JPL Scientist Contributes to Groundbreaking Studies on Earth’s Slowing Rotation

Days on Earth are growing slightly longer, and that change is accelerating.

In a pair of recent studies, scientists, including a researcher from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, have shed new light on the subtle yet significant changes in Earth’s rotation and axis movement, linking them to climate-related shifts in mass across the planet’s surface.

Dr. Surendra Adhikari, a geophysicist at JPL, co-authored two papers led by Mostafa Kiani Shahvandi, a doctoral student at ETH Zurich. The research, partially funded by NASA, reveals that Earth’s days are incrementally lengthening, with the rate of change accelerating in recent years.

“The common thread between the two papers is that climate-related changes on Earth’s surface, whether human-caused or not, are strong drivers of the changes we’re seeing in the planet’s rotation,” Dr. Adhikari explained in an interview at JPL’s campus.

The first study, published recently in Nature Geoscience, examined the phenomenon known as polar motion — the wandering of Earth’s rotational axis.

By analyzing 120 years of data using machine-learning algorithms, the researchers discovered that nearly all periodic fluctuations in the axis’ position could be attributed to changes in groundwater, ice sheets, glaciers, and sea levels.

Historically, scientists tracked polar motion by measuring the apparent movement of stars, later switching to very long baseline interferometry and satellite laser ranging. The study found that 90% of recurring fluctuations between 1900 and 2018 were explained by surface mass changes, with the remainder mostly resulting from Earth’s interior dynamics.

Building on this foundation, the team’s second paper, published on July 15 in Proceedings of the National Academy of Sciences, focused on the increasing length of Earth’s days. Their findings indicate that since 2000, days have been lengthening at a rate of about 1.33 milliseconds per century — faster than at any point in the previous hundred years.

The researchers link this acceleration to the melting of glaciers and ice sheets in Antarctica and Greenland, exacerbated by human-induced greenhouse gas emissions. This redistribution of mass from poles to equator acts like an ice skater extending their arms, effectively slowing the planet’s rotation.

The studies utilized satellite observations from the GRACE and GRACE-FO missions to measure mass changes. Looking ahead, the researchers project that the lengthening of days due to ice and groundwater changes could decelerate by 2100 under a scenario of severely reduced emissions. However, if emissions continue to rise, the day-lengthening effect could surpass that of lunar tidal friction, which has been the primary cause of Earth’s day-length increase for billions of years.

While the changes in day length — mere milliseconds over a century — might seem negligible, they have significant implications for technologies that rely on precise timekeeping, such as GPS systems.

“In barely 100 years, human beings have altered the climate system to such a degree that we’re seeing the impact on the very way the planet spins,” Adhikari said.