It is the nightmare of scientists and IT professionals: the introduction of a negative leap second. But due to climate change, this now appears to be necessary at least three years later than planned. And that gives us a little more time to prepare for it.
As the Earth warms, the ice near the poles is melting. And that contributes to a slower rotation of the Earth, so that it does not seem necessary to introduce a negative leap second – for the very first time – in the coming years. Researchers write this in the magazine Nature. It’s just a stay of execution; because around 2029 the negative leap second seems to really have to be introduced. But climate change does give us a little more time to prepare for this rather drastic import.
Leap second
We all know leap day. Less well known, however, is the leap second, where a minute once lasts 61 seconds instead of 60 seconds. The leap second was first used in 1972. And in the years and decades that followed, it was also regularly taken out of the stable.
Atomic clocks and the rotation of the Earth
And there was a good reason for this: the leap second was necessary to prevent time measurements made by atomic clocks from getting too far out of step with time measurements based on the Earth’s rotation.
How exactly does that work?
Today, the passage of time is measured using atomic clocks. These clocks use vibrations in atoms to measure time. Because a second is defined by atomic clocks as an x number of vibrations in an atom and that number of vibrations is constant and is not influenced by the environment, all atomic clocks worldwide always run the same and they all indicate the ‘real time’.
UTC
But global society uses Coordinated World Time (UTC). And it is only partly based on time measurements by atomic clocks. The rotation speed of the Earth is also partly taken into account. There is only one problem: the Earth – unlike those atomic clocks – is not a Swiss clock (see box).
A day has 24 hours. That was not just invented by someone, but based on the rotation speed of the earth. It takes our planet roughly 24 hours (or 86,400 seconds) to rotate around its own axis. But the Earth is not a Swiss clock, as super-accurate atomic clocks from the 1960s onwards showed. The rotation speed appeared to vary. Sometimes the Earth needed just a little more time – on the order of milliseconds – to complete a rotation around its own axis. And sometimes it actually took less time. This is based on a complex set of different factors: winds, friction with the atmosphere, mass movements in the Earth: it can all cause the Earth to sometimes rotate a little slower or sometimes a little faster.
Over the years, atomic clocks have shown that the Earth is generally spinning more slowly. It results in days that are – again by the order of milliseconds – longer. To prevent the rotating Earth from getting too far out of step with our atomic clocks over the years, regular interventions have been carried out since the 1970s. For example, a leap second is regularly inserted. Clocks will then be given a one-second extra second at the end of December or end of June, to correct for the slower rotation of the Earth.
Positive leap seconds
We are talking about positive leap seconds: an extra second is added. This was necessary in 1972, for example, because a day defined by the Earth’s rotation lasted 2500 microseconds longer than a day defined by atomic clocks. And over a year, the difference between the two increased to almost 1 second. To avoid an even bigger difference, a leap second was used. But that is certainly not necessary every year. For example, in 2023 the difference between the duration of a day as measured by atomic clocks and as the Earth’s rotation dictates was only 80 microseconds. As a result, the difference over a year only amounted to 0.03 seconds; too little to justify the use of a leap second.
Melting ice
But why is that? As mentioned, there are variations in the rotation of the Earth and this has different causes. Some of those causes change the Earth’s rotation at the same rate every year. But there are two phenomena that change the Earth’s rotation speed to varying degrees every year: the melting of land ice at higher latitudes and movements in the Earth’s liquid core. Let’s start with the melting land ice – which has been given a significant boost by global warming. When land ice melts, the meltwater runs into the ocean, causing global sea levels to rise. It is in fact a mass shift: first the mass was near the poles, now it is moving closer to the equator. And that causes the earth to rotate more slowly.
Earth core
And then there is the liquid core of the Earth: an enormous amount of molten iron in the heart of our planet. Very slow movements in that iron soup ensure that our planet can generate a magnetic field, but also influence the speed at which the Earth rotates, as researchers discovered seventy years ago. This allows the rotation to be slowed down, but also accelerated. And measurements have shown that the liquid core itself has actually promoted acceleration since 1972. This makes the days shorter and in the long term you would have to consider introducing a negative leap second, so that a minute would not last 60, but 59 seconds.
Delay
If that trend continues and we ignore the melting of the land ice caused by global warming, a negative leap second should be introduced somewhere around 2026. But because the melting land ice is slowing down the rotation somewhat, that moment is now expected to wait three years longer.
It is undoubtedly a relief for scientists and IT professionals. Because introducing a positive leap second is already quite drastic, as researcher Erik Dierikx, who works at the national metrological institute VSL, previously told Scientias.nl. “We can program our atomic clocks to include the leap second. But with other equipment in our laboratory – including equipment that transmits time to computer networks – this is difficult and remains exciting.” Experience shows that computer networks can be quite upset by such a correction, especially if it is not implemented completely correctly. “Computers are often told what time it is from multiple clocks. And when one clock says: it’s so late. And the other clock transmits a different time, such a system can switch itself off for safety reasons. For example, the check-in system of a major airport has sometimes failed during the input of a leap second.” But introducing a negative leap second – something that has never happened before – is probably much more drastic. “It poses an unprecedented problem for computer networks,” writes researcher Duncan Carr Agnew Nature.
But thanks to climate change, we can prepare for this a little longer. However, that small advantage that global warming appears to have for our registration of time does not nearly outweigh the enormous disadvantages it has for our planet and humanity in many other areas, Agnew emphasizes. In fact, the fact that climate change is already affecting the speed at which our earth rotates indicates that we humans influence our planet in an unparalleled way.
Whether a negative leap second in 2029 is really necessary remains to be seen. It depends on what the Earth’s rotation will do in the coming years. If the introduction of the leap second is ultimately postponed even further, this may eventually be adjusted. Because a few years ago, countries worldwide agreed to abolish the leap second by 2035. The decision followed years of discussion about the usefulness of the leap second. “Some people say: just abolish it,” Dierikx said in 2020 Scientias.nl. “Others want to keep the leap second. It is certain that there are few technical arguments for the introduction of the leap second. It is more based on a feeling that people like the middle of the day to coincide with 12:00 noon and midnight actually being at 00:00.” But in the end, that sentiment turned out not to outweigh the disadvantages that the introduction of a leap second – positive or negative – has for computer networks in particular. And so this will be a thing of the past from 2035 onwards.
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