Critical Thinking

Introduction a few years”. This essay will explore



Scientists have made a number of different
interpretations and definitions in regards to ‘climate’s flickering switch’
however a suitable definition used in this essay, as described by Barker, is
the concept that “the climate system may be capable of flickering between
glacial and near-interglacial conditions in periods of just a few years”. This
essay will explore further this concept and its definition as well as examining
the discussions around what the cause of it could be. Evidence which has been used
to demonstrate this switch and to highlight examples include the analysis of ice
cores using methods such as electrical conductive measurement to study the ratio
of acids and bases (Taylor et al, 1993) and ratios of oxygen isotopes (Steffensen
et al, 2008) and the examination of sediment and lithic grains . Key case
studies include the Dansgaard-Oeschger events and especially the transition from
the Bølling warming and the Younger-Dryas which provide evidence of the
‘flickering’ nature of the Earth’s previous climate.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!

order now


the term


The concept of the ‘flickering switch’ arose
through the work of geographers such as Denton and Karlén who suggested that “Holocene
climate 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 between
researchers. For example, whilst the climate patterns shown can be described as
‘oscillations’ (Barker, 2005), Broeker
et al (1985) have also referred it as ‘long periods of glacial build up
suddenly terminated with rapid warmings’. Figure
1, taken from Broeker et al, visualises the oscillating patterns of the
switch from the evidence of two ice cores (Camp Century in Greenland and Byrd
Station in Antarctica) which shows the temperature of the air above the ice
cap. The oscillating lines show varying levels in the Oxygen 18 isotope (as
will be discussed in a later paragraph) throughout the time period of the
glacial, showing that during the period there was a ‘flickering’ climate. 



Whilst there is somewhat of a discussion around
the cause of these events, in a journal

written by Rasmussen et al (2016) a suggested
cause is the thermohaline circulation in the Atlantic. The authors describe how
cold periods are related to times of limited convection currents which meant
that the North Atlantic was cooled greatly due to a lack of warm water
travelling north (Rasmussen et al, 2016). On the other hand, the return of
convection currents align with the interstadial conditions; Rasmussen et al
(2016) refer to this as a “bipolar seesaw” (page 1). Furthermore, as noted by Bond
et al (1993), the temperature oscillations examined in evidence relate closely
to 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 water
due to icebergs melting (Broecker, 1994). Evidence for this ocean surface
cooling can be found through the analysing the rise in amounts of Globigerina
quinqueloba which is a species of plankton found primarily in the cooler waters
of the Arctic (Bond et al, 1997). Figure 2, which has been sourced from Boyle, highlights the Heinrich events
on a climatic record taken from two ice cores (GISP2 and V23-81), showing how
they take place parallel with cooler oscillations. Whilst a number of
geographers have discussed this link between thermohaline circulation and
oscillations, Boyle argues that there is limited evidence which shows a “strong
link between…circulation and…climate transitions” (Boyle. 256).




Research into the Greenland Ice Sheet Projects
showed that there were “large, rapid climate fluctuations throughout the last
glacial period” (Grootes et al, 1993) which have been named the
Dansgaard-Oeschger events (Bond et al, 1993), illustrating the concept of a
“flickering switch”. These events can be described as “an abrupt warming to
warm interstadial conditions followed by a more gradual cooling… to very cold
stadial conditions” (Rasmussen et al, 2016, p1).  A particular oscillation from the Dansgaard-Oeschger
events is known as the Bølling-Allerød to Younger Dryas Oscillation. The
Bølling-Allerød was an interstadial period which took place during a glacial
14,700 years ago (Thiagarajan, 2014) with warm conditions lasting approximately
2000 years before entering a 1200 year-long glacial period known as the Younger
Dryas which ended with the beginning of the Holocene (Broeker, 2000). These
Dansgaard-Oeschger events can be named as a key case study when researching the
‘flickering switch’.




Cores : Dust


Projects, such as the North Greenland Ice Core
Project, have been used to investigate ice

cores in order to investigate previous climates.
Figure 3, taken from a journal by
Steffenson et al (2008) demonstrates the location of four different ice cores:
Greenland Ice Sheet Projects 1 and 2, North Greenland Ice Core Project and the
DYE 3 ice core.

Taylor et al. (1993) described polar ice as
having a “unique record of past climate variations” (432), specifically the use
of Greenland ice cores, as have been examined in the Greenland Ice-Sheet
Project 2, as evidence for both interstadial and cooler periods. One way in
which these can be examined is through electrical conductive measurement (ECM)
which can be used to measure the existence of acids and basis in ice, depending
on the transfer on electrical current within the ice (Taylor et al, 1993) which
can illustrate the concentration of acids such as sulphuric or nitric which can
be diminished due to the presence of alkaline dust during glacial periods which
neutralise them.


Cores: Oxygen Isotopes


The cores can also be measured for oxygen
isotopes as the concentrations of these change depending on the temperature of
their location (Steffensen et al, 2008). This form of analysis is namely
studying the ratio between O18 and O16; as described by
Bond 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 isotope
levels- namely O18 during periods of cooling (Figure 3) during past
climates. An explanation for the varying concentrations of O18 and O16
is due to the fact that oxygen-16 is ‘lighter’ and oxygen 18 (Riebeek,
2005). This means that during glacial periods the ‘lighter’ oxygen evaporates
from the water, returning into the ocean through the form of freshwater during interglacial
periods as the icebergs melt under warmer temperatures (Riebeek, 2005).




