Earth’s rotation speed fluctuates over decades, shifting day lengths by milliseconds. A study published on September 23 in Nature reveals that a gravitational tug-of-war between the planet’s solid inner core and rocky mantle drives these multidecadal changes, outcompeting other deep interior forces.
We treat a day as a neat 24 hours, but our planet keeps time rather loosely. For decades, geophysicists have tracked tiny, multidecadal variations in the speed at which Earth turns on its axis. These fluctuations amount to only a few milliseconds, making them mostly imperceptible to people going about their daily lives on the surface. Yet they point to massive, churning dynamics hidden thousands of miles beneath our feet.
Shorter-term shifts in day length are governed by atmospheric and oceanic patterns, while gravitational tidal tugs from the Moon sculpt day lengths over millions of years. But the intermediate timescales spanning several decades have long baffled scientists trying to find the missing driver.
How the Inner Core and Mantle Trade Momentum
Scientists have known for roughly thirty years that the planet’s liquid core does not rotate at a constant rate. Magnetic field data shows that the liquid core gradually speeds up over several decades before slowing back down in the decades that follow. To keep the planet’s total angular momentum constant, the massive rocky mantle responds in reverse, speeding up when the core slows and dragging when the core accelerates.
Observational data indicates that core-driven processes shortened the length of the day by several milliseconds between the early 1970s and 2021 overall. Pinpointing how momentum transfers across these massive interior boundaries proved difficult because friction between the core and mantle is far too weak on its own to drive the shift outward.
The Gravitational Tug-of-War Revealed in Nature
To solve the mystery, University of Alberta physics doctoral student Huifeng Zhang and professor Mathieu Dumberry revisited angular momentum exchange concepts first modeled in 1988. They tested three distinct coupling mechanisms operating at the base of the mantle against six decades of observational records: electromagnetic coupling, topographic coupling, and gravitational coupling.
Their statistical models demonstrated that gravitational coupling exerts the most influence on multidecadal rotation changes by far. The solid inner core rotates at a slightly different speed than the rest of the planet, leaving it offset from mass irregularities tucked inside the mantle above it.
“The gravity of the inner core pushes rotation in one direction, while other interactions between the core and the mantle oppose it. The result of this competition, accumulated over decades, would be small accelerations and decelerations in the planet’s spin.”
Mathieu Dumberry, Geophysicist at the University of Alberta
While gravity attempts to pull the inner layers back into a harmonious alignment, electromagnetic drag and topographic friction at the core-mantle boundary act as opposing buffers against those changes. Dumberry noted in a press release that gravity still wins
the competition, driving the slow accelerations observed at the surface over time.
Probing Deformable Deep Interior Layers
Reconstructing rotational changes between 1964 and 2019 using seismic estimates of inner core rotation and liquid metal flow models, the researchers found their gravitational framework accurately reproduced both the timing and the magnitude of historical day-length variations. The findings also suggest the solid inner core deforms viscously on roughly a ten-year cycle, proving that Earth’s deepest reaches respond dynamically to internal forces.
“What I find particularly exciting is that these different pieces of information can come together to provide a more coherent picture of Earth’s deep interior, a region that is extremely difficult to observe directly.”
Huifeng Zhang, Doctoral Student at the University of Alberta
Because the physical properties governing these deep mechanisms remain imperfectly understood, the new constraints give geophysicists a reliable benchmark to refine future interior models of our planet.
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