A paper in Nature explains how human-induced melting of polar ice sheets might delay the need for introducing a negative leap second. Dr Patrizia Tavella, Director of the BIPM Time Department, discusses the topic of timekeeping and the link to earth rotation in the associated edition of Nature News & Views Forum.

Leap seconds have been used since 1972 to maintain synchronization between UTC and Earth's rotation. For the first time since then, earth rotation specialists anticipate the possible need to use a negative leap second to compensate for the gradual acceleration of the Earth’s rotation. However, leap seconds - whether positive or negative - pose considerable risks to the synchronization of world infrastructures.

A thoroughly considered solution to this challenge was put forward by the CCTF and formally developed in Resolution 4 of the 27th CGPM (2022) deciding to extend the tolerance between UTC and the Earth rotational angle, to ensure the continuity of UTC for at least a century. Resolution 4 was subsequently endorsed by the Radiocommunication Sector of the International Telecommunication Union at the 2023 World Radiocommunication Conference.