PLIO-PLEISTOCENE GLACIAL CYCLES AND
MILANKOVITCH VARIABILITY
K. H. Nisancioglu, Bjerknes Centre for Climate
Research, University of Bergen, Bergen, Norway
& 2009 Elsevier Ltd. All rights reserved.
Introduction
A tremendous amount of data on past climate has
been collected from deep-sea sediment cores, ice
cores, and terrestrial archives such as lake sediments.
However, several of the most fundamental questions
posed by this data remain unanswered. In particular,
the Plio-Pleistocene glacial cycles which dominated
climate during the past B2.8 My have puzzled scientists. More often than not during this period large
parts of North America and northern Europe were
covered by massive ice sheets up to 3 km thick, which
at regular intervals rapidly retreated, giving a sea
level rise of as much as 120 m.
The prevalent theory is that these major fluctuations in global climate, associated with the glacial
cycles, were caused by variations in insolation at
critical latitudes and seasons. In particular, ice sheet
growth and retreat is thought to be sensitive to high
northern-latitude summer insolation as proposed by
Milankovitch in his original astronomical theory.
Brief History of the Astronomical
Theory
Long before the first astronomical theory of the ice
ages, the people of northern Europe had been puzzled
by the large erratic boulders scattered a long way
from the Alpine mountains where they originated.
Based on these observations the Swiss geologist and
zoologist Louis Agassiz presented his ice age theory at
a meeting of the Swiss Society of Natural Sciences in
Neuchatel in 1837, where he claimed that the large
boulders had been transported by Alpine glaciers
covering most of Switzerland in a past ice age.
A few years later, the French mathematician
Joseph Alphonse Adhemar was the first to suggest
that the observed ice ages were controlled by variations in the Earth’s orbit around the Sun. At this
point it was known that there had been multiple
glaciations, and Adhemar proposed that there had
been alternating ice ages between the North and the
South Pole following the precession of the equinoxes.
504
Indeed, the winter is warmer when the Earth is at the
point on its orbit closest to the Sun, and colder when
the Earth is furthest from the Sun. Adhemar correctly
deduced that the precession of the equinoxes had a
period of approximately 21 000 years, giving alternating cold and warm winters in the two hemispheres every 10 500 years.
In 1864, James Croll expanded on the work by
Adhemar and described the influence of changing
eccentricity on the precession of the equinoxes. He
assumed that winter insolation controlled glacial
advances and retreats, and determined that the precession of the equinoxes played an important role in
regulating the amount of insolation received during
winter. Based on this, he estimated that the last ice
age lasted from about 240 000 to 80 000 years ago.
Croll was aware of the fact that the amplitude of the
variations in insolation was relatively small, and
introduced the concept of positive feedbacks due to
changing surface snow and ice cover as well as
changes in atmosphere and ocean circulation.
In parallel to the work of Croll, geologists in
Europe and America found evidence of multiple
glacial phases separated by interglacial periods with
milder climate similar to that of the present day, or
even warmer. These periodic glaciations were consistent with Croll’s astronomical theory. However,
most geologists abandoned his theory after mounting
evidence from varved lake sediments in Scandinavia
and North America showed that the last glacial
period ended as late as 15 000 years ago, and not
80 000 years ago as suggested by Croll.
Milankovitch’s Astronomical Theory of
the Ice Ages
Following Croll’s astronomical theory there was a
period where scientists such as Chamberlin and
Arrhenius tried to explain the ice ages by natural
variations in the atmospheric content of carbon dioxide. The focus of the scientific community on an
astronomical cause of the ice ages was not renewed
until the publication of Milankovitch’s theory in a
textbook on climate by the well-known geologists
Wladimir Köppen and Alfred Wegener in 1924. This
was the first comprehensive astronomical theory of the
Pleistocene glacial cycles, including detailed calculations of the orbitally induced changes in insolation.
