vortex expands and meridional circulation increases, and thus represents an increase in cold and windy conditions. The periodicity found by Mayewski and colleagues (O’Brien et al., 1995) in GISP2 salts is close to 2600 years (figure 52 b). They noticed a good correlation between the atmospheric maxima and clusters of three grand solar minima (GSM) of the Maunder- and Spörer-type patterns, with the most recent one taking place during the LIA (O’Brien et al., 1995; figure 52 c).
Figure 52. Holocene North Atlantic and Arctic atmospheric changes. a). GISP2 polar circulation index, a time series of the dominant empirical orthogonal function, EOF1, of the major ions in the ice core, that provides a relative measure of the average size and intensity of polar atmospheric circulation. PCI values increase (e.g., more continental dust and marine contributions) during colder portions of the record. b). Main periodic component of the sea salt Na flux in GISP2 ice core with a quasi-2600-year periodicity. c). ∆14C intervals that present Maunder- and Spörer-type patterns occurring in clusters. Sources: S.R. O’Brien et al., 1995. Science, 270, 1962-1964. Ice Core Working Group. The National Ice Core Laboratory. d). Mean grain size of eolian soil deposition at Hólmsá, Iceland, indicative of wind strength. 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. Source: M. G. Jackson et al., 2005. Geology 33, 509-512. e). 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. Black curve, main periodic component of the data. Source: J-H. Kim et al. 2007. Geology 35, 387-390. Light grey trace, inferred NAO circulation pattern from the principal component analysis of redox parameters (Ca/Ti and Mn/Fe ratios) from a Greenland lake sediment record. Source: J. Olsen et al. 2012. Nature Geoscience, 5, 808-812.
The atmospheric reorganization that takes place at the lows of the Bray cycle and causes increased polar circulation is partially evident in eolian soil sediments in southern Iceland (Jackson et al., 2005; figure 52 d). Some of the biggest grain sizes transported by the strongest winds are associated with cold periods and coincide with some of the lows of the Bray cycle (B3 & B4, figure 52 d). The authors of the work underscore the wind pattern similarity to the North Atlantic drift-ice Bond record.
The changes in polar circulation and wind strength are suggestive of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). The AO reflects the state of the atmospheric circulation over the Arctic, through a positive phase, featuring below average geopotential heights, and a negative phase in which the opposite is true. 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). Therefore, cold Arctic air and storm tracks move farther south, causing a drop in northern hemisphere temperatures and changes in precipitation patterns. 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. Fluctuations in the strength of these pressure centers alter the alignment of the jet stream affecting temperature and precipitation distribution. A NAO negative phase is produced when the weakening of the Iceland low and the Azores high reduces the pressure gradient resulting in weaker more southern westerlies producing colder conditions over much of North America and Northern Europe while moving the storm tracks southward towards the Mediterranean. A NAO negative phase usually features more frequent and longer blocking conditions when a stationary pressure pattern allows cold Arctic air to spill over mid-latitudes.
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). 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). 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. The lows of this NAO cycle are characterized by NAO negative dominant conditions that produce northern hemisphere cooling and precipitation changes. Rimbu et al. (2004), have argued that during the Holocene, the AO/NAO atmospheric circulation was the dominant climate mode at millennial time scales.
The oceanic 2400-year climate cycle
Given the strong coupling between atmospheric and oceanic variability, it is not surprising that the ~ 2400-year climate cycle is prominently displayed by some oceanic current proxies, particularly in the North Atlantic. Oppo et al. (2003) used an established deepwater proxy, the carbon-isotope composition of benthic foraminifera, to evaluate Holocene deepwater variability at a sediment core in the NE subpolar Atlantic. Low 13C values are indicative of a reduction in the 13CO2 rich North Atlantic Deep Water (NADW) contribution. Oppo et al. (2003) identify the largest reductions in NADW at 9.3, 8.0, 5.0 and 2.8 kyr ago. The latest three periods correspond with Bray lows 2 to 4 (figure 53 a). Significant reductions in 13C indicative of reduced NADW production have also been reported at 10,300 BP (B5) by Bond et al. (1997), and at the LIA (B1) by Keigwin and Boyle (2000). This means that all the lows in the Bray cycle had been identified as periods of reduced NADW contribution by different authors. Such periods might see a reduction in the northward flux of warm near surface waters in the North Atlantic to maintain mass balance (that could be the cause of the NADW reduction), and would result also in the cooling of North Atlantic high latitudes.
