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.
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 Ko ¨ ppen 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
433
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.
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 Ko ¨ ppen 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
433