Milutin Milankovitch was of Serbian origin, born
in 1879. He obtained his PhD in Vienna in 1904 and
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
techniques had made it possible to recover climate records covering the last 500 000 years. By studying the
variations in oxygen isotopes of foraminifera in deepsea sediment cores as well as by reconstructing past sea
level from terraces of fossil coral reefs, new support
was emerging for an astronomical phasing of the glacial cycles. The oxygen isotope data from the long
deep-sea cores presented in a paper in 1976 by Hays
et al. were considered as proof of the Milankovitch
theory, as they showed cycles with lengths of roughly
20 000 and 40 000 years as well as 100 000 years in
agreement with Milankovitch’s original calculations.
was later appointed Professor of Applied Mathematics at the University of Belgrade. He was captured during World War I, but allowed to work at the
Hungarian Academy of Sciences, where he completed his calculation of the variations of the orbital
parameters of the Earth and their impact on insolation and climate. Milankovitch’s basic idea was
that at times of reduced summer insolation, snow
and ice could persist at high latitudes through
the summer melt season. At the same time, the cool
summer seasons were accompanied by mild winter
seasons leading to enhanced winter accumulation
of snow. When combined, reduced summer melt and
a slight increase in winter accumulation, enhanced
by a positive snow albedo feedback, could eventually
lead to full glacial conditions.
During World War II, Milankovitch worked on a
complete revision of his astronomical theory which
was published as the Kanon der Erdbestrahlung in
1941. However, the scientific establishment was critical
of Milankovitch, and his theory was largely rejected
until the early 1970s. By this time, great advances
in sediment coring, deep-sea drilling, and dating
(a)
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Orbital Parameters and Insolation
The Earth’s orbit around the Sun is an ellipse where
the degree to which the orbit departs from a circle is
measured by its eccentricity (e). The point on the orbit
closest to the Sun is called the perihelion, and
the point most distant from the Sun the aphelion
(Figure 1). If the distance from the Earth to the
Sun is rp at perihelion, and ra at aphelion, then the
eccentricity is defined as e ¼ ðra # rp Þ=ðra þ rp Þ.
Spring equinox
Today
Spring
Win
ter
Summer solstice
!
Winter solstice
Perihelion
Aphelion
"
Sum
mer
Fall
Fall equinox
(b)
11 000 years ago
Fall equinox
Winter solstice
Sun
!
Perihelion
Summer solstice
Aphelion
Earth
Spring equinox
Figure 1 Sketch of the Earth’s orbit around the Sun today and at the end of the last glacial cycle (11 000 years ago), showing the
positions of the solstices and equinoxes relative to perihelion. The longitude of perihelion (o) is measured as the angle between the
line to the Earth from the Sun at spring equinox and the line to the Earth at perihelion.
506
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
Variations in the eccentricity of the Earth’s orbit follow cycles of 100 000 and 400 000 years giving a
change in annual mean insolation on the order of
0.2% or less. This change in insolation is believed to
be too small to produce any notable effect on climate.
A more significant change in insolation is caused
by variations in the seasonal and latitudinal distribution of insolation due to obliquity. Obliquity (e) is
the angle between Earth’s axis of rotation and the
normal to the Earth’s plane around the Sun (Figure 1). This angle is 23.51 today, but varies between
values of 22.11 and 24.51 with a period of 41 000
years. A decrease in obliquity decreases the seasonal
insolation contrast, with the largest impact at high
latitudes. At the same time, annual mean insolation
at high latitudes is decreased compared to low latitudes. An example of the effect of obliquity variations on seasonal insolation is shown in Figure 2(a).
During times when obliquity is small, high-latitude
summertime insolation decreases, whereas midlatitude wintertime insolation increases. The magnitude of the change in high-latitude summer
insolation due to obliquity variations can be as large
as 10%.
