  The time series of northern hemisphere (NHSI) and southern hemisphere (SHSI)
sea-ice extent are submitted to singular spectral analysis (SSA). The
components are analyzed with Laplace's formulation of the Liouville-Euler
system. As already shown in a previous work, the trends observed in the time
series are quasi linear, decreasing for NHSI and increasing for SHSI. The
amplitude of annual variations in the Antarctic is double that in the Arctic,
they are in phase opposition, modulated. The semi-annual components are in
quadrature. The first 3 components of both NHSI and SHSI at 1, 1/2 and 1/3 yr
account for more than 95% of the signal variance. We complement previous
analyses of variations in pole position (PM = m1, m2) and length of day (lod).
Whereas SSA of lod is dominated by the same first 3 components as sea-ice, SSA
of PM contains only the 1 yr and the Chandler components. The 1 yr component of
NHSI is in phase with that of lod and in phase opposition with m1. The reverse
holds for the 1 yr component of SHSI. We note that the semi-annual component
appears in lod not in m1. The annual and semi-annual components of NHSI and
SHSI are much larger than the trends observed since 1978, that leads us to test
whether a first order geophysical or astronomical forcing should not be
preferred to the mechanisms generally suggested as a forcing factor of the
trends. The lack of modulation of the largest forced component suggests an
alternate mechanism. In Laplace's paradigm, the torques exerted by the Moon,
Sun and planets play the leading role as the source of forcing of many
geophysical phenomena. These forces lead to changes in the inclination of the
Earth's rotation axis, setting Earth masses in motion and resulting in thermal
dissipation. It is variations in inclination of the rotation axis that lead to
the large annual components of melting and re-freezing of sea-ice.
