3.1.5.5. Secular Variations

Observations of the secular trend in the orbital excitations contain a mixture of signals from the solid Earth's return toward isostatic equilibrium after the removal of large ice loads of the last glacial maximum and the signals from current ice-sheet mass balance changes [James and Ivins, 1995] and the related change in global mean sea level. Current results from Lageos-1 [Eanes, 1995] indicate that

$J$2 + 0.037 J4 + 0.08 J6 + 0.01 J8 = (- 2.4 ± 0.2 ) x 10-11yr-1

and from Starlette [Eanes and Bettadpur, 1995]
$J$2 + 0.04 J4 - 0.55 J 6 - 0.15 J 8 + ... = ( - 2.9 ± 0.3 ) x 10-11yr-1

Cheng et al. [1995] attempted to separate the constraint equations from these two satellites and 4 others into estimates of the secular changes in the individual harmonics. The results are
$J$2 = - 2.7 ± 0.5 , J4 - 1.3 ± 1.0 x 10-11yr-1

Analogous results for the odd-degree zonal harmonics have been obtained; however, these results are more strongly affected by nongravitational perturbations, and efforts to establish realistic error estimates are now in progress.