The third and last variable affecting insolation
is the longitude of perihelion (o). This parameter is
(a)
90
−40
60
Latitude (deg)
defined as the angle between the line to the Earth
from the Sun at spring equinox and the line to the
Earth at perihelion (Figure 1). It determines the direction of the Earth’s rotational axis relative to the
orientation of the Earth’s orbit around the Sun,
thereby giving the position of the seasons on the orbit
relative to perihelion. Changes in the longitude of
perihelion result in the Earth being closest to the Sun
at different times of the year. Today, the Earth is
closest to the Sun in early January, or very near winter
solstice in the Northern Hemisphere. All other things
being equal, this will result in relatively warm winter
and cool summer seasons in the Northern Hemisphere, whereas the opposite is the case in the
Southern Hemisphere. At the time of the last deglaciation, 11 000 years ago the Earth was closest to the
Sun at summer solstice, resulting in extra warm
summers and cool winters in the Northern Hemisphere. An example of the effect of changes in precession on seasonal insolation is shown in Figure 2(b).
If the Earth’s orbit were a circle, the distance to the
Sun would remain constant at all times of the year and
it would not make any difference where on the orbit
the seasons were positioned. Therefore, the impact of
variations in the longitude of perihelion depends on
the eccentricity of the Earth’s orbit and is described by
30
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SE
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SE −60 0 60
(W m−2)
WS
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SE
SS
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−80
FE
Time of year
0
0
40
−4
40
Latitude (deg)
FE
Time of year
90
−40
(b)
SS
WS
SE −60 0 60
W m−2
Figure 2 Insolation difference in units of W m # 2 as a function of latitude and season: (a) when decreasing obliquity from 24.51 to 221
in the case of a perfectly circular orbit (e ¼ 0); and (b) for a change in precession going from summer solstice at perihelion to summer
solstice at aphelion while keeping obliquity at today’s value (e ¼ 23.51) and using a mean value for eccentricity (e ¼ 0.03). The annual
mean insolation difference is shown to the right of each figure and the seasons are defined as follows: FE, fall equinox; SE, spring
equinox; SS, summer solstice; WS, winter solstice.
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
the precession parameter (e sin o). The combined effect of eccentricity and longitude of perihelion can
give changes in high-latitude summer insolation on
the order of 15% and varies with periods of 19 000
and 23 000 years, but is modulated by the longerperiod variations in eccentricity. Figure 3 shows the
variations in obliquity (e), eccentricity (e), and the
precession parameter (e sin o).
507
(a)
25
24
23
22
0
200
400
600
800
1000
Time before present (millenia)
Plio-Pleistocene Glacial Cycles
Some of the longest continuous records of past climate come from deep-sea sediment cores. Ocean
sediments are laid down over time, and by drilling
into the seafloor, layered sediment cores can be extracted containing valuable information about the
conditions at the time when the layers were formed.
By studying the relative abundance of oxygen isotopes
in shells of tiny marine organisms (foraminifera)
found in the sediments, it is possible to estimate the
amount of water tied up in the continental ice sheets
and glaciers. This is because water molecules containing the lighter isotope of oxygen (16O) are more
readily evaporated and transported from the oceans to
be deposited as ice on land. Thus, leaving the ocean
water enriched with the heavy oxygen isotope (18O)
during glacial periods. However, the fractionation of
the oxygen isotopes when forming the shells of the
foraminifera also depends on the surrounding water
temperature: low water temperature gives higher
d18O values (the ratio of 18O and 16O relative to a
standard). Therefore records of d18O are a combination of ice volume and temperature. By analyzing
benthic foraminifera living on the seafloor where the
ocean is very cold, and could not have been much
colder during glacial times, the contribution of temperature variations to the d18O value is reduced.
The benthic d18O ice volume record of Hays et al.
from 1976 was one of the very first continuous
records of the late Pleistocene extending back to
the Brunhes–Matuyama magnetic reversal event
(780 000 years ago), making it possible to construct a
timescale by assuming linear accumulation rates.
Analysis of the data showed cycles in ice volume with
periods of about 20 000 years and 40 000 years, with
a particularly strong cycle with a period of roughly
100 000 years. Later studies extended the record past
the Brunhes–Matuyama reversal, showing that the
late Pliocene (3.6–1.8 Ma) and early Pleistocene
records (1.8–0.8 Ma) were dominated by smalleramplitude cycles with a period of 41 000 years,
rather than the large 100 000 years cycles of the late
Pleistocene (0.8–0 Ma).
