Recently, some media reported that five years from now, in 2029, one minute may only be 59 seconds.
This claim comes from a study by scientists at the University of California, San Diego, which was published in the journal Nature.
Advertisement
Scientists have discovered that due to global warming, the Arctic and Antarctic ice sheets are melting on a large scale, changing the shape of the Earth and causing the Earth's rotation to slow down faster than before. This change may cause a global timing crisis within five years, such as large-scale interruptions in computer communications and telecommunications networks.
In fact, this statement is not accurate. The correct statement should be: In 2029, a minute may be shortened to 59 seconds, and the melting of the ice sheet has delayed the arrival of this day.
What is going on here? Why does one minute decrease to 59 seconds? Isn't time fixed? What impact will this have on our lives?
Advertisement
Our Timing System
Although time appears to pass evenly, we've actually been adjusting it for decades – inserting a leap second every few years. To better understand this, we first need to understand our timekeeping system.
To determine the time, we now have three common time systems, namely:
Universal Time (UT1) based on the Earth's rotation period; Ephemeris Time (ET) based on the Earth's revolution period around the Sun; International Atomic Time (French: Temps Atomique International, TAI) based on the frequency of electromagnetic oscillations emitted by electron energy level transitions inside atoms.
Universal Time (UT1) is a time standard that is determined by counting the time it takes the Earth to rotate once per day. It divides the time scale based on the change in the angle of the Earth relative to the mean Sun. Universal Time plays an important role in the fields of navigation and astronomy, as well as astro-geodesy.
However, because the speed of the Earth's rotation is not constant, Universal Time is not stable enough to fully meet the needs of modern scientific research and technological applications for extremely high precision, such as astronomical observations and the Global Navigation Satellite System (GNSS).
In order to meet the actual needs of higher precision, scientists introduced atomic time. Specifically, atomic time is achieved through atomic clocks, which use the period of electromagnetic oscillation inside atoms to measure time, and this oscillation period is very stable.
Therefore, atomic time has extremely high accuracy and stability, and can provide extremely precise time standards, and is thus widely used in scientific research, navigation systems, communication networks and other fields.
Cesium atomic clock Image source: Wikipedia
In the Space Strontium Atomic Optical Clock Laboratory of the National Time Service Center of the Chinese Academy of Sciences, the measuring instrument shows relevant experimental signals. Photo by Xinhua News Agency reporter Zhang Bowen
In 1967, the 13th International Conference on Weights and Measures decided to change the definition of the second from the astronomical second to the atomic second, and set the time duration of 9192631770 cycles of radiation corresponding to the undisturbed ground state hyperfine energy level transition of the cesium-133 atom as 1 second. In other words, the time duration of 9192631770 vibrations of the radiation emitted by the cesium-133 atom was set as 1 second, which is called the SI second.
This decision marked the official establishment of atomic time and laid the foundation for the development of subsequent time measurement systems.
It is worth mentioning that in order to achieve independent calibration of our country's standard time, scientists represented by Researcher Zhang Shougang from the National Time Service Center of the Chinese Academy of Sciences have been rooted in the west for a long time, willing to be lonely, and have been working on key issues for more than ten years. They have successfully developed a highly stable and continuously operating cold atom cesium fountain reference clock, reducing the deviation between my country's standard time and the international standard time from 100 nanoseconds to within 5 nanoseconds.
Why does one minute become 59 seconds?
Universal time, which is based on the rotation of the Earth, has always been one of the important parameters for the generation of international standard time. A day is divided into 24 hours, 1 hour and 60 minutes, and 1 minute and 60 seconds. Universal time reflects the rotation angle of the Earth relative to the cosmic background, which is very important.
While adopting atomic time is a very accurate and unchanging way to define time, it also comes with a troubling consequence: atomic time doesn't quite match up with universal time defined by the rotation of the Earth.
The difference between atomic time and universal time. Image source: Literature[1]
Over the centuries, the stability of time measurements has continued to increase, allowing us to see that the Earth's rotation speed is not constant, which causes a difference between atomic time and universal time.
In order to take both needs into account, the Coordinated Universal Time (UTC) system was introduced. When the difference between the International Atomic Time and the World Time reaches 0.9 seconds, the Coordinated Universal Time (UTC) needs to be adjusted, that is, add or subtract 1 second to get as close to the World Time as possible. This is the so-called leap second (negative leap second, the last minute is 59 seconds; positive leap second, the last minute is 61 seconds). This world time with added leap seconds is the Coordinated Universal Time, also known as the World Standard Time, which is currently the most widely used time system.
Since the official use of UTC in 1972, the Earth's rotation has been on a slowing trend, and 27 leap seconds have been added to Coordinated Universal Time, all of which are positive leap seconds. However, since mid-2020, the Earth's rotation rate has shown a trend of accelerating.
