We all know that an extra day is added in February once every four years – known as a leap year.
But every few years, a ‘leap second’ is also added, usually at the end of June or the end of December.
That’s because the speed at which Earth rotates around its axis fluctuates slightly – meaning one full rotation is not always precisely one day.
Now, a scientist in the US is calling for a ‘negative’ leap second – the removal of a second – for the first time.
Professor Duncan Agnew, a geophysicist at UC San Diego, says the negative leap second should happen in 2029 to account for the Earth spinning too fast, but he warns this could lead to ‘unprecedented’ problems for smartphones and computers.
To make up for irregularities in the Earth’s spin, leap seconds are traditionally added – but for the first time, a second could be taken away, possibly in 2029
A leap second was last added on December 31, 2016. Here is a screenshot from the time.gov website with the leap second added. You can see how this would cause confusion for computers. Professor Duncan Agnew at UC San Diego suggests removing a second in 2029 – so the final time reading of the year would be 23:59:58
‘Extrapolating the trends for the core and other relevant phenomena to predict future Earth orientation shows that UTC as now defined will require a negative discontinuity by 2029,’ says Professor Agnew in his new paper.
‘This will pose an unprecedented problem for computer network timing and may require changes in UTC to be made earlier than is planned.’
Coordinated Universal Time (UTC) is defined by sophisticated, ultra-precise ‘atomic clocks’ around the world, which tick precisely and continuously.
However, these atomic clocks do not align exactly with observed ‘solar time’, which historically defines days as a single rotation of the Earth.
The time it takes for a single planetary rotation changes every so often due to the pull of the moon, causing the two time systems to drift apart.
Since 1972, a leap second has been added 27 times, the last of which was in 2016.
However, the Earth’s rotation has reportedly been speeding up since 2020 – not slowing down.
A leap second may therefore need to be removed, rather than added, in the future, in order to keep observed solar time in sync with clocks.
Coordinated Universal Time (UTC) is defined by sophisticated, ultra-precise ‘atomic clocks’ around the world, which tick precisely and continuously. Experts are pictured here with the NIST-F2 atomic clock in the US
A ‘positive’ leap second is the addition of a second when the Earth’s rotation has been too slow, while a ‘negative’ leap second is the removal of a second when the Earth’s rotation has been too fast.
‘Even a few years ago, the expectation was that leap seconds would always be positive, and happen more and more often,’ said Professor Agnew.
‘But if you look at changes in the Earth’s rotation, which is the reason for leap seconds, and break down what causes these changes, it looks like a negative one is quite likely.’
Anyone who has forgotten to set their clocks forward or backward an hour knows how confusing it is to be out of sync with everyone else’s time.
The problem is, the same confusion can occur with leap seconds, especially for today’s modern technology systems.
If a computer network doesn’t know about them, it will become out of sync with everything else.
For example, if an online store’s computer and your bank’s computer keep different times, clicking the ‘order’ button will appear to create two actions at different times, not just one.
‘One second doesn’t sound like much, but in today’s interconnected world, getting the time wrong could lead to huge problems,’ said Professor Agnew.
‘Many systems now have software that can accept an additional second, but few if any allow for removing a second, so that a negative leap second is expected to create many difficulties.’
Several factors cause the Earth’s rotation rate to vary from year to year, and one of them is Earth’s liquid core – a mass of molten iron inside the solid part of the planet.
Several factors cause the Earth’s rotation rate to vary from year to year, and one of them is Earth’s liquid core – a mass of molten iron inside the solid part of the planet (stock image)
Very slow motions of different parts of the core interact to produce the Earth’s magnetic field.
These changing motions also cause Earth to spin faster or slower.
In his new study, Professor Agnew claims that the melting of ice at high latitudes due to global warming is an additional factor that slows down Earth’s rotation.
When the ice melts, the water spreads out over the whole ocean, which increases Earth’s ‘resistance’ to angular acceleration and in turn slows the planet down.
So while the Earth’s rotation has been speeding up overall, global warming slows it down, delaying the need for a negative leap second.
Without the slowing of Earth’s rotation caused by melting ice, the negative leap second would be needed three years earlier, in 2026, Professor Agnew reports.
Each day on Earth contains 86,400 seconds, but the rotation isn’t uniform, which means over the course of a year, each day has a fraction of a second more or less
This postponement, while giving more time to prepare, is only a trivial benefit compared to massive problems from global warming, he adds.
‘Increased melting of ice in Greenland and Antarctica, measured by satellite gravity, has decreased the angular velocity of Earth more rapidly than before,’ he says in his study, published in the journal Nature.
‘Global warming and global timekeeping have become inextricably linked and may be more so in the future.’
In 2022, scientists voted to scrap the leap second system entirely, but this might not happen until 2035 – by which point a negative leap second may already have been needed.
