Great Geologists | 93
The key orbital parameters that make up Milankovitch cyclicity.
First, the shape of the Earth’s orbit around the Sun changes
periodically, being sometimes more circular and at other times
more elliptical. The degree of ellipticity of Earth’s orbit around
the Sun is known as its eccentricity. A more elliptical orbit has
high eccentricity. The length of one eccentricity cycle is about
100,000 years. Extreme variations in eccentricity occur with
a cycle time of 405,000 years. At its most elliptical, the extra
distance from the Sun can cut the amount of insolation by as
much 30% compared to when the Earth and Sun are at their
closest. For this reason, Milanković considered eccentricity of
prime importance compared to the other two factors. Recent
studies have noted that the 405,000 year eccentricity cycle is
very stable and can be detected in the rock record for at least
the last 250 million years. Driven by the gravitational interactions
between Jupiter and Venus, it behaves as a metronome, forming
one of the primary drivers of cyclicity seen in sedimentary
sequences. Recently, ‘grand cycles’ with a periodicity of 1.2
million years and 2.4 million years have been recognised, the
result of interactions between Earth and Mars.
Second, the angle or obliquity of the Earth’s axis of rotation
changes periodically. Today this angle is 23.5°, but it cycles
between 21.5° and 24.5° with a periodicity of about 41,000
years.
Third, Earth’s axis of rotation wobbles like a spinning top, so
that axis of rotation draws out a cone over time, giving rise to a
pattern of variation known as precession. This has a periodicity
of around 23,000 years. The main impact comes from the fact
that the seasons occur at different points on the eccentric orbit,
changing the lengths of summer and winter.
The harmonic interference between the different periodicities of
cyclicity was understood by Milanković as being critical in driving
insolation changes and thus climate variations through time.
Milanković argued that these cycles would control the waxing
and waning of polar ice sheets and be a primary control on
the Earth’s climate fluctuations. These ideas were vindicated
in the 1970’s when oxygen isotope records (a proxy for
paleotemperature) from deep-sea sediment cores demonstrated
cyclicity in keeping with Milankovitch’s periodicities. Subsequent
to this, studies on ice cores from the Antarctic that show
temperature and atmospheric greenhouse gases (as determined
from gas bubbles within the ice cores) vary with a periodicity
completely in accord with the orbital cyclicity noted by
Milanković.
In 1939, to collect his scientific work on the theory of orbitally-
driven variations in insolation and their impact on past climates