By SETH BORENSTEIN (AP Science Writer)
The changes in Earth's rotation may affect our perception of time, clocks, and modern society, but only by a small amount.
For the first time, experts may need to remove a second from our clocks in a few years because the planet is spinning slightly faster. According to a study in the journal Nature, clocks may need to skip a second, known as a “negative leap second,” around 2029.
“This is an extraordinary situation and a significant development,” explained study lead author Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego. “While it won't lead to any catastrophic events, it is noteworthy. This is another sign of the unusual time we're in.”
The melting of ice at both of Earth’s poles has been counteracting the planet’s increased speed and may have postponed this global second adjustment by about three years, Agnew suggested.
“We are moving towards a negative leap second,” stated Dennis McCarthy, retired director of time for the U.S. Naval Observatory, who was not involved in the study. “It's just a matter of when.”
It’s a complex situation involving physics, global power dynamics, climate change, technology, and two types of time.
The Earth takes approximately 24 hours to complete one rotation, but the key word here is approximately.
For thousands of years, the Earth has been gradually slowing down, with the rate fluctuating over time, according to Agnew and Judah Levine, a physicist for the time and frequency division of the National Institute of Standards and Technology.
The slowing down is mainly caused by the impact of tides, which are influenced by the gravitational pull of the moon, noted McCarthy.
This wasn't a problem until atomic clocks became the official time standard over 55 years ago. These clocks did not slow down.
This established two versions of time — astronomical and atomic — and they did not align. Astronomical time fell behind atomic time by 2.5 milliseconds each day. This meant that the atomic clock would display midnight a fraction of a second earlier than astronomical time, Agnew explained.
These daily fractions of seconds accumulated to whole seconds every few years. Beginning in 1972, international timekeepers decided to add a “leap second” in June or December to allow astronomical time to catch up to atomic time, known as Coordinated Universal Time or UTC. Instead of midnight turning 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 go from 11:59 and 58 seconds directly to midnight, skipping 11:59:59.
Between 1972 and 2016, 27 separate leap seconds were added as Earth slowed down. However, the rate of slowing was decreasing.
“Around 2016 or 2017, or possibly 2018, the rate of slowing had decreased to the point where the Earth was actually speeding up,” Levine said.
Earth is speeding up due to its hot liquid core — “a large ball of molten fluid” — which behaves in unpredictable ways, with varying eddies and flows, as Agnew explained.
Agnew stated that for approximately 50 years, the Earth's core has been causing the rotation to speed up. However, the rapid melting of ice at the poles since 1990 has hidden this effect. The melting ice transfers the Earth's mass from the poles to the equator, which slows down the rotation, similar to how a spinning ice skater slows down by extending their arms.
If it weren't for the melting ice, Earth would have needed a negative leap second in 2026 instead of 2029, according to Agnew's calculations.
Astronomers have been adding leap seconds to keep universal and astronomical time aligned for many years. However, technology operators have found it challenging to incorporate these additions into the precise technology the world now depends on. In 2012, certain computer systems mishandled the leap second, causing issues for Reddit, Linux, Qantas Airlines, and others, experts noted.
McCarthy questioned the necessity of making time adjustments that lead to numerous problems.
However, the use of astronomical time by Russia's satellite system means that eliminating leap seconds would create issues, as noted by Agnew and McCarthy. Astronomers and others wanted to maintain the system that adds a leap second whenever the difference between atomic and astronomical time approaches one second.
In 2022, the world's timekeepers decided to revise the standards for adding or subtracting a leap second in the 2030s, making it much less likely.
Tech companies such as Google and Amazon independently implemented their own solutions to address the leap second issue by gradually adding fractions of a second over a full day, according to Levine.
Levine remarked that the conflicts are significant because the stakes are small.
Agnew also highlighted the challenging aspect of needing to subtract, rather than add, a leap second. McCarthy pointed out that skipping a second is likely to be more difficult because software programs are designed to add, not subtract, time.
McCarthy observed that the trend toward requiring a negative leap second is evident, but believes it's more related to the Earth becoming more round as a result of geological shifts from the end of the last ice age.
Three other external scientists concurred that Agnew's study is logical, describing his evidence as compelling.
However, Levine expressed doubt about the necessity of a negative leap second. He stated that the overall slowing trend from tides has persisted for centuries and continues, but the shorter trends in Earth's core fluctuate.
Levine conveyed the idea that this process is not one where the past can accurately predict the future. He emphasized the uncertainty of making long-term predictions about the future.
___
Read more of AP’s climate coverage at http://www.apnews.com/climate-and-environment
___
Follow Seth Borenstein on X at @borenbears
___
The Associated Press’ climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org.