Figure 53. Holocene North Atlantic and Arctic oceanic currents changes. a). Benthic Cibicidoides wuellerstorfi δ13C variations, at a marine sediment core in the subpolar NE Atlantic, record variations in δ13C of the total amount of CO2 in bottom waters, as a proxy for δ13C-rich North Atlantic deepwater (NADW) contribution. The lows in the Bray cycle (blue bars), correspond to periods of reduced NADW contribution. Source: D.W. Oppo et al., 2003. Nature, 422, 277-278. b). Salinity reconstruction at the base of the thermocline by paired Mg/Ca–δ18O measurements from Globigerina inflata from a marine sediment core south of Iceland. During the early Holocene, the sub-thermocline was saltier, but underwent a freshening at a time when the ice sheets were still contributing meltwater. The glacial freshwater discharge event of 8.2 kyr ago can be recognized. Warm saline sub-thermocline conditions took place at 0.3, 1.0, 2.7 and 5.0 kyr ago, coinciding with known climatic perturbations in the North Atlantic region. Source: D.J.R. Thornalley et al., 2009. Nature, 457, 711-714. c). Quantitative wt% mineralogical (quartz and feldspars) detrended variations from 16 cores from the Iceland shelf (thick line), as a proxy for drift ice from the Arctic Ocean and East Greenland, fitted to a fourth-order polynomial (thin line). Five peaks in residuals from the data are defined by the 2500-year cycle. Source: J.T. Andrews, 2009. J. Quat. Sci. 24, 664-676. d). Detrended (grey) and smoothed (black) Gephyrocapsa muellerae abundance (nº x 108/g) record as a proxy of warmer Atlantic water flow through the Iceland-Scotland strait of the Nordic Seas from a sediment core off Norway. The low abundance during the LIA (B1) might be due to Atlantic waters being too cold during summers for this warm-loving species. Source: J. Giraudeau et al., 2010. Quat. Sci. Rev. 29, 1276-1287.
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 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. 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). They propose solar variability as the forcing behind these oscillations. The increased salinity of the Atlantic inflow observed at the times of reduced NADW formation identified by Oppo et al. (2003; figure 53 a) may have limited the reduction, or helped restart a stronger AMOC.
Andrews (2009) analyzed the distribution of foreign mineral species by drift ice in Icelandic shelf waters. 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. Periods of high drift ice coincide with the lows of the Bray cycle (Andrews, 2009; figure 53 c). As is the case with the Bond series, there is variability in drift ice records, since some cold events do not belong to the Bray cycle.
A more detailed look at millennial-scale oceanic transport changes that took place at the Iceland-Scotland Ridge further confirms the oceanic ~ 2400-year cycle. Abundance of coccolith species in a marine core off Norway reflects major Holocene changes in Atlantic water transfer to the Nordic Seas with a 2400-year periodicity (Giraudeau et al., 2010; figure 53 d). Millennial-scale Holocene episodes of increased advection of heat by Atlantic waters off Norway are associated with enhanced winter precipitation over Scandinavia, increased sea-salt fluxes over Greenland, and strengthened wind over Iceland. Thereby suggesting common atmospheric forcings. This may be the location and intensity of the westerlies and the associated changes in mid- to high-latitude pressure gradients. Such atmospheric processes are thought to explain the observed coupling between periods of excess drift ice delivery to Northern Iceland (Andrews, 2009; figure 53 c), and intervals of maximum inflow of warm Atlantic water to the Norwegian Sea (Giraudeau et al., 2010; figure 53 d) throughout the last 11,000 years.
The hydrological 2400-year climate cycle
Precipitation is affected by multiple factors, and in many cases determined by regional or even local climatic and weather patterns. It is clear however that the atmospheric reorganizations that have accompanied the 2400-year Bray climate cycle are reflected in precipitation changes in several locations. For decades Michael Magny has been studying Holocene mid-European lake level fluctuations and their impact on prehistoric human settlements (Magny et al., 2004). His research shows very clearly the impact of Holocene climatic change. There is a general trend to increasing dryness during the Neoglacial, after a wetter HCO. Overlapping this general trend attributable to Milankovitch forcing, the 2400-year cycle is characterized by strong transitions from low to high lake levels (Magny et al., 2004; figure 54 a), indicating greatly increased precipitation at the lows of the Bray climatic cycle.
