Excuse me, what time is it?
The answer might be more surprising than you imagine. Two new studies reveal that the melting of glaciers and rising sea levels - consequences of the climate crisis - are significant enough to alter Earth's rotation speed and the global standard time. Previously, such changes were attributed primarily to the gravitational influence of massive bodies like the Moon and the dynamics of Earth's core.
Tides
The Earth rotates on its axis constantly, giving rise to sunrises and sunsets, day and night. Each rotation lasts exactly one day - 24 hours, which is equivalent to 86,400 seconds, during which Earth completes one full spin on its axis. At least, that's what we thought.
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Tides create friction between the oceans and Earth's crust, slowing the planet's rotation. The Moon’s influence on Earth's tides
(Siberian Art, Shutterstock)
However, Earth's rotation is not constant but changes over time. For instance, about half a billion years ago, a day lasted only about 22 hours, meaning Earth completed approximately 400 rotations around its axis during each orbit around the Sun. In other words, a year back then had 400 days. Since that time, friction caused by oceanic tides has gradually slowed Earth's rotation and slightly pushed the Moon farther away from us.
Thus for instance, tides generate water currents along the ocean floor, creating friction between the oceans and Earth's crust. The Moon’s gravitational pull is the primary force driving tidal movements, which follow the Moon's position as it shifts at a slightly different pace from Earth's crust.
Leap Second
The tides, ebbs, and other factors still influence Earth's rotation rate, some in a consistent manner and others in variable and unpredictable ways. While these changes are minimal, the modern world demands extreme precision, and these unpredictable shifts make it challenging to synchronize global clocks with Earth's rotation rate. Therefore, in 1972, global clocks began relying on the atomic clock, the accuracy of which is nothing short of remarkable.
However, the duration of a standard second measured by atomic clocks was defined based on Earth's rotation rate in the 19th century, when the length of a second was established, rather than the current rate. As a result, each day now averages 86,400 seconds plus a few milliseconds. While this might seem a bit nitpicky, in a world where countless independent computers perform billions of operations per second and must remain synchronized to manage our communication and commerce networks, precision is critical.
Consequently, since 1972, 27 leap seconds have been introduced - seconds added to specific minutes to ensure synchronization between standard timekeeping, based on atomic clocks, and timekeeping based on Earth's rotation. Without adding these leap seconds, over the course of many years the clock would gradually drift out of alignment with the actual times of sunrise and sunset. This is comparable to adding a day to the Gregorian calendar on February 29 during leap years, to account for the fact that a year is not exactly 365 days long, thereby preserving a consistent relationship between the seasons and calendar dates.
Climate Change Alters Time
When we discuss Earth's rotation rate, we are referring to the speed at which its surface rotates. However, Earth is not entirely rigid—its core can rotate at a different speed, as indeed happens.
Until now, tides have been the primary driver of changes in Earth's rotation rate. However, it now appears that human activity is altering this rotation to a comparable extent. As the climate crisis intensifies, the rates of glacier melting and global sea-level rise have increased. A recent study indicates that these processes are also slowing Earth's rotation. The physical mechanism underlying this phenomenon arises from changes in the distribution of mass on Earth's surface. Recent measurements reveal how meltwater from glaciers in Greenland and Antarctica flows toward the equator, adding mass to equatorial regions. The addition of water flowing toward the equatorial region increases Earth's mass in the area farthest from its axis of rotation, slowing down its rotation.
"It's like when a figure skater does a pirouette, first holding her arms close to her body and then stretching them out," explained Benedikt Soja, one of the study’s authors, in an interview with ScienceAlert, "the initially fast rotation becomes slower because the masses move away from the axis of rotation, increasing physical inertia."
The researchers measured minute changes in Earth's rotational speed using a technique called interferometry, which detects differences in the arrival times of electromagnetic waves from space at various points on Earth's surface. The measured differences in wave arrival times at each point allowed scientists to calculate changes in Earth's rotational speed and the duration of a full rotation. Another tool used was the Global Positioning System (GPS), which enables measurements of Earth's rotational period with millisecond-level precision.
How the Earth Spins
The distribution of mass across Earth's surface is changing. Estimates based on observations from 1900 to the present suggest that climate change has lengthened the day by an average of 0.8 milliseconds. With continued glacier melting, researchers predict that climate change could extend the length of a day by approximately 2.62 milliseconds by the end of the 21st century. In a more optimistic scenario, where greenhouse gas emissions are significantly reduced in the coming years, they estimate that the day would lengthen by about one millisecond by 2100
This new study's forecast is not an outlier. A study published in March used precise satellite measurements to examine the effects of mass movement on Earth's surface—such as the shifting of water and ice—and other factors influencing Earth's rotation rate. This study also found that glacier melting has become a driver of changes in Earth's rotation. "This [slowdown] didn’t exist 30 years ago," explained Duncan Agnew, the study's author, in an interview with The Naked Scientist podcast. However, the study also found that the liquid part of Earth's core slows its rotation, which increases the rotation speed of Earth's surface and even counteracts the decelerating effects of tides and glacier melting.
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As a result, by 2029, we may need to do something unprecedented—subtract a second from one of the days instead of adding a leap second, as has been done so far. According to Agnew, even a one-second mismatch in global timekeeping could cause significant issues in communication systems, computing, and financial markets, making it essential to prepare for this adjustment.
Earth's core alternates between moving faster and slower than the mantle, but the continuous melting of glaciers consistently exerts a unidirectional effect, indicating that its impact may grow over time. As Agnew noted, "One of the impressive results of this paper is the realization that global warming is causing the entire Earth to slow down. It's not just some local effect that we could actually see in our very precise measurements, another sign of the fact that global warming is causing unprecedented things to happen.”