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Figure 1- the primary amplitudes of the three orbital forcing
phenomena, the eccentricity amplitude is increasing according to
longer wavelength oscillations difficult (although not impossible) to
observe in the stratigraphic record (Herbert 1997; Elkibbi & Rial
2001)
Why are there multiple cycles of glaciations within the Quaternary?
The Quaternary represents one of the most exciting epochs for researchers in the field of
paleoclimatology, with extensive waxing and waning of the Northern Hemisphere ice sheets,
dramatic changes in the climate and characteristics of the Earth system. During the last glacial
maximum for example, the Northern Hemisphere ice masses were up to 15 times their current size.
Much of Northern Europe and the North American Continent was covered with 2-3km of ice (Mix et
al. 2001) lowering sea level by 123m (Ashkenazy et al. 2012). Temperatures during the glaciations
were about 6°C lower and during interglacials atmospheric CO2 concentrations were 80-100ppm
more than during glacials (Ashkenazy et al. 2012). Although the Quaternary was a time of massive
change for the earth system, the change came and went with a regular oscillation, leading many to
suggest an external mechanism was forcing the growth and decline of ice masses. Throughout the
Quaternary ice sheets waxed and waned with a periodicity of around 40kyr before 0.9Ma BP and
100kyr thereafter (figure 1) (Pisias & Moore 1981). These fluctuations are characterised by a gradual
buildup of ice sheet volume before a rapid collapse event (Denton 2000; Wallmann 2014), evidence
for which can be gathered from the marine and terrestrial stratigraphic record (Lisiecki & Raymo
2005). This essay will present some of the proposed mechanisms behind this, including Milankovitch
cycles (and associated variations of that hypothesis), the Thermohaline Circulation and sea ice.
Milankovitch cycles have long been hailed as the primary forcing agent on earth’s climate (Hays et al.
1976) and the hypothesis focuses on
the variation in insolation the earth
receives from the sun. This variation
occurs due to three phenomena,
obliquity, precession and eccentricity of
the earth’s orbit with periods of 23,000,
42,000, and approximately 100,000
years respectively (figure 1)(Hays et al.
1976). This cyclic change in the Earths
geometry has led to many (e.g Hays et al.
1976; Pisias & Moore 1981; Berger et al.
2005) to conclude that orbital forcing is the
key control on glaciation during the
Quaternary. One key caveat to this theory however is the change in the period of glacials from 40kyr

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Figure 2- multiple proxy records taken from the Vostok Ice core, all correlate
strongly with each other and show strong 100kyr periodicity. Ice volume also
exhibits the asymmetry proposed by (Denton 2000). Taken from (Petit et al.
1999)
to 100ky around 0.9 million years ago, with no discernible change in the Milankovitch cycles (Pena &
Goldstein 2014). This raises questions about the role of Milankovitch forcing at these time intervals.
With such a disagreement within the scientific community as to the importance of orbital forcing in
causing the glacial cycles a number of hypothesis have been presented to explain why the northern
hemisphere ice sheets grew and decayed in the way they did. Many however are in agreement that
orbital forcing considered in isolation has very little (<10%) influence on ice sheets (Wunsch 2004;
Herbert 1997; Denton 2000) and that the true cause lies elsewhere.
With such an apparently small influence, it has been proposed that a climatic feedback mechanism
may be responsible for amplifying the effects of orbital forcing. Stratigraphic data from the Vostok
ice core in Antarctica indicates that CO2 and CH4 concentration (using the deuterium/ hydrogen ratio
in ice) changes with a strong 100kyr cyclicity (figure 2) (Petit et al. 1999; Augustin et al. 2004). Similar
trends have been
observed in a core
from the European
Project for Ice Coring
in Antarctica (EPICA)
going back even
further than Vostok
(Augustin et al. 2004).
This periodicity is not
however accounted
for by traditional
principal Milankovitch
forcing. While eccentricity
does operate on this period its
its effect on the Earth’s
energy balance is extremely small, and not nearly enough to explain the large scale climatic changes
observed (Ganopolski & Calov 2011). This cycle must therefore represent internal oscillations within
the Earth systems for example the ocean-cryosphere system (Gildor & Tziperman 2001; Tziperman &
Gildor 2003; Gildor & Tziperman 2003; Gildor et al. 2013) or the carbonate-climate system (Paillard
& Parrenin 2004). Both of these systems can be modelled and produce synchronous oscillations
phase locked to the eccentricity cycles. Climatic variations on the 100kyr timescale could arise from
the amplitude modulation of precession by eccentricity, which produces well-defined 100, 400 and