Figure 54. Holocene northern hemisphere precipitation changes. a). Holocene mid-European lake-level reconstruction from a data set of 180 radiocarbon, tree-ring and archaeological dates of higher and lower lake-level events based on multiple lines of evidence, obtained from sediment sequences of 26 lakes in the Jura mountains, the northern French Pre-Alps and the Swiss Plateau. The score indicates how well registered the lake-level event is, not its intensity. With a resolution of 50 years, episodes of higher lake-level are defined by a collapse of lower lake-level scores followed by a peak in higher lake-level scores. Source: M. Magny, 2004. Quat. Internat. 113, 65-79. b). Winter precipitation reconstruction at Bjørnbreen glacier in Jotunheimen, southern Norway. Precipitation is reconstructed from the known relation between variations in the equilibrium line altitude (ELA, the boundary between the ablation and accumulation areas) and mean July temperature variations reconstructed from palynological data. Winter precipitation is more important than summer temperature for glacier expansion episodes. Source: J.A. Matthews et al., 2005. Quat. Sci. Rev. 24, 67-90. c). Holocene summed probability plot for Spanish fluvial system paleofloods, fine to medium sands deposits on the sides of narrow bedrock canyons that resulted from floods of similar or greater magnitude to those of the largest floods recorded in the instrumental series and are considered evidence of past extreme floods. Source: V.R. Thorndycraft & G. Benito, 2006. Quat. Sci. Rev. 25, 223–234. d). 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. Under humid conditions trees were unable to grow on wetter bogs. Source: C. Turney et al., 2005. J. Quat. Sci. 20, 511-518. e). 25-year average sedimentary varve thickness record at a marine core in the Santa Barbara Basin as a proxy for annual rainfall in the area. Thin line represents lowpass filter to emphasize millennial scale fluctuations. Data is missing around the 8.2 kyr event when the basin entered a bioturbated non-varved interval similar to glacial stadials. Source: A.J. Nederbragt & J. Thurow, 2005. Palaeo. 221, 313-324. f). 5-year-resolution δ18O isotope record from Dongge Cave (southern China) stalagmite DA as a proxy for the strength of the Asian monsoon over the past 9000 years. Yellow bars denote the timing of Bond events 0 to 5 in the North Atlantic. Two grey bars indicate two other notable weak Asian monsoon events that can be correlated to ice-rafted debris events. Source: Y. Wang et al., 2005. Science 308, 854-857.
A winter precipitation reconstruction from Norway’s coastal glaciers shows periods of increasing precipitation at the lows of the Bray cycle (Matthews et al., 2005; figure 54 b). Besides feeding glacier advances at these times (figure 51 a), the Norway glacier-derived winter precipitation record matches almost exactly the Norway marine-derived Atlantic warm-water inflow record (figure 53 d), supporting a causal relationship.
Spanish fluvial chronology also supports a 2400-year cycle in precipitation (Thorndycraft & Benito, 2006; figure 54 c). Three of the five main flooding periods highlighted by the authors coincide with B1, B2, and B5 lows in the Bray cycle. In addition, B3 and B4 lows are also characterized by significant episodes of slackwater floods or paleofloods, that record periods of increased flood frequency related to Holocene climatic variability (Thorndycraft & Benito, 2006; figure 54 c). They are fine-grained sediments produced by large magnitude floods, preserved in valley side rock shelters in bedrock gorge reaches. The last 1300 years register a large increase in the frequency of floods in Spanish rivers. The authors propose an increased preservation potential and/or increased human impact on the landscape as likely cause.
Holocene Ireland hydrology has been reconstructed from oaks and pines collected from bogs. 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). Although Ireland hydrology shows a complex pattern over the increasingly wet Neoglacial trend, lows in the Bray cycle are associated with periods of increased precipitation (figure 54 d). This is in contrast with a Neoglacial drying trend in much of the rest of Europe and the world
The hydrological changes caused by the 2400-year climatic cycle are not restricted to the North Atlantic region. 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). 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.
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). 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). Most of the centennial and millennial variability in the Asian and Indian monsoons has traditionally been linked by multiple authors to solar variability (Wang et al., 2005; Neff et al., 2001).
The temperature 2400-year cycle
Although temperature variations should not dominate climate change analysis, they are an important indicator of abrupt climate changes, and therefore one would expect to find traces of the 2400-year climatic cycle in temperature proxy records. And indeed, they are clearly there. In the previous article I reviewed the 73 global proxies analyzed by Marcott et al. (2013; Holocene climate variability). When properly averaged every low of the Bray cycle coincides with a period when temperatures were experiencing a significant decrease when compared to the previous trend (figure 55 a). Even B5, when the world was still experiencing the fast warming that led to the HCO, shows a significant departure from the warming trend of the previous centuries.
