NEW YORK — The Earth’s evolving rotation is poised to play havoc with our perception of time, our timekeeping devices, and our digital society in a way we’ve never experienced before — albeit only for a fleeting second.
In an unprecedented event, global timekeepers may need to contemplate deducting a second from our clocks in the coming years. This is due to the Earth spinning slightly quicker than it has in the past. According to a study published in the journal Nature on Wednesday, we may need to implement a “negative leap second” around 2029, causing our clocks to skip a beat.
“This is a groundbreaking situation and a significant event,” stated Duncan Agnew, the study’s lead author and a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego. “While it’s not a drastic change in the Earth’s rotation that will lead to a disaster, it’s certainly noteworthy. It’s another sign that we’re living in extraordinary times.”
Agnew explained that the melting ice at both of Earth’s poles has been counterbalancing the planet’s increased speed, likely postponing this global moment of reckoning by approximately three years.
“We are on the path towards a negative leap second,” commented Dennis McCarthy, the retired director of time for the U.S. Naval Observatory, who was not involved in the study. “It’s just a question of when.”
This complex situation involves a mix of physics, international power politics, climate change, technology, and two distinct types of time.
While the Earth takes roughly 24 hours to rotate, the keyword here is ‘roughly’.
The Concept of a ‘Leap Second’
For millennia, the Earth has been gradually slowing down, with the rate fluctuating over time, explained Agnew and Judah Levine, a physicist for the time and frequency division of the National Institute of Standards and Technology.
The slowing is primarily due to the effect of tides, which are caused by the gravitational pull of the moon, McCarthy added.
This discrepancy didn’t become an issue until atomic clocks were established as the official time standard over 55 years ago. These clocks, unlike the Earth, did not slow down.
This led to the creation of two versions of time — astronomical and atomic — which did not align. Astronomical time lagged behind atomic time by 2.5 milliseconds each day. This meant that while the atomic clock would indicate midnight, the Earth would reach midnight a fraction of a second later, Agnew explained.
These daily fractions of seconds accumulated to whole seconds every few years. Starting in 1972, international timekeepers decided to add a “leap second” in June or December for astronomical time to catch up to the atomic time, also known as Coordinated Universal Time or UTC. Instead of transitioning from 11:59 and 59 seconds to midnight, there would be an additional second at 11:59 and 60 seconds. A negative leap second would jump from 11:59 and 58 seconds directly to midnight, bypassing 11:59:59.
The Unpredictability of Earth
Between 1972 and 2016, 27 separate leap seconds were added as the Earth continued to slow. However, the rate of slowing was diminishing.
“Around 2016, 2017, or perhaps 2018, the rate of slowdown had decreased to the point that the Earth was actually accelerating,” Levine noted.
The Earth’s acceleration is due to its hot liquid core — “a large ball of molten fluid” — behaving in unpredictable ways, with varying eddies and flows, Agnew said.
Agnew explained that the core has been causing a speedup for about 50 years, but the rapid melting of ice at the poles since 1990 has concealed this effect. Melting ice redistributes Earth’s mass from the poles to the bulging center, which slows the rotation much like a spinning ice skater slows when extending their arms out to their sides, he said.
Without the effect of melting ice, Earth would require that negative leap second in 2026 instead of 2029, Agnew calculated.
For decades, astronomers had been synchronizing universal and astronomical time with those handy little leap seconds. However, computer system operators argued that these additions are not easy for all the precise technology the world now depends on. In 2012, some computer systems mishandled the leap second, causing issues for Reddit, Linux, Qantas Airlines, and others, experts noted.
“Why do we need this time adjustment when it causes so many problems?” McCarthy questioned.
However, Russia’s satellite system depends on astronomical time, so eliminating leap seconds would cause them issues, Agnew and McCarthy pointed out. Astronomers and others wanted to maintain the system that would add a leap second whenever the difference between atomic and astronomical time approached a second.
On ‘Very, Very Shaky Ground’
In 2022, the world’s timekeepers decided that starting in the 2030s they’d change the standards for inserting or deleting a leap second, making it much less likely.
Tech giants like Google and Amazon independently implemented their own solutions to the leap second issue by gradually adding fractions of a second over a full day, Levine said.
“The disputes are so intense because the stakes are so small,” Levine remarked.
Then there’s the “strange” effect of subtracting, not adding a leap second, Agnew said. It’s likely to be more challenging to skip a second because software programs are designed to add, not subtract time, McCarthy said.
McCarthy stated that the trend towards needing a negative leap second is clear, but he believes it’s more to do with the Earth becoming more round from geologic shifts from the end of the last ice age.
Three other external scientists said Agnew’s study makes sense, calling his evidence compelling.
However, Levine doesn’t think a negative leap second will be necessary. He said the overall slowing trend from tides has been around for centuries and continues, but the shorter trends in Earth’s core come and go.
“This is not a process where the past is a good prediction of the future,” Levine said. “Anyone who makes a long-term prediction on the future is on very, very shaky ground.”
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Disagree – Earth’s rotation speed shouldn’t impact global clocks.
Agree – It’s important for global clocks to accurately reflect Earth’s rotation speed for consistency and precision in timekeeping.