For the past couple of decades, Earth’s rotation around its axis has been speeding up. Consequently, the days have grown shorter every day by just a tiny fraction of a second. This very slight shortening of the day has caused a lot of headaches to world timekeeping. The bandaid-like solution has been to add a “leap second” during some years at irregular intervals to keep the “official” and “astronomical” time in sync.

But a new study suggests this pattern has been broken. Melting ice at Earth’s poles is redistributing water across the world’s oceans, counteracting some of our planet’s recent bursts of speed. This means, the researchers argue, that clocks would have to skip a second around 2029. This would make it the first “negative leap second” in modern timekeeping history.

“This is an unprecedented situation and a big deal,” study lead author Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego, told AP. “It’s not a huge change in the Earth’s rotation that’s going to lead to some catastrophe or anything, but it is something notable. It’s yet another indication that we’re in a very unusual time.”

Time is not timeless

The earliest civilizations relied on the natural world to measure time. Ancient Egyptians used sundials during the day and water clocks at night. These devices were based on the apparent movement of the Sun across the sky and the steady flow of water, respectively. However, these methods varied with the seasons and were not universally standardized.

In the Middle Ages, mechanical clocks began to show up in the towers of Western European cities. These were powered by weights and gears, marking a significant step towards more precise and consistent timekeeping. Yet, the lack of standardization meant that noon occurred at slightly different times in different locations. This was never a big deal until the railway system was invented in the 19th century. Trains needed to be on time.

Thus came the adoption of “Railway Time” in Britain, a precursor to time zones. Finally, Greenwich Mean Time (GMT) was introduced in 1884. It was based on the mean solar time at the Royal Observatory in Greenwich, London. GMT became the world’s time standard, with time zones defined relative to it.

Atomic timing

The pinnacle of timekeeping came with the invention of atomic clocks in the 1980s. Time has no physical properties to measure, unlike mass for instance. So, when we say we measure time, what we are really measuring is time intervals, or the duration between two events.

Using atomic clocks, scientists have standardized the united of time, the second, to the duration it takes for cesium-133 atoms to vibrate 9,192,631,770 times in reaction to microwave radiation. These clocks are accurate to within 1/15,000,000,000 of a second per year. These clocks would not have gained or lost a second even if they started running at the dawn of the universe billions of years ago. For comparison, your quartz wristwatch may be accurate to within about 15 seconds per month. 

By this mind-bogglingly precise definition of a second, one day on Earth lasts 86,400 atomic seconds or so — that “or so” is the problem. While the vibrations of cesium atoms may be totally predictable, Earth’s rotation around its axis is not nearly as clockwork.

The Coordinated Universal Time (UTC), the current standard for timekeeping, combines the precision of atomic time with the Earth’s rotation, adjusted by leap seconds to account for slight variations in the Earth’s rotation speed. The first leap second was added in 1972. Up until 2016, 27 separate leap seconds have been added as Earth slowed.

Leap seconds can cause a lot of havoc. In 2012, a leap second caused Reddit to go offline for 30 to 40 minutes. The addition of this second confused the site’s high-resolution timer, leading to server overload and freezing the CPUs. Cloudflare experienced its own leap second troubles in 2017, which disrupted its public DNS service. Airlines have shut down because of it. Leap seconds are a big deal.

One second to midnight

It’s not entirely clear why Earth’s rotation is slowing down. It seems to be due to a combination of factors, including tidal forces between Earth and the Moon, glacial rebound from the last ice age from 20,000 years ago, core-mantle interactions, and changes in atmospheric and ocean currents. The Moon used to be much more important in this equation. During Earth’s early history, when the moon was much closer than it is today, a day lasted for only four hours.

Even with its decreased effect, it remains one of the factors. And all of these things add up such that in June 2022, we had the shortest day on record — when Earth completed one spin in 1.59 milliseconds less than 24 hours — over the past half a century.

Despite the record, since 2022 this steady speedup has intriguingly switched to a slowdown. Previously, scientists have suggested this is probably due to interactions deep within the planet. In any event, these developments have suggested the unprecedented prospect of adding a “negative leap second” — a skipped second, rather than adding an extra one as has been the norm. This prospect is dreaded by everyone because it could cause even more disruption. Our current software is designed to add seconds, not subtract them.

“We do not know how to cope with one second missing. This is why time metrologists are worried,” Felicitas Arias, former director of the Time Department at the International Bureau of Weights and Measures in Sèvres, France, told Nature.

Losing moments

In a new study, Agnew and Judah Levine, a physicist for the time and frequency division of the National Institute of Standards and Technology, have added a new piece to the jigsaw puzzle. Their computer modeling shows that melting ice in Greenland and Antarctica due to climate change is decreasing Earth’s angular velocity.

As the ice melts, it turns into water that is redistributed throughout the oceans by currents. These changes in mass from the poles to the bulging center change the planet’s moment of inertia. The angular momentum is always conserved, so Earth’s rotation slows down — similar to how an ice skater slows their rotation by extending their arms out to their sides. Greenland has seen an alarming mass reduction, shedding an average of 279 billion tons of ice per year between 1993 and 2019. Meanwhile, Antarctica’s ice loss has accelerated to an average of 148 billion tons annually over the same period.

What all of this means is that, in effect, climate change is countering the recent trend of slowdown in planetary spin. The net result is still a decelerating Earth, but the researchers believe the need for a negative leap second can be postponed by three years. This means that a negative leap second could be added in 2029 — thankfully this could be the last leap second of any time as metrologists have agreed to get rid of leap-second corrections in 2035.

It’s quite astonishing to contemplate how far-reaching the effects of climate change can be. A force so strong, that it alters time itself.

The new findings appeared in the journal Nature.