Figure 55. Holocene temperature proxies and reconstruction. a). Global average temperature reconstruction from Marcott et al., 2013, using proxy published dates, and differencing average, with temperature anomaly rescaled as discussed here. Source: Marcott et al., 2013. Science 339, 1198-1201. b). Earth’s axis obliquity is shown to display a similar trend to Holocene temperatures. c). 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. Temperatures expressed as anomaly relative to the temperature at 1850-1880 CE. Shaded bands represent ±1 SD. Source: Y. Rosenthal et al., 2013. Science 342, 617-621. d). Sea Surface Temperature reconstruction at the Davao Gulf, south of Mindanao, from Mg/Ca levels in the surface foraminifer Globigerinoides ruber. Dark grey band corresponds to the 2000–3000 years band-pass filter of the data, with the light grey area the 90% confidence level. Source: D. Khider et al. 2014. Paleoceanography 29, 143–159. e). Holocene variations in subtropical Atlantic SST from marine sediment core 658C. 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. Superimposed on this trend are millennial-scale SST variations coherent with some of the North Atlantic ice-rafting events defined by Bond et al. 2001, including the lows of the Bray cycle (blue bars). Source: P. deMenocal et al., 2000. Science 288, 2198-2202. f). Ice-rafted debris stack (inverted) from four North Atlantic sediment cores. 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. Source: G. Bond et al., 2001. Science 294, 2130-2136.
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. (2013) reconstruction of intermediate water temperatures at the equatorial Indo-Pacific Warm Pool, the warmest oceanic region in the world. Their reconstruction displays a very similar profile to the global reconstruction of Marcott et al. (2013), and shows that every Bray cycle low coincides with a significant downward departure from the general temperature trend (figure 55 c). This is confirmed also by the finding in the same area (south of Magindanao) that Holocene SST display variability in the 1000, 1500, and 2500 periodicities, and the 2500 periodicity coincides very well with the Bray cycle (Khider et al., 2014; figure 55 d). Khider et al. measure the water surface temperature changes associated with the Bray cycle at the Indo-Pacific Warm Pool as 0.3°C, and calculate a climate sensitivity to millennial solar cycles of 9.3-16.7 °C/Wm–2, an order of magnitude higher than the estimated sensitivity to the 11-year solar cycle.
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. 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).
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. (2004), using 18 alkenone records. The principal mode of variability reflects Milankovitch forcing, delayed in the case of the North Atlantic by the melting of the ice sheets. 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). The main disagreement is with B4 due to the 8.2 kyr event, that affected SST in the North Atlantic as early as 8.4 kyr BP, but seems to have had a delayed effect in the tropics around 8.1 kyr BP, possibly preempting the effect of B4 a few centuries later. By analogy with the instrumental period records and the analysis of a long-term integration of a coupled ocean-atmosphere general circulation model, the authors suggest that the AO/NAO is one dominant mode of climate variability at millennial time scales. This conclusion agrees well with the other evidence shown here for the Bray climate cycle.
Figure 56. Holocene millennial-scale sea-surface temperature variability. a) and c). Marine sediment core positions for the 8 tropical region (25°S to 25°N) cores and the 10 North Atlantic realm cores analyzed, respectively. b) and d). Time coefficient (Principal Component Analysis) for the second empirical orthogonal function of the alkenone-based SST variability. The dominant modes of tropical and North Atlantic Holocene SST display a 2.3 kyr cycle linked to the strength of AO/NAO during the Holocene, showing that this cycle has a global character. Source: N. Rimbu et al., 2004. Clim. Dyn. 23, 215-227.
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). Most, if not all, Bond events have been linked to cooling and abrupt climate change outside the North Atlantic area. The Bond record also reflects the 2400-year Bray cycle as the lows in the Bray cycle coincide with Bond events 0 (LIA), 2a, 4a, 5a, and 7 (figure 55 f).
We can conclude that the 2400-year Bray climate cycle is very well established in the proxy record of past climate changes in the North Atlantic region, but affects the entire planet. It is the most important climatic cycle of the Holocene. Although the Bray climate cycle is present in the chemical record of Greenland ice cores, it is not easily seen or, maybe, absent in the Greenland and Antarctic ice core temperature records. This is one of the reasons why it has been ignored for so long despite being present in multiple proxies and recognizable since 1912. Paleoclimatology has come to depend too much on the very reliable and precisely dated polar ice cores at the expense of the often contradictory, unreliable, and imprecisely dated climate proxies. This has had the result that whatever is not prominently displayed in polar ice cores is considered unreal. Another complicating factor is that the Bray cycle is not the only cause of climate change during the Holocene and thus proxies are full of signals whose origin is often difficult to ascertain, creating much confusion among researchers that results in contradictory reports.
Continues in Part B.
Conclusions
1) A 2600-year climate cycle was first proposed in the late 1960s by Roger Bray based on vegetation transitions and major glacier re-advances, and linked to solar activity.
2) This climate cycle is clearly evident in numerous proxies from the North Atlantic region and other places in the world that reflect ~ 2400 year-periodic changes in wind patterns, oceanic currents strength and salinity, drift ice, precipitation, and temperatures.
Acknowledgements
I thank Andy May for reading the manuscript and improving its English.
Bibliography [link]
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