Many records generated since this time have confirmed these early observations, namely:
(b)
0.06
0.04
0.02
0
0
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1000
Time before present (millenia)
Summer
Winter
(c)
0.05
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-0.05
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Time before present (millenia)
Summer
Winter
Figure 3 The three most important cycles regulating insolation
on Earth are obliquity, eccentricity, and precession: (a) obliquity, or
tilt of the Earth’s axis varies with a period of 41 000 years;
(b) eccentricity of the Earth’s orbit varies with periods of 100 000
years and 400 000 years; and (c) precession of the equinoxes has
a dominant period of 21 000 years and is modulated by eccentricity.
1. from about 3 to 0.8 Ma, the main period of ice
volume change was 41 000 years, which is the
dominant period of orbital obliquity;
2. after about 0.8 Ma, ice sheets varied with a period
of roughly 100 000 years and the amplitude of
508
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
2.0
41 ka
~100 ka
2.5
#18 O (per mil)
3.0
3.5
4.0
4.5
5.0
5.5
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−2500
−2000
−1500
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−500
0
Time before present (millenia)
Figure 4 Benthic foraminiferal d18O ice volume record from the North Atlantic plotted to a paleomagnetic timescale covering the last
3 My. The transition from a dominant 41 000 to a 100 000-year periodicity in ice volume occurs close to the Brunhes–Matuyama
magnetic reversal event (B780 000 years ago).
oscillations in d18O increases, implying growth of
larger ice sheets.
The long benthic d18O record from a deep-sea
sediment core extracted from the North Atlantic
shown in Figure 4 illustrates both of these points.
This isotope record is plotted with a paleomagnetic
timescale determined by the depth of magnetic field
reversals recorded by ferromagnetic grains in the
sediment core. Using this simple timescale, which is
not biased by orbital tuning, one can clearly observe
the 41 000-year periodicity of the late Pliocene and
early Pleistocene (3.0–0.8 My), as well as the dominance of the stronger B100 000-year periodicity of
the late Pleistocene (last 800 000 years).
Note that the main periods of orbital precession
(19 000 and 23 000 years) are of less importance in the
benthic ice volume record, whereas it is known that
they increase in strength after about 800 000 years (the
mid-Pleistocene transition). The lack of an imprint
from orbital precession in the early part of the record
and the reason for the dominance of roughly 100 000
years periodicity in the recent part of the record are
some of the major unanswered questions in the field.
Only eccentricity varies with periods matching the
roughly 100 000 years periods observed in the late
Pleistocene. Although eccentricity is the only orbital
parameter which changes the annual mean global
insolation received on Earth, it has a very small impact. This was known to Croll and Milankovitch,
who saw little direct importance in variations in eccentricity and assumed that changes in precession
and obliquity would dominate climate by varying the
amount of seasonal, rather than annual mean insolation received at high latitudes. Milankovitch postulated that the total amount of energy received from
the Sun during the summer at high northern latitudes
is most important for controlling the growth and
melt of ice. To calculate this insolation energy, he
divided the year into two time periods of equal
duration, where each day of the summer season received more insolation than any day of the winter
season. The seasons following these requirements
were defined as the caloric summer and caloric
winter half-years. These caloric half-years are of
equal duration through time and the amount of insolation energy received in each can be compared
from year to year.
For Milankovitch’s caloric summer half-year insolation (Figure 5),obliquity (e) dominates at high latitudes (4651 N), whereas climatic precession (e sin o)
dominates at low latitudes (o551 N). In the mid-latitudes (B55 # 651 N), the contribution by obliquity
and climatic precession are of similar magnitude. In
the Southern Hemisphere, variations in caloric halfyear insolation due to obliquity are in phase with the
Northern Hemisphere and could potentially amplify
the global signal, whereas variations due to climatic
precession are out of phase. By taking into account the
positive snow albedo feedback, Milankovitch used his
caloric insolation curves to reconstruct the maximum
extent of the glacial ice sheets back in time (Figure 6).
