Glaciers Melt – Days Grow Longer

Excuse me, what time is it? It turns out the answer is beyond imagination. Two new studies reveal that the melting of glaciers and rising sea levels, consequences of the climate crisis, are significant enough to alter the Earth's rotation speed and the agreed-upon global time—a phenomenon previously attributed mainly to the influence of massive bodies like the Moon and Earth's core.

Tides

The Earth spins on its axis constantly, leading to sunrises, sunsets, day, and night. Each rotation takes exactly one day, or 24 hours, equivalent to 86,400 seconds, during which the Earth completes one full spin on its axis.

At least, that's what we thought.

The Earth's rotation rate is not constant but changes over time. For example, about half a billion years ago, a day lasted around 22 hours, meaning Earth completed approximately 400 rotations on its axis for each orbit around the Sun. Back then, each year had about 400 days. Since then, friction caused by oceanic tides has slowed Earth's rotation and slightly pushed the Moon farther away from us.

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 tides, so tidal movements follow the Moon's position, which shifts at a different pace from Earth's crust.

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.

Leap Second

Tides, tidal patterns, and other factors continue to alter the Earth's rotation speed—some in a consistent manner and others in unpredictable ways. While these changes are minimal, the modern world demands extreme precision, and these unexpected shifts make it challenging to align global clocks with Earth's rotation rate. Therefore, in 1972, global clocks began relying on the atomic clock, whose accuracy is nothing short of remarkable.

However, the duration of a standard second measured by atomic clocks was determined based on Earth's rotation rate in the 19th century when the length of a second was established, not on the current rate. As a result, each day now averages 86,400 seconds plus a few milliseconds. It may seem trivial, but in a world where countless computers execute billions of operations per second and must remain synchronized to manage communication and commerce networks, precision is critical.

Consequently, since 1972, 27 leap seconds have been declared—seconds added to the clock to ensure synchronization between standard timekeeping, based on atomic clocks, and timekeeping based on Earth's rotation. If these seconds were not periodically added, over many years, the discrepancy would grow, causing the clock to lose alignment with the 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 precisely 365 days long, thereby maintaining a consistent relationship between the seasons and calendar dates.

Without adding leap seconds to the clock, we would lose synchronization between clock time and the actual times of sunrise and sunset. A digital clock displaying an added second | AlexLMX, Shutterstock.

Climate Change Alters Time

When discussing Earth's rotation rate, we are referring to the speed at which Earth's surface rotates, as the planet 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 influencing this rotation to a similar extent. As the climate crisis intensifies, the rate of glacier melting and sea-level rise has increased. A new study indicates that these processes are also slowing Earth's rotation.

The physical mechanism behind this phenomenon stems 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. This added mass at the furthest point from the axis of rotation increases Earth's resistance to spinning, thereby slowing 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."

Researchers measured tiny changes in Earth's rotational speed using a technique called interferometry. This method detects differences in the arrival times of electromagnetic waves from space at different points on Earth's surface. These differences allowed scientists to calculate changes in Earth's rotational speed and the length of a day. Another tool used was the Global Positioning System (GPS), which can measure 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, the researchers estimate the day would lengthen by about one millisecond by 2100.

This new study's forecast aligns with other research. A study published in March used precise satellite measurements to examine the impact of mass movement—such as the shifting of water and ice—and other factors on Earth's rotation rate. This study also found that glacier melting has become a major factor in altering Earth's spin. "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 noted that the fluid part of Earth's core is speeding up, counteracting the slowing effects of tides and glacier melting, and even accelerating Earth's surface rotation.

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.

While Earth's core alternates between faster and slower rotations compared to its mantle, the ongoing glacier melting only slows Earth's spin. This means its long-term effects may become more pronounced. 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, and we can measure it."