A new spin on global warming.

Apparently global warming will slow the rotation of the planet. Water melting at the poles will migrate toward the equator, thus increasing the moment of inertia of the planet (ie, moving some of the mass away from the axis of rotation). To conserve angular momentum, the planet will slow a little. Days will become approximately one-tenth of a second longer, so don't lose sleep over this. 

Source: "Do Polar Bears Get Lonely", published by New Scientist.