Climate change to alter leap seconds trend

Based on the earth’s current rotation rate, a second might need to be required to be subtracted every ten years to time, writes B K Singh.

Historically, time has been measured by the rotation of the earth. The average Earth Day contains 86,400 seconds. The earth’s rotation has varied slightly depending upon the movement of the earth’s core, oceans, and atmosphere. A ‘positive leap second’ is a second added to our clocks to keep them synchronised with the earth’s rotation. The clock would go from 23:59:58 seconds directly to 00:00:00 (midnight.)

Adding a second to the clock once every few years brought earth rotation measurements into sync with time. In a study published in Nature recently, scientists concluded that the earth’s rotation is speeding up due to the melting of glaciers in Greenland and Antarctica due to the global rise in temperature. Though 27 leap seconds have been added to clocks since 1972, the time has come to subtract a second, known as a ‘negative leap second’, to sync it with the time from the atomic clock.

In 1967, the world’s timekeepers embraced atomic clocks to switch to a more precise timekeeping system. However, sailing in the sea was practically dependent on the Sun and stars for navigation, and taking a cue from this, experts/ scientists decided that the connection between the earth’s rotation and time must be retained.

Since the earth was moving slightly slower than the atomic t+ime, the two times were not in sync, and it was decided to add a leap second to the clock whenever the difference in two timings approached 0.9 of a second. Adding 1 second to the clock would mean adding it to the coordinated universal time (UTC), an internationally agreed standard, and we all set our clocks based on this. 

With an error of only 1 second in 100 million years, the atomic clocks are the most accurate time-keeping device in history. International System of Units defines the second as the time it takes a Caesium (Cs133) in a precisely defined state and its free electron to oscillate 9,192,631,770 times. Officially, the second is the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the Caesium (Cs133) atom at rest at -2730 Celsius.

Cs133 is the most stable isotope of Caesium, with 55 protons and 78 neutrons in the nucleus and 55 electrons, including a free electron around its nucleus. Four hundred atomic clocks worldwide contribute to calculating international atomic time, which determines UTC and local time. GPS also relies on this time measurement to calculate positions accurately.

Timekeepers’ records show that leap seconds have been added 27 times since 1972, the last one being added on the eve of January 1, 2016. In other words, 27 seconds were added to synchronise the two times obtained from earth rotation and the atomic clock in this period, and on average, a positive leap second was added every 18 months. On each occasion, computer systems worldwide have been adjusted to the shock inflicted by it.

The rotation period has been varying each year by a fraction of a millisecond. Based on the current rotation rate, a negative leap second might be required to be adjusted every ten years. The study shows that the timekeepers’ program to adjust a negative leap second in 2026 will be postponed to 2029 due to the relative slowing down of the earth’s rotation.

The first-ever introduction of a negative leap second may disrupt computer systems worldwide. Of course, experts would handle it at their level. Duncan Agnew, a researcher at the University of California and the study’s lead author, said, “This has never happened before and poses a major challenge to ensuring that all parts of global timing infrastructure show the same time.”

The havoc on computer systems worldwide is being postponed by three years, from 2026 to 2029. Agnew added, “Ice melting at both of earth’s poles has been counteracting the planet’s burst of speed and is likely to have delayed this global second of reckoning by about three years.”

Agnew reported in his study that human-induced melting of polar ice slows the Earth’s rotation, effectively delaying a decision on the need for a negative leap second. Agnew added in his paper that earth rotation results from three geophysical processes on a millennial timescale. (1) Friction between Sea water and its floor can slow the rotation—the effect is also known as tidal dissipation; (2) Shape adjustment undergone by earth since the last ice age that can speed up rotation and (3) Coupling between earth’s iron core and rocky mantle and crust – the change in angular momentum of the core has to be balanced by angular momentum driving mantle and crust.

(The author is a retired principal chief conservator of forests (Head of Forest Force) Karnataka and post-graduate in Physics)