I Picked this comment up on a recent WUWT sea level thread. LOD is Length of Day, a measurement of the Earth’s speed of rotation, which varies for several reasons. For some reason yet to be fully explained, its variation seems to correlate well with the changing disposition of mass in the solar system above and below the solar equatorial plane (SSBz), and detrended global average temperature.
Jens Bagh says:
May 16, 2012 at 1:03 am
Were sea levels to be rising this would slow down the rotation of the earth. Has any change been noted in the rate of change of rotation and if so how does this compare to the present study?
Oddly enough, LOD responds to sea level rise differently depending on whether the source is thermal expansion or melting ice. Of course thermal expansion adds no mass, but it does move it further from the center of gravity so that it flows toward expanding shallow coasts. It just so happens that shallow coasts are concentrated nearer to the poles than would be expected at random, much reducing the rate at which thermal expansion increases LOD or even reversing it, whereas rise due to melted ice should increase LOD by about .1ms per cm. So to determine the overall effect we have to know what fraction of rise is due to which source, when in fact the rise is so miniscule that neither satellites nor gauges can measure the combined effect accurately.
Still, LOD places limits on possible ice mass transfer, but core/mantle coupling is suspected of having a greater decadal effect on earth rotation. The earth’s loss of angular momentum can be calculated by measuring the rate of lunar recession. Secular deceleration of earth rotation can be calculated from ancient astronimical observations. The former gives a rate of 2.3ms/century and the latter yields 1.7. The difference is attributed to isostatic adjustment. The past couple of years have seen LOD hang around 1ms over the 1800 standard, meaning .5ms/century, a third of the normal. And since the advent of the atomic clock LOD has hardly increased at all, though it fluctuates on a number of time scales.
The core/mantle excuse is only good up to a point–time wise and amplitude wise: catastrophic sea level rise would certainly increase LOD to an easily measured rate, and this is not happening. J2 –a measure of earth curvature–can theoretically distinguish between core angular momentum contribution and sea level contribution to LOD, hence between thermal and mass expansion of the sea, but as with temperature and tide gauge measurements, noise predominates.
I might add that the LIA and MWP can be roughly inferred from reconstructed LOD. –AGF