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2,400 kyr cycles (evidence for which can be obtained from sedimentation patterns in ocean
cores)(Herbert 1997). In the case of the eccentricity-modulated precession amplitude varies about a
mean according to a lower frequency value (Herbert 1997), which in itself would not be present in
the record. However non-linear systems such as the carbonate-climate system can significantly
increase the energy available from amplitude-modulated forcing and therefore could produce an
outcome with significant energy at the modulating periods (Clemens & Tiedemann 1997).
In such a system an increased eccentricity modulation of precession produces 23kyr CO2 and CH4
maxima that lead to warming and therefore serve to enhance the summer melt as a result of higher
summer insolation in the Northern Hemisphere (Ruddiman 2003). Climate is then driven deeper into
an interglacial period. When eccentricity modulation subsequently decreases, the ice sheets steadily
accumulate due to obliquity forcing and initiate a CO2 feedback mechanism around 41kyr long and
the climate is driven into a glacial. This alteration as a result of eccentricity-modulation is thought to
be responsible for the 100kyr cycles (Ruddiman 2003). When modelled this mechanism can also give
an explanation for the Mid Pleistocene Transition (MTP Pena & Goldstein 2014) 0.9myr BP as a
gradual cooling trend throughout the Pleistocene permitted ice sheets to survive during weak
precession insolation maxima where they would previously have terminated and then go on to grow
large to initiate strong CO2 feedback mechanisms (Ruddiman 2003; Wallmann 2014)
A number of hypothesis have arisen as to the role the ocean system and specifically its interaction
with the Cryosphere plays in modulating glaciations over the Quaternary. The Thermohaline
circulation (THC) has been touted as a potential explanation for the quasi-periodicity observed in the
stratigraphic record (Alonso-Garcia et al. 2011; Pena & Goldstein 2014) and so has sea ice extent
around the primary ice caps (Ashkenazy et al. 2012). By using Nd isotopes to infer the strength of the
THC during the Quaternary evidence has been found for a significant THC disruption during the MPT
which led to exceptional weakening around 0.9Ma BP; a mode the THC has not emerged from since
(Pena & Goldstein 2014). This weakening coincided with a unique situation in which during an
interglacial the THC did not strengthen and could be considered a transglacial (glacial-like
interglacial). At the same time the stability of major ice sheets was increasing leading to a positive
feedback effect (Pena & Goldstein 2014). It’s thought this feedback effect, coupled with the
increased drawdown of atmospheric CO2 due to the reduced mixing and exchange between deep
and surface water, and the anomalously low southern hemisphere insolation values 0.9 Ma BP
helped the climate system to reach the threshold for the post-MPT 100kyr cycles (Pena & Goldstein
2014).
The hypothesis that sea ice acts as a control on Quaternary cycles of glaciation focuses on the
switching of states between extensive sea ice and minimal sea ice cover around a constant landmass

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and its impact on the climate (Ashkenazy et al. 2012). This alternating ‘switch’ acts as a control on
the moisture flux over ice caps and glaciers. For example when the ice extent is at its maximum,
albedo is increased, atmospheric temperatures decrease, the airs ability to transport moisture is
reduced and local evaporation decreases. This starves the terrestrial ice of precipitation, and the sea
ice switch is said to be “on” (Tziperman & Gildor 2003). With a reduced accumulation rate, terrestrial
ice masses retreat over a time frame of around 10,000 years (Ashkenazy et al. 2012). As terrestrial
ice retreats, albedo decreases and the sea ice switch is deemed to be “off”, precipitation increases
and the ice sheets grow again over 90,000 years (Ashkenazy et al. 2012). Simple modelling has
produced the phenomena described by this theory, although it does fail to produce some of the
features observed in the palaeoclimatic record including the temperature and the behaviour of the
Southern Hemisphere ice masses (Gildor & Tziperman 2001). With more modern and complex
modelling, including more processes, the differences in sea ice modes are insufficient to explain the
glacial changes observed, and further research is still needed (Gildor et al. 2013).
With several potential mechanisms forcing glacial cycles during the quaternary, present
understanding and limited modelling capability it may impossible for some time to conclusively say
what causes these oscillations. Although as more processes are being studied, some hypotheses are
being ruled out to a degree. For example it’s no longer widely accepted that Milankovitch cycles are
the dominant causes of glaciation (Wunsch 2004). Rather as of yet poorly understood feedback
mechanisms within the earth system are responsible for forcing the amplitude of change observed
(Larson & Mylroie 2013). Using statistical methods pioneered in the field of economics, the
possibility that a bivariate cause may in fact be unlikely has been raised, with a multivariate
hypothesis incorporating a range of systems a far more likely explanation having removed the bias
inherent in bivariate models (Kaufmann & Juselius 2013).
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