A 70-year Earth-rotation cycle does not point to an imminent negative leap second
A study links a long-term pattern in the length of Earth’s day to changes in the planet’s fluid core, while cautioning that future time adjustments cannot be predicted from the model alone.
Editorial illustration — not from the study.
Researchers removed modeled effects from tides, surface fluids and long-term tidal braking from records of the length of Earth’s day, then compared the remaining variation with an equivalent rotation signal inferred from geomagnetic observations. Both records contained a cycle close to 70 years over the shared 1883–2022 interval.
The results are consistent with a contribution from the core to slow, multidecadal changes in Earth’s rotation. The authors stress that the analysis is a geophysical assessment rather than an operational forecast of future changes to Coordinated Universal Time, the international time standard.
A cycle in Earth’s spin
Harmonic fits gave periods of 69.7 years for the residual length-of-day record and 71.8 years for the core-related equivalent record. The fitted amplitudes were 2.87 milliseconds and 1.94 milliseconds, respectively, while spectral analysis found peaks near 67.8 and 70.5 years. The two annual series had a zero-lag correlation of 0.918, with the strongest numerical correspondence when the core-related series led by about two years. Because both records are strongly autocorrelated, the researchers used these correlations to describe their correspondence, not to establish predictive significance.
The findings are consistent with a core-related contribution to low-frequency changes in Earth’s rotation. They do not distinguish among electromagnetic, topographic, gravitational and viscous mechanisms that can transfer angular momentum between the core and mantle. Within the fitted model, the multidecadal cycle alone does not show sustained near-term shortening of the day that would, by itself, require a negative leap second.
Why leap seconds matter
A negative leap second could eventually be considered if Earth’s rotation stayed fast enough relative to the time standard. The study indicates that the recent acceleration can be viewed in the context of a longer cycle linked to the core, rather than treated as evidence by itself of a continuing near-term trend toward such an adjustment.
This matters for interpreting recent changes in Universal Time and Earth’s rotation, but the result does not determine future leap-second decisions. Those decisions depend on how rotation evolves beyond the period analyzed and on operational measurements and assessments.
Evidence and caveats
The analysis used annual length-of-day data and a core angular-momentum record inferred from geomagnetic observations over their common 1883–2022 interval. Researchers applied harmonic regression, spectral analysis and lagged comparisons after removing modeled tidal, surface-fluid and secular contributions.
The historical length-of-day record before the space-geodetic era is less precise, and complete surface-fluid corrections were available only from 1976 onward. Unresolved low-frequency surface effects may therefore remain in the earlier residual record. The core signal relies on assumptions used to infer flow from geomagnetic data, and the study cannot uniquely identify the physical coupling mechanism. Its conclusion about leap seconds is model-dependent and is not an operational prediction of future UTC adjustments.
// Source
Earth Planets and Space · 2026 · DOI: 10.1186/s40623-026-02529-1
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