The changing rotation of the Earth threatens to tinker with our perception of time, clocks and computerized society in unprecedented ways – but only for a second.
For the first time in history, world timekeepers may have to consider removing a second from our clocks in a few years because the planet is spinning a little faster than before. Clocks may have to skip a so-called “negative leap second” around 2029, according to a study published Wednesday in the journal Nature.
“This is unprecedented and a major event,” said study lead author Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego. “This isn’t a huge change in the Earth’s rotation that would cause a catastrophe or anything, but it’s something worth noting. “This is another indication that we are in very unusual times.”
Melting ice at both Earth’s poles has prevented the planet from exploding and will likely delay this moment of global reckoning by about three years, Agnew said.
“We’re heading towards a negative leap second,” said Dennis McCarthy, retired time manager for the U.S. Naval Observatory, who was not part of the study. “It’s a matter of time.”
It’s a complex situation involving physics, global power politics, climate change, technology, and two types of time.
It takes about 24 hours for the Earth to rotate, but the key word is approximate.
Agnew and Judah Levine, a physicist in the time and frequency division of the National Institute of Standards and Technology, said the Earth has been generally slowing down for thousands of years, and that speed has changed from time to time.
McCarthy said the slowdown was mostly due to the tidal effect of the moon receding.
This didn’t matter until atomic clocks were adopted as the official time standard 55 years ago. These have not slowed down.
This created two versions of time, astronomical and atomic, and they did not agree. Astronomical time fell 2.5 milliseconds behind atomic time every day. This means that the atomic clock will say it is midnight, and on Earth it will be midnight in less than a second, Agnew said.
The sum of these daily fractions of seconds equaled a full second every few years. Beginning in 1972, international timekeepers decided to add a “leap second” in June or December for astronomical time to catch up with atomic time, called Coordinated Universal Time, or UTC. Instead of 11:59 and 59 seconds turning into midnight, there would be one more second at 11:59 and 60 seconds. A negative leap second will skip 11:59:59 and go directly from 11:59 and 58 seconds to midnight.
Between 1972 and 2016, 27 separate leap seconds were added as the Earth slowed down. However, the rate of deceleration was gradually decreasing.
“In 2016 or 2017 or maybe 2018, the rate of deceleration slowed down to the point where the Earth was actually accelerating,” Levine said.
Agnew said the Earth is accelerating because its hot liquid core (“a big ball of molten liquid”) is moving in unpredictable ways with variable eddies and flows.
The core has been triggering an acceleration for about 50 years, but rapid melting of polar ice since 1990 has masked that effect, Agnew said. Melting ice shifted the Earth’s mass from the poles toward the bulging center, slowing the spin like a spinning ice skater slowing down as she extends her arms out to her sides, he said.
Agnew calculated that without the impact of melting ice, Earth would need this negative leap second in 2026 instead of 2029.
For decades, astronomers had kept universal and astronomical time together with handy little leap seconds. But computer system operators said those additions weren’t easy given the technologies the world now relies on. In 2012, some computer systems mishandled leap seconds, causing problems for Reddit, Linux, Qantas Airlines and others, experts said.
“What is the need for this adjustment over time when it causes so many problems?” McCarthy said.
But Agnew and McCarthy said Russia’s satellite system is based on astronomical time, so eliminating leap seconds would cause problems. Astronomers and others wanted to preserve the system that would add a leap second whenever the difference between atomic and astronomical time approached one second.
In 2022, the world’s timekeepers decided to reduce this possibility by changing the standards for adding or deleting leap seconds starting in the 2030s.
Tech companies like Google and Amazon have unilaterally introduced their own solutions to the leap second problem by gradually adding fractions of a second over the course of a full day, Levine said.
“The fights are so serious because the risks are so small,” Levine said.
Then, instead of adding a leap second, add the “weird” effect of subtraction, Agnew said. McCarthy said it would probably be harder to skip a second because software programs are designed to add time, not reduce it.
McCarthy said the trend toward needing a negative leap second is clear, but he thinks it has more to do with the Earth becoming rounder due to geological changes since the end of the last ice age.
Three outside scientists said Agnew’s study made sense and his evidence was convincing.
But Levine doesn’t think a negative leap second will actually be needed. He said the overall slowing trend from tides has been around for centuries and continues, but shorter trends in the Earth’s core come and go.
“This is not a process where the past is a good predictor of the future,” Levine said. “Anyone making long-term predictions about the future is on very, very shaky ground.”
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