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Climate change is messing with time itself.

The melting of polar ice due to global warming is affecting Earth’s rotation and could have an impact on precision timekeeping, according to <a href="https://www.nature.com/articles/s41586-024-07170-0" target=_blank>a paper </a>published Wednesday in the journal Nature.

The planet is not about to jerk to a halt, nor speed up so rapidly that everyone gets flung into space. But timekeeping is an exact science in a highly technological society, which is why global authorities more than half a century ago felt compelled by the slight changes in Earth’s rotation to invent the concept of the “leap second.”

Climate change is now making these calculations even more complicated: In just a few years it may be necessary to insert a “negative leap second” into the calendar to get the planet’s rotation in sync with Coordinated Universal Time.

“Global warming is managing to actually measurably affect the rotation of the entire Earth,” said study author Duncan Agnew, a geophysicist at the University of California at San Diego. “Things are happening that have not happened before.”

Timekeeping has traditionally had an astronomical basis. Earth is a type of a clock. In simpler times, the planet would spin one full revolution on its axis, and everyone would call it a day.

Technologists, however, demand excruciating levels of exactitude. Atomic clocks and not sundials now tell us what time it is. In atomic time, a second is defined as 9,192,631,770 oscillations of a Cesium atom. The goal of the people who want to get things exactly right is to make sure that atomic time is perfectly in sync with astronomical time.

For example, GPS satellites need to know exactly where Earth is beneath them — and precisely what time it is — to accurately get you from your house to the nearest Arby’s.

But Earth doesn’t spin at a perfectly constant speed. Our planet is in a complicated gravitational dance with the moon, the sun, the oceanic tides, Earth’s own atmosphere and the motion of the planet’s <a href="https://www.washingtonpost.com/science/2023/01/23/earth-core-slowing/" target=_blank>solid inner core</a>.

Unhelpfully, Earth’s core is unavailable for close scrutiny and “is a bit of a black box,” as Agnew noted. Geophysicists can infer some details about the planet’s interior by <a href="https://www.washingtonpost.com/science/2023/06/06/drill-earth-mantle-rocks/" target=_blank>drilling </a>into select areas of the sea floor. And as The Post reported last year, scientists have discerned changes in Earth’s rotation that appear to match 70-year oscillations in the rotation of the core.

But when scientists try to describe what Earth is doing at any given moment, they have to factor in a lot of slop and wobble.

The planet’s fluctuating spin rate is carefully tracked by the International Earth Rotation and Reference Systems Service (formerly the only slightly less bombastically named International Earth Rotation Service). In the early 1970s, Earth was clearly slowing down in its rotation, and a gap was forming between atomic time and astronomical time. Thus was born the “leap second” to adjust for the fact that the “day” was getting a bit longer.

Twenty-seven leap seconds have been added to Universal Coordinated Time since 1972. The addition of a leap second happens at the last tick of the clock on the night of Dec. 31 or June 30.

But hold on: Earth is not slowing down anymore. It’s actually been speeding up a bit. In fact, there hasn’t been a leap second added since the end of 2016.

Here is where the casual reader’s head might start spinning. The melting of the ice caps in Antarctica and Greenland shifts mass — meltwater — toward the equator. That process increases the equatorial bulge of the planet. Meanwhile, at the poles, the land that had been pressed down by ice rises, and Earth becomes more spherical.

These two changes in the planet’s shape have opposite effects on Earth’s rotation, according to Judah Levine, a physicist at the National Institute of Standards and Technology in Boulder, Colo.

The new paper by Agnew contends that, although the core is causing the planet to spin faster, the planetary shape changes caused by a warming climate are slowing that process. Absent this effect, Agnew wrote, the overall acceleration of the planet’s rotation might require timekeepers to insert a “negative leap second” at the end of 2026. Because of climate change, that might not be necessary until 2029, he found.

Levine, who is not part of this new study, said the science is credible, but he was not prepared to sign off on the conclusion because it is inherently tricky to predict what Earth is going to do.

“There’s a very great uncertainty about this,” Levine said. “A few years ago, there were predictions in the other direction.”

Nick Stamatakos, head of the U.S. Naval Observatory’s Earth Orientation Department, said in an email that, although it’s more likely a negative leap second will need to be adopted, there’s no way to know for sure what the planet will do.

“It is like a weather prediction of a big storm or hurricane. We can’t say for certain what will happen and when. However, we can say there is a higher percentage chance of a negative leap second than say 25 years ago,” Stamatakos wrote.

He noted that his brother-in-law teases him every time a leap second gets added, declaring, “Wow that was a long year — an entire extra second!” Stamatakos’s response: “Well, in that one second, the Earth rotated about four football fields.”

A previous version of this article stated that leap seconds are added on Dec. 31. They have also been added on June 30. The article has been corrected.

Princeton University, politics, 1982

Joel Achenbach covers science and politics for the National desk. He has been a staff writer for The Post since 1990. 

Joel Achenbach

The leap second is being affected by climate change, scientists warn

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