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Introduction Scientists have made a number of differentinterpretations and definitions in regards to ‘climate’s flickering switch’however a suitable definition used in this essay, as described by Barker, isthe concept that “the climate system may be capable of flickering betweenglacial and near-interglacial conditions in periods of just a few years”. Thisessay will explore further this concept and its definition as well as examiningthe discussions around what the cause of it could be. Evidence which has been usedto demonstrate this switch and to highlight examples include the analysis of icecores using methods such as electrical conductive measurement to study the ratioof acids and bases (Taylor et al, 1993) and ratios of oxygen isotopes (Steffensenet al, 2008) and the examination of sediment and lithic grains . Key casestudies include the Dansgaard-Oeschger events and especially the transition fromthe Bølling warming and the Younger-Dryas which provide evidence of the’flickering’ nature of the Earth’s previous climate.  Exploringthe term  The concept of the ‘flickering switch’ arosethrough the work of geographers such as Denton and Karlén who suggested that “Holoceneclimate was much more variable than implied” by records (Bond, 1997 page 1257).Whilst physical geographers have a general coherence about the concept of the’flickering switch’, there are subtle differences in approaches betweenresearchers.

For example, whilst the climate patterns shown can be described as’oscillations’ (Barker, 2005), Broekeret al (1985) have also referred it as ‘long periods of glacial build upsuddenly terminated with rapid warmings’. Figure1, taken from Broeker et al, visualises the oscillating patterns of theswitch from the evidence of two ice cores (Camp Century in Greenland and ByrdStation in Antarctica) which shows the temperature of the air above the icecap. The oscillating lines show varying levels in the Oxygen 18 isotope (aswill be discussed in a later paragraph) throughout the time period of theglacial, showing that during the period there was a ‘flickering’ climate.   CausationWhilst there is somewhat of a discussion aroundthe cause of these events, in a journal written by Rasmussen et al (2016) a suggestedcause is the thermohaline circulation in the Atlantic. The authors describe howcold periods are related to times of limited convection currents which meantthat the North Atlantic was cooled greatly due to a lack of warm watertravelling north (Rasmussen et al, 2016). On the other hand, the return ofconvection currents align with the interstadial conditions; Rasmussen et al(2016) refer to this as a “bipolar seesaw” (page 1).

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Furthermore, as noted by Bondet al (1993), the temperature oscillations examined in evidence relate closelyto Heinrich events which signify a “sea surface cooling” (Bond, 1993 page 145).The sudden cooling of the ocean is likely to be due to a release of fresh waterdue to icebergs melting (Broecker, 1994). Evidence for this ocean surfacecooling can be found through the analysing the rise in amounts of Globigerinaquinqueloba which is a species of plankton found primarily in the cooler watersof the Arctic (Bond et al, 1997). Figure 2, which has been sourced from Boyle, highlights the Heinrich eventson a climatic record taken from two ice cores (GISP2 and V23-81), showing howthey take place parallel with cooler oscillations. Whilst a number ofgeographers have discussed this link between thermohaline circulation andoscillations, Boyle argues that there is limited evidence which shows a “stronglink between…circulation and…climate transitions” (Boyle. 256).  Dansgaard-Oeschgerevents Research into the Greenland Ice Sheet Projectsshowed that there were “large, rapid climate fluctuations throughout the lastglacial period” (Grootes et al, 1993) which have been named theDansgaard-Oeschger events (Bond et al, 1993), illustrating the concept of a”flickering switch”. These events can be described as “an abrupt warming towarm interstadial conditions followed by a more gradual cooling… to very coldstadial conditions” (Rasmussen et al, 2016, p1).

 A particular oscillation from the Dansgaard-Oeschgerevents is known as the Bølling-Allerød to Younger Dryas Oscillation. TheBølling-Allerød was an interstadial period which took place during a glacial14,700 years ago (Thiagarajan, 2014) with warm conditions lasting approximately2000 years before entering a 1200 year-long glacial period known as the YoungerDryas which ended with the beginning of the Holocene (Broeker, 2000). TheseDansgaard-Oeschger events can be named as a key case study when researching the’flickering switch’.    IceCores : Dust Projects, such as the North Greenland Ice CoreProject, have been used to investigate ice cores in order to investigate previous climates.