Milankovitch’s predicted cold periods occurred
roughly every 40 000–80 000 years, which fit reasonably well with the glacial advances known to
geologists at that time. However, as the marine
sediment core data improved, it became clear that
the last several glacial periods were longer and had a
preferred period of roughly 100 000 years (Figure 4),
which was not consistent with Milankovitch’s original predictions. Based on these observations, and
without knowledge of the 41 000 years cycles of the
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
509
6
80° N
60° N
4
(J m−2) × 106
40° N
2
0
2
4
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Time before present (millenia)
Figure 5 Caloric summer half-year insolation following Milankovitch’s definition plotted for the latitudes of 401 N, 601 N, and 801 N.
Caloric half-years are periods of equal duration where each day of the summer half-year receives more insolation than any day of the
winter half-year.
early Pliocene and late Pleistocene, scientists reasoned that climatic precession and its modulation by
eccentricity must play the leading role in past climates. Following Andre Berger and others, who recalculated and improved the records of orbital
insolation, most researchers replaced the caloric halfyear insolation as a driver of glacial climate by midmonth, or monthly mean insolation, for example,
June or July at 651 N (Figure 7).
As can be seen from Figure 7, monthly mean insolation is dominated by precession. As insolation
time series at a given time of the year (e.g., June or
July) are in phase across all latitudes of the same
hemisphere, the proxy records could be compared
equally well with insolation from other latitudes
than the typical choice of 651 N shown here. This
means that any direct response of climate at high
latitudes to monthly or daily insolation requires a
strong presence of precession in the geologic record.
Although both the frequencies of precession and
obliquity are clearly found in the proxy records, a
simple linear relationship between summer insolation and glacial cycles is not possible. This is particularly true for the main terminations spaced at
roughly 100 000 years, which must involve strongly
nonlinear mechanisms.
The strong positive feedback on global climate
caused by greenhouse gases, such as CO2, was
pointed out as early as 1896 by the Swedish physical
chemist Svante Arrhenius. Shortly thereafter, the
American geologist Thomas Chamberlin suggested a
possible link between changing levels of CO2 and
glacial cycles. From the long ice cores extracted from
Antarctica, covering the last 740 000 years, it is now
known that atmospheric levels of CO2 closely follow
the glacial temperature record (Figure 8). Although
greenhouse gases, such as CO2, cannot explain the
timing and rapidity of glacial terminations, the
changing levels of atmospheric greenhouse gases
clearly contributed by amplifying the temperature
changes observed during the glacial cycles.
Modeling the Glacial Cycles
Following the discovery of the orbital periods in the
proxy records, a considerable effort has gone into
modeling and understanding the physical mechanisms
involved in the climate system’s response to variations
in insolation and changes in the orbital parameters. In
this work, which requires modeling climate on orbital
timescales (410 000 years), the typical general circulation models (GCMs) used for studying modern climate and the impact of future changes in greenhouse
gases require too much computing power. These
GCMs can be used for simulations covering a few
thousand years at most, but provide valuable equilibrium simulations of the past climates, such as the
Last Glacial Maximum (LGM).
Instead of the GCMs, it has been common to use
Energy Balance Models (EBMs) to study changes in
climate on orbital timescales. These types of models
can be grouped into four categories: (1) annual mean
atmospheric models; (2) seasonal atmospheric models with a mixed layer ocean; (3) Northern Hemisphere ice sheet models; and (4) coupled climate–ice
sheet models, which in some cases include a representation of the deep ocean.
Studies with the first type of simple climate models
were pioneered by the early work of Budyko in the
1960s, who investigated the sensitivity of climate to
changes in global annual mean insolation. However,
changes in the Earth’s orbital parameters result in a
510
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
Latitude equivalent
60°
65°
Millenia
Würm I
70°
Riss II
75°
Riss I
300 290 280 270 260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100
60°
Millenia
Mindel II
65°
Mindel I
70°
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Günz I
Latitude equivalent
Figure 6 Milankovitch’s reconstructed maximum glacial ice extent for the past 600 000 years. From Milankovitch M (1998) Canon of
Insolation and the Ice-Age Problem (orig. publ. 1941). Belgrade: Zavod za Udzbenike I Nastavna Sredstva.
