The number of leap seconds required is determined by the Earth's rotation speed, which isn't constant. In the same way that an ice skater extending his arms slo

The number of leap seconds required is determined by the Earth's rotation speed, which isn't constant. In the same way that an ice skater extending his arms slows down, shifts in mass on the Earth can alter its rotation speed. Earthquakes, icemelt, atmospheric warming, and even the filling of the Three Gorges Dam can have an effect at the scale required for GPS synchronization.