Mathez, Edmond A. ed. "Profile: Milutin Milankovitch: Seeking the

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INTERPLANETARY
ROCKS
Milutin Milankovitch:
Seeking the Cause
of the Ice Ages:Profile
Portrait of Milutin Milankovitch (1879-1958) by
Paja Jovanovic, 1943.
What causes an ice age? Will another one
occur? Since it had become generally
recognized by the mid-nineteenth-century that
much of Europe had once been covered by a
great sheet of ice, scientists have been
wondering what could cause such vast shifts in
the Earth’s climate. Some began looking for
underlying astronomical causes. They already
knew that the tilt of Earth’s axis caused
seasonal change, and also that small variations
in Earth’s orbit, over tens of thousands of years,
affected the amount of solar energy reaching
Earth. Several scientists had proposed the
existence of a cycle of global winters, but none
of their figures seemed accurate, and testing
their reliability was difficult. Each theory was
eventually shelved.
In 1911 a young Serbian mathematician, Milutin
Milankovitch, decided to chart the ice ages of
the Pleistocene. (The Pleistocene is the epoch
that began 1.8 million years ago and ended
about 11,500 years ago. It was characterized
by lengthy ice ages, when glaciers covered
large regions of the continents, interrupted by
short interglacial periods, when the climate
was temperate.) All Milankovitch’s calculations
were done by hand, and he worked at them
obsessively for the next thirty years. He
incorporated new information about small
variations in the tilt of the Earth’s axis, and
factored in small orbital changes caused by the
gravitational tug of other planets. Each of these
orbital variations has its own time scale, and
consequently they interact in different ways over
time, but each one is regular. Going back
600,000 years in his computations, he carefully
calculated the effect of these factors on
incoming solar radiation across the Northern
Hemisphere. The charts and tabulations
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Milankovitch created are still used today. He
also measured summer solar radiation curves in
high northern latitudes, where the ice age
glaciers originated, linking certain low points
with four previous European Pleistocene ice
ages. Ultimately, the mathematician arrived at a
complete astronomical theory of glaciation.
The horizontal striations in this outcrop, in New York’s
Central Park, were caused by scouring from boulders
imbedded beneath an advancing glacier.
On the basis of his analysis, Milankovitch
concluded that Earth’s orbit changes in three
cycles of different lengths. The shape of Earth’s
orbit around the Sun changes from less to more
and back to less elliptical in about 96,000
years. The Earth is tilted on its axis of rotation
relative to the solar plane, currently at an angle
of 23.5°. This tilt changes, however, from 21.5°
to 24.5° and back again in about 41,000 years.
Finally, the Earth’s axis of spin wobbles with a
period of 23,000 years. The challenges for
Milankovitch were to understand when the three
cycles were coincident with each other and how
they worked together to influence insolation (the
amount of solar radiation received by the Earth).
Based on his computations, Milankovitch
theorized variations of more than twenty
percent in the amount of sunshine reaching the
northern latitudes. In his 1941 account, Canon
of Insolation and the Ice Age Problem, he
suggested that this caused the waxing and
waning of the great continental ice sheets.
Like that of several predecessors,
Milankovitch’s work was greeted with
considerable excitement, but was then largely
dismissed. Ice ages are difficult to date, partly
because each erases much of the traces of its
predecessor. However, the tables were turned
by the late 1960s. Technical advances made it
possible for geologists to study deep-sea
sediment cores that contain a climate record
going back millions of years. This climate
record shows remarkably regular variations,
which correlate with the mathematician’s figures
and which are now known as Milankovitch
cycles. However, it is also clear that
astronomical factors alone cannot cause the
large changes that the Earth experienced.
Other factors must also influence climate but
scientists still do not know how.