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
W m−2
(a)
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Figure 7 Summer solstice insolation at (a) 651 N and (b) 251 N for the past 500 000 years.
300
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CO2 (ppmv)
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Time (years before 1950)
Figure 8 Variations in deuterium (dD; black), a proxy for local temperature, and atmospheric concentrations of the greenhouse
gases carbon dioxide (CO2; red), and methane (CH4; green), from measurements of air trapped within Antarctic ice cores. Data from
Spahni R, Chappellaz J, Stocker TF, et al. (2005) Atmospheric methane and nitrous oxide of the late Pleistocene from Antarctica ice
cores. Science 310: 1317–1321 and Siegenthaler U, Stocker TF, Monnin E, et al. (2005) Stable carbon cycle-climate relationship
during the late Pleistocene. Science 310: 1313–1317.
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PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
redistribution of insolation with latitude and time of
the year, with a negligible impact on global annual
mean insolation. Therefore, annual mean models are
not adequate when investigating the impact of orbital
insolation on climate, as they cannot capture the
parts of the insolation variations which are seasonal
and translate them into long-term climate change.
The second type of models includes a representation
of the seasonal cycle, and has been used to investigate
the orbital theory of Milankovitch. In this case, the
seasonal variations in orbital insolation are resolved.
However, as for the first type of models, past changes
in ice cover are assumed to follow the simulated
variations in the extent of perennial snow. This approach assumes that ice cover and the powerful ice
albedo feedback are governed only by temperature, as
the extent of snow in these models is fixed to the
latitude with a temperature of 0 1C. In reality, the
growth and decay of land-based ice sheets are governed by the balance of accumulation and ablation.
Therefore, when investigating changes in ice cover, it is
necessary to include an appropriate representation of
the dynamics and mass balance of ice sheets in the
model.
The third type of models improves upon this by
focusing on modeling past changes in mass balance
and size of typical Northern Hemisphere ice sheets,
such as the Laurentide. This type of studies was
initiated by Weertman in the 1960s who used simple
ice sheet models, forced by a prescribed distribution
of accumulation minus ablation, to predict ice
thickness versus latitude. These models do not calculate the atmospheric energy balance in order to
estimate snowfall and surface melt; instead, changes
to the prescribed distribution of net accumulation
follow variations in mean summer insolation.
The fourth type of models include zonal mean
seasonal climate models coupled to the simple
Weertman-type ice sheet model, as well as earth
models of intermediate complexity (EMICs) coupled
to a dynamic ice sheet. These models give a more
realistic representation of the climate as compared
with the simpler models.
Partly due to the lack of good data on variations in
global ice volume older than about half a million
years, most model studies have focused on understanding the more recent records dominated by the
B100 000 years glacial cycles. All of these models
respond with periods close to the precession and obliquity periods of the insolation forcing. However, the
amplitude of the response is in most cases significantly
smaller than what is observed in the proxy records. At
the same time, the dominant B100 000 year cycles of
the ice volume record, characterized by rapid deglaciations, are only found when including a time lag in
the response of the model. Such an internal time lag
can be produced by taking into account bedrock depression under the load of the ice, or by adding a
parametrization of ice calving into proglacial lakes, or
marine incursions at the margin of the ice sheet. Alternatively, the B100 000-year cycles have been explained as free, self-sustained oscillations, which might
be phase-locked to oscillations in orbital insolation.
One of the very few model studies that have investigated variations in ice volume before the late
Pleistocene transition (B800 000 years ago) used a
two-dimensional climate model developed at
Louvain-la-Neuve in Belgium. It falls within the definition of an EMIC and includes a simple atmosphere
coupled to a mixed layer ocean, sea ice and ice sheets.
By forcing this model with insolation and steadily
decreasing atmospheric CO2 concentrations, the
model reproduces some of the characteristics of the
ice volume record. The 41 000-year periodicity is
present in the simulated ice volume for most of the
past 3 My and the strength of the 100 000-year signal
increases after about 1 My. However, a longer
400 000-year year period is also present and often
dominates the simulated Northern Hemisphere ice
volume record.
