Scientists used more than 120 years of data to unravel how melting ice, receding groundwater and rising sea levels are changing the planet’s axis of rotation and lengthening the days.

Recent studies show that Earth’s days are getting longer, and that this trend is accelerating due to climate change, such as melting glaciers and depleting groundwater. These changes in mass distribution are not only shifting the planet’s axis, but also slowing its rotation. Using advanced measurement techniques spanning the past century, studies detail how human-induced changes are amplifying these natural events, with potential long-term impacts on timekeeping and the technologies that rely on accurate timekeeping.

Days are getting longer due to climate change

Days on Earth are getting slightly longer, and the change is accelerating. This is due to the same mechanisms that have caused the planet’s axis to tilt by about 10 meters (30 feet) over the past 120 years. The findings come from two new NASA-funded studies that looked at how climate’s redistribution of ice and water affects Earth’s rotation.

This redistribution occurs when ice sheets and glaciers melt more than they are replenished by snowfall, and aquifers lose more groundwater than they are replenished by precipitation. These changes in mass cause the planet to wobble as it rotates, shifting its axis; this phenomenon is called polar motion. They also slow the Earth’s rotation, as measured by the lengthening of the day. Both have been recorded since 1900.

Polar Movement: A Ten-Year Analysis

Scientists who have been analyzing the pole’s movement for 12 years have explained almost all of the periodic fluctuations in the axis position by changes in groundwater, ice sheets, glaciers and sea level. According to a recently published paper Nature GeologyMass fluctuations in the 20th century were mostly the result of natural climate cycles.

The same researchers teamed up with a follow-up study focused on day length. They found that days have gotten longer by about 1.33 milliseconds per 100 years since 2000, a faster rate than at any time in the past century. The reason: rapid melting of glaciers and ice sheets in Antarctica and Greenland due to anthropogenic greenhouse gas emissions. Their results were published July 15. Proceedings of the National Academy of Sciences.

“What the two papers have in common is that climate-related changes at the Earth’s surface, whether human-caused or not, are strong drivers of the changes we see in the planet’s rotation,” said Surendra Adhikari, co-author of both papers and a geophysicist at NASA’s Jet Propulsion Laboratory (JPL) in Southern California.

Technological tracking of polar motion

In the early days, scientists tracked the motion of the poles by measuring the apparent motion of stars. They later moved on to very-long-baseline interferometry, which analyzes radio signals from quasars, or satellite laser locators, which direct lasers at satellites.

Researchers have long assumed that the poles’ motion is the result of a combination of processes in Earth’s interior and on its surface. What’s less clear is how much each process shifts the axis and what effect each has—whether cyclical movements repeat over weeks or decades, or a steady drift over centuries or millennia.

In their paper, the researchers used machine learning algorithms to examine the 120-year record. They found that 90% of the repeated fluctuations between 1900 and 2018 could be explained by changes in groundwater, ice sheets, glaciers, and sea levels. The rest is mostly the result of Earth’s internal dynamics, such as fluctuations caused by the tilt of the inner core relative to the planet’s mass.

Patterns of polar motion associated with changes in surface mass have recurred several times during the 20th century, about every 25 years, and leading researchers have suggested that these are mostly due to natural climate changes. Previous work has linked recent polar motions to human activities, including Adhikari’s work, which linked the sudden eastward shift of the axis (starting around 2000) to faster melting of the Greenland and Antarctic ice sheets and depletion of groundwater in Eurasia.

This study focused on the past two decades, during which groundwater and ice mass loss and sea level rise, all measured by satellites, have been strongly linked to human-caused climate change.

“It’s true to some extent” that human activities affect the movement of the poles, said Mostafa Kiani Shahwandi, lead author of both papers and a doctoral student at ETH Zurich in Switzerland. “But there are natural regimes in the climate system that have the main impact on the fluctuations in the movement of the poles.”

Acceleration of the increase in day length

For the second paper, the authors used data from satellite observations of mass change from the GRACE mission (short for Gravity Recovery and Climate Experiment) and its follow-up program GRACE-FO, as well as data from previous mass balance studies that analyzed the contribution of gravity to changes in groundwater, ice sheets and glaciers prior to the 20th-century sea level rise, to reconstruct changes in day length due to these factors from 1900 to 2018.

Thanks to historical records of eclipses, scientists know that the length of the day has increased over the millennia. While the delay is almost imperceptible to humans, it is important to take into account that many modern technologies, including GPS, rely on accurate time.

The rapid melting of ice sheets in recent years has shifted mass from the poles to the equatorial ocean. This flattening causes the Earth to slow down and the day to lengthen, similar to a skater lowering and extending their arms to slow their spin.

Shortly after 2000, the authors observed an increase in the rate of day lengthening; this change correlates closely with independent observations of the alignment. Between 2000 and 2018, the rate of day length increase due to ice and groundwater movement was 1.33 milliseconds per century, faster than any period in the previous 100 years, when it varied between 0.3 and 1.0 milliseconds per century.

The researchers note that the extension, driven by changes in ice and groundwater, could slow by 2100 under a climate scenario that projects significant emissions reductions. (Even if emissions were stopped today, gases released earlier, particularly carbon dioxide, would persist for decades.)

If emissions continue to rise, the length of the day could reach 2.62 milliseconds per century due to climate change, exceeding the moon’s effect on tides, increasing the length of the Earth’s day by an average of 2.4 milliseconds per century. This effect, called lunar tidal friction, has been the main cause of the lengthening of the Earth’s day for billions of years.