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George is right in suggesting that the recent speeding up of the Earth's
rotation is likely due to the core but I wouldn't call this a sudden change.
The core is believed to affect the Earth's rotation on decadal time scales so
even within a 10-year period, we should not be surprised to see significant
variations in LOD. In the recent past (the mid 1920s to the mid 1930s), the
LOD was even shorter than 86,400 seconds (see Fig.1 in
). Should this
happen again, we might even have to add a negative leap second to UTC. The
provision to do this actually exists.
For more information on the core and the Earth's rotation, see


and, specifically

-- Richard Langley

On Wed, 28 Jul 2004, George Huxtable wrote:

>Thanks for information from Richard Langley and Andres Ruiz about this topic.
>
>Perhaps I can summarise the situation as follows-
>
>Right through most of the 20th century, up until 1994 or so, the difference
>between Ephemeris Time and Universal Time , or delta-T, increased each year
>by about 0.7 seconds. But over the last six years or so, this quantity has
>stopped increasing and at present remains roughly constant. This must be
>the result of the Earth's spin speed having increased, by about 1 part in
>40 million, rather suddenly, within a very few years.
>
>I know that delta-T has fluctuated significantly in the past, but this time
>there seems to have been a rather sudden increase in spin speed, by quite a
>large amount. What could be the cause of this rather dramatic change? One
>part in 40 million may not seem a lot at first sight, but in view of the
>immense angular momentum of the Earth, some large torques must have been at
>work.
>
>I would like to muse about some possibilities, aware that it's somewhat
>off-topic for navigation, but asking for a bit of indulgence on that score.
>
>It can't possibly be due to tidal friction, which causes angular momentum
>to be transferred from the Earth to the Moon, can it? That is strictly a
>one-way process, which can only ever act to slow the Earth's rotation, and
>never to speed it up.
>
>Could the Earth have shrunk slightly, perhaps? That would speed up the
>spin, as it does for an ice-skater doing a pirouette, as she draws in her
>outstretched arms. If the Earth radius were to shrink by one part in eighty
>million, with all its inner layers shrinking in proportion, that would be
>enough to do the trick. That needs no more than 3 inches or so shrinkage in
>radius. I suspect geodesists would be able to detect such an unlikely event
>using modern technology.
>
>It couldn't possibly be due to interaction with the moving atmosphere,
>which doesn't carry nearly enough mass to have sufficient effect.
>
>If the oceans were to acquire a change in East-to-West drift right around
>the Earth, including the Drake passage round the Horn, that would speed up
>the Earth's rotation, which is in the opposite direction. But to have
>sufficient effect, such a current drift would have to be sufficiently large
>that it would have been perceived already, according to my rough estimates.
>
>We keep getting told that the polar ice caps in Antarctica and Greenland
>are melting. However, this displacement of mass, from being locked in the
>ice caps near the Earth's axis, where it has little effect on the Earth's
>rotational inertia, to disperse around the oceans where its effect on
>inertia would be greater, could only slow the Earth's spin, not speed it
>up.
>
>The only other possibility that I'm aware of (and listed by Richard
>Langley) is a change in the circulating currents within the Earth's liquid
>interior. We know that these exist, and change, because these motions in
>the conductive liquid result in the ever-changing magnetic field pattern of
>the Earth. It seems most likely, to me, that there has been a significant
>increase over recent years, and a rather sudden one, in the component of
>overall swirl in the opposite direction to the rotation of the Earth. I
>wonder whether any such changes may be detectable in terms of a sudden
>effect on the Earth's magnetic field over recent years. It's just a
>thought.
>
>I admit to knowing little about geophysics, and hope that others, wiser
>about these matters, will correct any errors in my speculations.
>
>George.
>
>================================================================
>contact George Huxtable by email at george@huxtable.u-net.com, by phone at
>01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
>Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>================================================================
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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