As a result, the relative distribution of water mass will shift away from the equator and closer to the poles. Enough water will move that Earth will actually spin slightly faster, like a figure skater who brings her arms closer to her body. "Earth's rotation rate changes if its moment of inertia is altered via redistribution of mass in the oceans," geophysicist Felix Landerer and his colleagues report in Geophysical Research Letters. The result: a day that is 0.12 millisecond—slightly more than one ten-thousandth of a second—shorter two centuries from now.
Of course, there are other factors at work. The model fails to account for additional water actually entering the oceans via the melting of the ice sheets in Greenland or Antarctica. "Ice melting would work opposite, moving mass away from the Earth's axis," says Richard Ray, a geophysicist at NASA Goddard Space Flight Center. "So it would partly cancel out this effect."
Changes in wind patterns could produce a similar small effect, either slowing or speeding the planet's spin depending on which way the wind blows. And the sloshing of molten material at the Earth's core as well as the tidal drag the moon inflicts slow Earth much more significantly. "That's on the order of about two milliseconds per century," Ray notes. "If you go back long enough, the day was 18 hours long."
In the extremely distant future, such lunar tidal friction will actually slow the Earth's spin until a day is as long as a month (though the sun might swallow up the Earth-moon system before that ever happens). In the shorter term, warming the world's oceans might actually make the day go by a little quicker.