Figure 3, taken from a journal bySteffenson et al (2008) demonstrates the location of four different ice cores:Greenland Ice Sheet Projects 1 and 2, North Greenland Ice Core Project and theDYE 3 ice core. Taylor et al. (1993) described polar ice ashaving a “unique record of past climate variations” (432), specifically the useof Greenland ice cores, as have been examined in the Greenland Ice-SheetProject 2, as evidence for both interstadial and cooler periods. One way inwhich these can be examined is through electrical conductive measurement (ECM)which can be used to measure the existence of acids and basis in ice, dependingon the transfer on electrical current within the ice (Taylor et al, 1993) whichcan illustrate the concentration of acids such as sulphuric or nitric which canbe diminished due to the presence of alkaline dust during glacial periods whichneutralise them.

 IceCores: Oxygen Isotopes The cores can also be measured for oxygenisotopes as the concentrations of these change depending on the temperature oftheir location (Steffensen et al, 2008). This form of analysis is namelystudying the ratio between O18 and O16; as described byBond et al (1993) there were ‘large decreases in planktic d18O’ during the Heinrich events (times of cooling)(Bond et al 1993, page 145). The North Greenland Ice Project ice core project,for example, provided evidence of the correlation of oxygen stable isotopelevels- namely O18 during periods of cooling (Figure 3) during pastclimates. An explanation for the varying concentrations of O18 and O16is due to the fact that oxygen-16 is ‘lighter’ and oxygen 18 (Riebeek,2005). This means that during glacial periods the ‘lighter’ oxygen evaporatesfrom the water, returning into the ocean through the form of freshwater during interglacialperiods as the icebergs melt under warmer temperatures (Riebeek, 2005).

 Sediment                                                                                                                                                                   Another method of examining the flickeringswitch of the Earth’s past climate is through the study of sediment whichaccumulate in the “drifts” found in northern Atlantic Ocean (Broecker, 2000),known as ‘ice-rafted detritus’ which is “sediment…entrained in floating ice” (Hemming,2004 page 3). An example of this are the sediments taken from the sedimentdrift named the Bermuda Rise which can be examined to study the Earth’s pastclimatic patterns shows the rise in sea surface temperature from 2° to 2.5° C when enteringwarmer oscillations (Sachs & Lehman, 1999).

Examples of the sediment whichcan be examined include lithic grains which Bond et al describes as “grainswith diameters greater than 150mm in 1g of core” (Bondet al, 1997 page 1257) and petrologic tracers, such as volcanic glass, which Bondet al describe as “percentages of certain types of lithic grains” (page 1257). Bondet al studied two ice cores in North Atlantic and the increases of these sedimenttypes act as evidence that of ice rafting occurred during the Holocene period meaningthat the climate “must have undergone a series of abrupt reorganizations” whichcould have caused the drift ice to travel to locations over 1000km apart (Bondet al, 1997 page 1257). As well as this, sediment has been examined inlocations such as the Santa Barbara Basin which demonstrate the concentrationsof O2 content which increase during cold periods (Broeker, 2000).  Conclusion The term ‘flickering switch’ refers to the oscillationswithin the Earth’s climate, which can be known as Dansgaard-Oeschger events, suchas can be seen within what was said to be a “relatively stable Holocene climate”(Bond et al, 1997, page 1257).

There is a discussion around what the causation ofsuch events are, with a leading idea being changes in the thermohalinecirculation including the impact of the release of freshwater during Heinrichevents. The switch can be recognised through past records, such as seen withthe transition to the Younger Dryas (Thiagarajan, 2014) and a number ofdifferent forms of evidence can be used to do so. Examples of evidence includethe analysis of ice cores to compare oxygen isotopes as well as dust samplesand the study of sediment. There is therefore strong evidence of the switch, asdemonstrated through the graphs provided in the figures of the appendix,illustrating that there has not been complete stability in the Earth’s pastclimate.         Bibliography Barker, S. (2005) The ‘flickering switch’ of late Pleistocene climate change revisited GeophysicalResearch Letters Vol.

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