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A global timekeeping problem postponed by global warming


The historical association of time with the rotation of Earth has meant that Coordinated Universal Time (UTC) closely follows this rotation1. Because the rotation rate is not constant, UTC contains discontinuities (leap seconds), which complicates its use in computer networks2. Since 1972, all UTC discontinuities have required that a leap second be added3. Here we show that increased melting of ice in Greenland and Antarctica, measured by satellite gravity4,5, has decreased the angular velocity of Earth more rapidly than before. Removing this effect from the observed angular velocity shows that since 1972, the angular velocity of the liquid core of Earth has been decreasing at a constant rate that has steadily increased the angular velocity of the rest of the Earth. Extrapolating the trends for the core and other relevant phenomena to predict future Earth orientation shows that UTC as now defined will require a negative discontinuity by 2029. This will pose an unprecedented problem for computer network timing and may require changes in UTC to be made earlier than is planned. If polar ice melting had not recently accelerated, this problem would occur 3 years earlier: global warming is already affecting global timekeeping.

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Fig. 1: Changes in J2.
Fig. 2: Changes in spin rate and their effects on timescales.

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Data availability

The J2 data are C20LongTerm.txt, downloaded from on 25 October 2023. The Earth rotation data are eopc0420.1962-now, downloaded from on 24 October 2023. The atmospheric angular momentum data are from, downloaded on 1 February 2023. Other parameters are taken from the papers referenced. Source data are provided with this paper.

Code availability

The code for analysing the residual series, est.noise v.1.2, was downloaded from on 25 June 2023. The seasonal-adjustment code stl was slightly modified from a version downloaded from in June 2008.


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I thank R. Ray, L. Morrison, A. Borsa, J. Mitrovica and M. King for their comments.

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Correspondence to Duncan Carr Agnew.

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Agnew, D.C. A global timekeeping problem postponed by global warming. Nature 628, 333–336 (2024).

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