This nicely illustrates the remaining questions in the
field. It is expected that models responding to the
100 000-year period will also respond to the longer
400 000-year period of eccentricity. However, this later
period is not present in the ice volume record. At the
same time, the late Pleistocene transition from a
dominance of 41 to B100 000-year period oscillations
in ice volume is not well understood. Explanations for
the transition which have been tested in models are: a
steady decrease in CO2 forcing and its associated slow
global cooling; or a shift from a soft to a hard sediment bed underlying the North American ice sheet
through glacial erosion and exposure of unweathered
bedrock. Neither of these changes are in themselves
abrupt, but could cause a transition in the response of
the ice sheets to insolation as the ice sheets grew to a
sufficiently large size.
In addition to the challenge of modeling the midPleistocene transition, no model has successfully reproduced the relatively clean 41 000-year cycles
preceding the transition. Following the transition,
the models only exhibit a good match with the observed glacial cycles when forced with reconstructed
CO2 from Antarctic ice cores together with orbital
insolation.
Summary
The Plio-Pleistocene glacial cycles represent some of
the largest and most significant changes in past
PLIO-PLEISTOCENE GLACIAL CYCLES AND MILANKOVITCH VARIABILITY
climate, with a clear imprint in terrestrial and marine
proxy records. Many of the physical mechanisms
driving these large cycles in ice volume are not well
understood. However, the pursuit to explain these
climate changes has greatly advanced our understanding of the climate system and its future response
to man-made forcing. New and better resolved proxy
records will improve our spatial and temporal picture
of the glacial cycles. Together with the advent of
comprehensive climate models able to simulate longer
periods of the glacial record, scientists will be able to
better resolve the interaction of the atmosphere,
ocean, biosphere, and ice sheets and the mechanisms
linking them to the astronomical forcing.
Nomenclature
e
ra
rp
d18O
e
o
eccentricity
aphelion
perihelion
oxygen isotope ratio (ratio of
16
O relative to a standard)
obliquity
longitude of perihelion
18
O and
See also
Deep-Sea Drilling Methodology. Deep-Sea Drilling
Results. Monsoons, History of. Oxygen Isotopes in
the Ocean. Satellite Remote Sensing of Sea
Surface Temperatures. Stable Carbon Isotope
Variations in the Ocean.
513
Further Reading
Bard E (2004) Greenhouse effect and ice ages: Historical
perspective. Comptes Rendus Geoscience 336: 603--638.
Berger A, Li XS, and Loutre MF (1999) Modelling
Northern Hemisphere ice volume over the last 3 Ma.
Quaternary Science Reviews 18: 1--11.
Budyko MI (1969) The effect of solar radiation variations
on the climate of the Earth. Tellus 5: 611--619.
Crowley TJ and North GR (1991) Paleoclimatology. New
York: Oxford University Press.
Hays JD, Imbrie J, and Shackleton NJ (1976) Variations in
the Earth’s orbit: Pacemakers of the ice ages. Science
194: 1121--1132.
Imbrie J and Imbrie KP (1979) Ice Ages, Solving the
Mystery. Cambridge, MA: Harvard University Press.
Köppen W and Wegener A (1924) Die Klimate Der
Geologischen Vorzeit. Berlin: Gebrüder Borntraeger.
Milankovitch M (1998) Canon of Insolation and the IceAge Problem (orig. publ. 1941). Belgrade: Zavod za
Udzbenike I Nastavna Sredstva.
Paillard D (2001) Glacial cycles: Toward a new paradigm.
Reviews of Geophysics 39: 325--346.
Saltzman B (2002) Dynamical Paleoclimatology. San
Diego, CA: Academic Press.
Siegenthaler U, Stocker TF, Monnin E, et al. (2005) Stable
carbon cycle–climate relationship during the late
Pleistocene. Science 310: 1313--1317.
Spahni R, Chappellaz J, Stocker TF, et al. (2005)
Atmospheric methane and nitrous oxide of the late
Pleistocene from Antarctica ice cores. Science 310:
1317--1321.
Weertman J (1976) Milankovitch solar radiation variations
and ice age ice sheet sizes. Nature 261: 17--20.