Global warming is expected to raise ocean levels and thereby effectively shift some ocean water from currently deep areas into shallower continental shelves, including a net transfer of water mass from the southern to the northern hemisphere. This in turn will bring just so much water closer to the Earth’s rotational axis, and this-like a figure skater speeding up as she folds her limbs inward-will shorten the diurnal period.
Not by much, though. According to Felix Landerer, Johann Jungclaus, and Jochem Marotzke, scientists at the Max Planck Institute for Meteorology in Hamburg, the day should shorten by 0.12 milliseconds over the next two centuries. (Recent issue of Geophysical Review Letters.)