Several-millisecond changes in Earth’s day length from the early 1970s to 2021 were best explained by gravitational torque between the core and mantle, according to a new Nature study.
Six decades of observations matched statistical models most closely when gravitational coupling dominated angular-momentum exchange, while electromagnetic and topographic effects pushed in the opposite direction and muted the net rotation change.
Researchers said the result helps resolve a long-standing question over what drives decadal rotation shifts, which are too small to notice in daily life but can reveal conditions thousands of kilometers below the surface.
The findings add a new tool for probing Earth’s deep interior, and the team is now testing whether similar core-mantle interactions could also explain a separate six-year cycle in day length.