Another method of examining the flickering
switch of the Earth’s past climate is through the study of sediment which
accumulate 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 sediment
drift named the Bermuda Rise which can be examined to study the Earth’s past
climatic patterns shows the rise in sea surface temperature from 2° to 2.5° C when entering
warmer oscillations (Sachs & Lehman, 1999). Examples of the sediment which
can be examined include lithic grains which Bond et al describes as “grains
with diameters greater than 150mm in 1g of core” (Bond
et al, 1997 page 1257) and petrologic tracers, such as volcanic glass, which Bond
et al describe as “percentages of certain types of lithic grains” (page 1257). Bond
et al studied two ice cores in North Atlantic and the increases of these sediment
types act as evidence that of ice rafting occurred during the Holocene period meaning
that the climate “must have undergone a series of abrupt reorganizations” which
could have caused the drift ice to travel to locations over 1000km apart (Bond
et al, 1997 page 1257). As well as this, sediment has been examined in
locations such as the Santa Barbara Basin which demonstrate the concentrations
of O2 content which increase during cold periods (Broeker, 2000).




The term ‘flickering switch’ refers to the oscillations
within the Earth’s climate, which can be known as Dansgaard-Oeschger events, such
as 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 of
such events are, with a leading idea being changes in the thermohaline
circulation including the impact of the release of freshwater during Heinrich
events. The switch can be recognised through past records, such as seen with
the transition to the Younger Dryas (Thiagarajan, 2014) and a number of
different forms of evidence can be used to do so. Examples of evidence include
the analysis of ice cores to compare oxygen isotopes as well as dust samples
and the study of sediment. There is therefore strong evidence of the switch, as
demonstrated through the graphs provided in the figures of the appendix,
illustrating that there has not been complete stability in the Earth’s past










Barker, S. (2005) The ‘flickering switch’ of late Pleistocene climate change revisited Geophysical
Research Letters Vol. 32


Bond, G., Broecker, W., Johnsen, S., McManus,
J., Labeyrie, L., Jouzel, J. & Bonani, G (1993) Correlations between climate records from North Atlantic sediments and
Greenland ice Nature Vol 365


Bond, G., Showers, W., Cheseby, M., Lottie, R.,
Almasi, P., deMenocal, P., Priore, P.,Cullen, H., Hajdas, I., Bonani, G (1997) A Pervasive Millennial-Scale Cycle in North
Atlantic Holocene and Glacial Climates Science Vol 278


Boyle, E.A
(2002) Is ocean thermohaline
circulation linked to abrupt stadial/interstadial transitions? Quaternary
Science Reviews vol 19


Broecker, W.S (1994) Massive iceberg discharges as triggers for climate change Nature
Vol 372


Broeker, W.S (2000) Abrupt climate change: casual constraints provided by the paleoclimate
record Earth Science Reviews 51 pg 137-154


Broeker, W.S., Peteet, D.M. & Rind, D (1985)
Does the ocean-atmosphere system have
more than one stable mode of operation? Nature Vol. 315


Grootes, P.M., Stuiver, M., White, J.W.C.,
Johnsen, S. & Jouzel, J. (1993) Comparison
of oxygen isotope records from the GISP2 and GRIP Greenland ice cores Nature
Vol 366


Hemming, S.R., (2004) Heinrich events: Massive Late Pleistocene Detritus Layers of the North
Atlantic and their Global Climate Imprint Rev. Geophys., vol 42


Rasmussen, T.L., Thomsen, E. & Moros, M
(2016) North Atlantic warming during
Dansgaard-Oeschger events synchronous with Antarctic warming and out-of-phase
with Greenland climate Scientific Reports 6, Article number: 20535


Riebeek, H., (2005) Paleoclimatology: the Oxygen Balance NASA Earth Observatory.
Accessible from:
(date: 29/01/18)


Sachs, J.P., Lehman, S.J (1999) Subtropical North Atlantic Temperatures
60,000 to 30,000 Years Ago Science Vol 286


Steffensen, J.P., Andersen, K.K., Bigler, M.,
Clausen, H.B., Dahl-Jensen, D., Fischer, H., Goto-Azuma, K., Hansson, M.,
Johnsen, S.J., Jouzel, J., Masson-Delmotte, V., Popp, T., Rasmussen, S.O.,
Röthlisberger, R., Ruth, U., Stauffer, B., Siggaard-Andersen, M.,
Sveinbjörnsdóttir, A.E., Svensson, A., White, J.W.C. (2008) High-Resolution Greenland Ice Core Data Show
Abrupt Climate Change Happens in Few Years Science Vol 321


Taylor, K.C., Lamorey, G.W., Doyle, G.A., Alley,
R.B., Grootes, P.M., Mayewski, P.A., White, JW.C. & Barlow, L.K (1993) The ‘flickering switch’ of late Pleistocene
climate change Nature Vol 361


Thiagarajan, N., Subhas, A.V., Southon, J.R.,
Eiler, J.M. & Adkins, J.F (2014) Abrupt
pre-Bølling-Allerød warming and circulation changes in the deep ocea Nature
Vol 511



I'm Simon!

Would you like to get a custom essay? How about receiving a customized one?

Check it out