Ice with everything - Antarctica: Global Science from a Frozen Continent - page 90

Box 3.2 A slowly changing orbit

The movement of our planet on its orbit is responsible for the distribution of incoming solar
energy and the contrasts between low and high latitudes and seasons. If there were only the Sun
and the Earth, the orbit of our planet would be a stable ellipse. However, the gravity of the other
planets of the solar system interfere with the gravity of the Sun and induce slow changes in the
orientation and shape of the orbit of the Earth. The eccentricity of the ellipse slightly varies over
periods of around 100 000 and 400 000 years, changing the magnitude of the seasonal contrasts
because of changing distances between the Earth and the Sun. The axis of rotation of the Earth has
a varying tilt (called obliquity) with respect to the plane of this orbit. Obliquity, varying with
periodicities around 40 000 years, modulates the gradient of insolation between low and high
latitudes, and between seasons. Finally, movements of precession (hula-hoop) of the ellipse and of
the axis of rotation of the Earth result in slow shifts of the position of the seasons on the orbit. The
resulting climatic precession modulates the contrasts between seasons with a periodicity around
19 and 23 000 years. The orbital theory of palaeoclimates suggesting that periodic shifts in
northern hemisphere summer insolation were driving ice ages was formulated in the 1940s by the
Serbian mathematician Milankovitch. The importance of Earth

'

s orbital parameters in driving
past climate variations has been demonstrated since the 1970s through quantitative palaeoclimate
reconstructions, and past climate simulations.

Figure 3.19 Schematic representation of the orbit of the Earth around the Sun, showing an exaggerated

view of the excentricity of the orbit (E), the tilt of the axis of rotation (T), and the movement of precession
(P). (Copyright IPCC 2007: WG1-AR4)