Global timekeeping: Climate change has slowed Earth's rotation

Wrestling with the concept of daylight savings can be baffling for many people, but imagine the task ahead for the world’s timekeepers as they investigate the impact of climate change on the spin of our planet and, consequently, our methods of time measurement. 

However, according to a recent study led by the Scripps Institution of Oceanography at the University of California San Diego (UCSD), it appears that the effects of global warming might inadvertently assist timekeepers by postponing the potential introduction of the first-ever “negative leap second” by three years. 

Negative leap second

The notion of integrating a negative leap second – essentially a 59-second minute – into our standardized time has sparked concerns over the potential chaos it could unleash on computerized systems worldwide.

Historically, time was measured by the rotation of the Earth. Yet, in 1967, a shift occurred when atomic clocks were adopted, heralding a new epoch of precision in timekeeping. 

Link between terrestrial rotation and timekeeping

Nonetheless, there remained a faction, including navigators of the seas dependent on celestial guidance, who insisted on preserving the link between terrestrial rotation and timekeeping, despite Earth’s less reliable time-telling tendencies. 

The disparity between Earth’s gradually lagging rotation and the unerring atomic clocks necessitated a workaround. The solution was the insertion of a “leap second” into Coordinated Universal Time (UTC) – the global consensus for time coordination – whenever the gap neared 0.9 seconds.

Earth has begun to outpace atomic time 

While the addition of 27 leap seconds to UTC from 1972 to 2016 might have gone unnoticed by many, a surprising turn of events has recently emerged. Earth has begun to outpace atomic time, suggesting that, for the first time, a negative leap second might need to be introduced to align the two measurements.

“This has never happened before, and poses a major challenge to making sure that all parts of the global timing infrastructure show the same time,” said author Duncan Agnew, a professor of geophysics at UCSD. “Many computer programs for leap seconds assume they are all positive, so these would have to be rewritten.”

Earth’s rotational speed

Agnew’s investigation into Earth’s rotational speed and its decelerating core, incorporating satellite observations, indicates that without the influence of climate change, the necessity for a negative leap second might have arisen as early as 2026. 

However, the dispersal of meltwater from ice sheets in Greenland and Antarctica since 1990 has slowed down the Earth’s rotation, pushing the requirement for a negative leap second to at least 2029. 

“When the ice melts, the water spreads out over the whole ocean; this increases the moment of inertia, which slows the Earth down,” Agnew explained.

“Here we show that increased melting of ice in Greenland and Antarctica, measured by satellite gravity, has decreased the angular velocity of Earth more rapidly than before,” wrote the study authors. 

“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.”

Postponing a global timekeeping problem 

The deferral of an “unprecedented” negative leap second is “welcome news indeed,” according to Patrizia Tavella, the director of the International Bureau of Weights and Measures, which is responsible for UTC. Nonetheless, Demetrios Matsakis, former chief scientist for time services at the US Naval Observatory, expressed skepticism toward Agnew’s analysis due to the unpredictability of the Earth’s rotation.

“It would not bring about the downfall of civilization, and given enough publicity some problems could be avoided. But I would not recommend being in an airplane at that time,” he said.

Global timekeeping authorities

The introduction of even positive leap seconds has previously led to difficulties for precision-dependent systems, which partly motivated the global timekeeping authorities’ decision in 2022 to eliminate the leap second by 2035. 

This approach will permit the discrepancy between atomic time and Earth’s rotation to expand to a minute, with a leap minute to synchronize them unlikely to be required this century. 

“A negative leap minute is very, very unlikely,” Agnew said, expressing hope that his findings will encourage timekeepers to consider dropping the leap second sooner than 2035, a sentiment shared by Tavella and Matsakis.

The study is published in the journal Nature.

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