An introduction to climate change - Climate Change: Biological and Human Aspects - page 22

Variations in the angle of the
Earth's tilt change with a cycle
of about 41 000 years.

Variations in the Earth's orbital
eccentricity (the deviations of the
orbit from a true circle to an ellipse)
change with a cycle of about
93 000 years .

Precession of the equinoxes
(the way the Earth's axis itself
spins slowly like a toy gyroscope),
which takes place with a cycle
of about 22 000 years.

Fig. 1.7 Milankovitchorbitalparametersofeccentricity,axialtiltandprecessionoftheequinoxes.Eachparameter

variesthroughacycleofdiferentperiod.EachafectsthewaytheEarthiswarmedbytheSun.

Afro-Eurasian continents restricts oceanic currents, and hence heat transport, about
the planet. Oceanic currents are further restricted at the poles, in the north by the pres-
ence of the North American and Euro-asian continents and in the south by Antarctica.
The way heat is accepted by and transported about the Earth under these constraints
makes the planet prone to glacials. Milankovitch calculated a theoretical energy curve
for changes over time based on the variations of three orbital parameters: angle of
axial tilt, or obliquity, which varies between 22 and 24.6 ◦ over a 41 000-year cycle;
orbital eccentricity (the degree of elliptical deviation from a true circle), which varies
every 93-136 000 years (so, approximately 100 000 years) between a true circle (an
eccentricity of 0) and an ellipse (of eccentricity 0.05) and back again (there is also a
400 000-year resonance); and precession of the equinoxes (the way the Earth's axis
spins slowly like a gyroscope), which takes place with a cycle of roughly 19 000-
23 000 years (effectively 22 000 years; see Figure 1.7). The climatic relevance of this
last, put simply, means that at a specific time of year, say mid-summer's day, the
Earth is closer or not to the Sun than on other mid-summer days. Using these factors
Milankovitch could predict when the Earth was likely to experience a cool glacial or
a warm interglacial (Milankovitch, 1920).