Therefore, scientists estimate that in 2029, humans may need to reduce 1 second to a “negative leap second” for the first time, corresponding to 1 minute with only 59 seconds, to keep the atomic clock time synchronized with the Earth's rotation period.
Why is the Earth's rotation speed not constant?
On millennial timescales, changes in the Earth's rotation speed are influenced by three geophysical processes.
First, the friction between the sea water and the seabed will gradually consume the kinetic energy of the Earth's rotation, thereby slowing down the Earth's rotation speed. This is the so-called tidal effect.
Secondly, due to the rebound after the ice age, the shape of the earth will change and become flatter, which will change the earth's moment of inertia and reduce its rotation speed. This is similar to the principle that ice skaters stretch their arms out to the sides of their bodies to slow down their rotation when spinning.
Finally, some processes inside the Earth, namely the interaction and mutual influence between the core and its outer layers (mantle and crust), such as changes in the geomagnetic field and mantle convection, can also cause changes in the Earth's rotation speed.
According to data from NASA and the International Earth Rotation and Reference Systems Service (IERS), the Earth's rotation speed is indeed slowly slowing down. Studies have shown that the Earth's rotation period increases by about 1.8 milliseconds per century. Although this change may seem small, its cumulative effect is very significant over a long time scale.
For example, there is a significant difference between the time of solar eclipses recorded by ancient astronomers and the time we calculate today. The time of a solar eclipse observed 2,500 years ago (around the Spring and Autumn Period and the Warring States Period) was about 4 hours off from the time of a modern clock.
Originally, scientists expected that due to these geophysical processes, the slowdown in the Earth's rotation would cause the first “negative leap second” to arrive in 2026.
However, satellite measurements show that as global warming intensifies, the ice sheets of Greenland and Antarctica are melting faster since 1986. This phenomenon causes the sea level to rise faster, further slowing down the Earth's rotation. Due to the dual effects of melting ice sheets and rising sea levels, the Earth's moment of inertia increases and its rotation becomes slower, thus delaying the arrival of negative leap seconds.
Polar ice is melting and moving toward the equator, slowing down the Earth's rotation. Image source: Literature[3]
What impact does a leap second have?
Leap seconds are usually implemented at 23:59:60 UTC on June 30 or December 31. The adjustment of leap seconds has little direct impact on daily life, and people often do not feel the changes brought about by leap seconds.
However, leap seconds have an important impact on technical systems and application areas that rely on precise time synchronization, such as computers, finance, aerospace, etc.
For example, the addition or deletion of leap seconds requires global synchronization, posing challenges to time management in computer systems.
In 2012, several large websites crashed their servers due to time synchronization errors, resulting in a brief service interruption. When the leap second came again in 2015, engineers fixed some of the problems that occurred in 2012, but discovered new problems. For example, every time a leap second is adjusted, the GNSS system needs to update the time data to ensure the timing accuracy. If the adjustment is not made in time, the navigation message may be inaccurate.
Unlike the traditional leap second, which adds one second, the unprecedented negative leap second will bring new challenges and uncertainties to many systems that rely on precise time synchronization. Computer and network systems, financial systems, etc. are often designed to handle the addition of positive leap seconds, but may not be adequately prepared for how to handle the reduction of negative leap seconds. Scientists are calling on all sectors to work together to fully prepare for the implementation of negative leap seconds to ensure the stability and security of global technical systems.
Although the original intention of leap seconds is to keep UTC synchronized with the Earth's rotation time UT1, the adjustment of leap seconds, especially the potential negative leap seconds, is increasing the complexity of the time synchronization system. Some people have proposed to implement larger corrections, such as leap minutes and leap hours, to extend the adjustment time to 100 years or 1,000 years; others have suggested stopping the correction and publishing the growing time difference between Universal Time and International Atomic Time.
The 27th General Conference on Weights and Measures in 2022 decided to abolish leap seconds and replace them with leap minutes no later than 2035, allowing the difference between the International Atomic Time and the World Time to be within 1 minute. It also required all parties to negotiate and propose a new plan that can maintain the “Coordinated Universal Time” for at least a hundred years.
With the development of science and technology, new time synchronization technologies will continue to emerge, such as more accurate optical clocks and more intelligent network time protocols, which may provide new ways to solve the leap second problem.
references
[1] Tavella, Patrizia, and Jerry X. Mitrovica. “Melting ice solves leap-second problem—for now.” (2024).
[2]Agnew, Duncan Carr. “A global timekeeping problem postponed by global warming.” Nature 628.8007 (2024): 333-336.
[3] Gibney, Elizabeth. “Climate change has slowed Earth's rotation—and could affect how we keep time.” Nature 628.8007 (2024): 243-244.