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Researchers used more than 120 years of data to decipher how melting ice, dwindling groundwater, and rising seas are nudging the planet's spin axis and lengthening days.
Days on Earth are growing slightly longer, and that change is accelerating. The reason is connected to the same mechanisms that also have caused the planet's axis to meander by about 30 feet (10 meters) in the past 120 years. The findings come from two recent NASA-funded studies focused on how the climate-related redistribution of ice and water has affected Earth's rotation.
This redistribution occurs when ice sheets and glaciers melt more than they grow from snowfall and when aquifers lose more groundwater than precipitation replenishes. These resulting shifts in mass cause the planet to wobble as it spins and its axis to shift location - a phenomenon called polar motion. They also cause Earth's rotation to slow, measured by the lengthening of the day. Both have been recorded since 1900.
Analyzing polar motion across 12 decades, scientists attributed nearly all of the periodic oscillations in the axis' position to changes in groundwater, ice sheets, glaciers, and sea levels. According to a paper published recently in Nature Geoscience, the mass variations during the 20th century mostly resulted from natural climate cycles.
The same researchers teamed on a subsequent study that focused on day length. They found that, since 2000, days have been getting longer by about 1.33 milliseconds per 100 years, a faster pace than at any point in the prior century. The cause: the accelerated melting of glaciers and the Antarctic and Greenland ice sheets due to human-caused greenhouse emissions. Their results were published July 15 in Proceedings of the National Academy of Sciences.
"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," said Surendra Adhikari, a co-author of both papers and a geophysicist at NASA's Jet Propulsion Laboratory in Southern California.
Decades of Polar Motion
In the earliest days, scientists tracked polar motion by measuring the apparent movement of stars. They later switched to very long baseline interferometry, which analyzes radio signals from quasars, or satellite laser ranging, which points lasers at satellites.
Researchers have long surmised that polar motion results from a combination of processes in Earth's interior and at the surface. Less clear was how much each process shifts the axis and what kind of effect each exerts - whether cyclical movements that repeat in periods from weeks to decades, or sustained drift over the course of centuries or millennia.
For their paper, researchers used machine-learning algorithms to dissect the 120-year record. They found that 90% of recurring fluctuations between 1900 and 2018 could be explained by changes in groundwater, ice sheets, glaciers, and sea level. The remainder mostly resulted from Earth's interior dynamics, like the wobble from the tilt of the inner core with respect to the bulk of the planet.
The patterns of polar motion linked to surface mass shifts repeated a few times about every 25 years during the 20th century, suggesting to the researchers that they were largely due to natural climate variations. Past papers have drawn connections between more recent polar motion and human activities, including one authored by Adhikari that attributed a sudden eastward drift of the axis (starting around 2000) to faster melting of the Greenland and Antarctic ice sheets and groundwater depletion in Eurasia.
That research focused on the past two decades, during which groundwater and ice mass loss as well as sea level rise - all measured via satellites - have had strong connections to human-caused climate change.
"It's true to a certain degree" that human activities factor into polar motion, said Mostafa Kiani Shahvandi, lead author of both papers and a doctoral student at the Swiss university ETH Zurich. "But there are natural modes in the climate system that have the main effect on polar motion oscillations."
Longer Days
For the second paper, the authors used satellite observations of mass change from the GRACE mission (short for Gravity Recovery and Climate Experiment) and its follow-on GRACE-FO, as well as previous mass-balance studies that analyzed the contributions of changes in groundwater, ice sheets, and glaciers to sea level rise in the 20th century to reconstruct changes in the length of days due to those factors from 1900 to 2018.
Scientists have known through historical eclipse records that length of day has been growing for millennia. While almost imperceptible to humans, the lag must be accounted for because many modern technologies, including GPS, rely on precise timekeeping.
In recent decades, the faster melting of ice sheets has shifted mass from the poles toward the equatorial ocean. This flattening causes Earth to decelerate and the day to lengthen, similar to when an ice skater lowers and spreads their arms to slow a spin.
The authors noticed an uptick just after 2000 in how fast the day was lengthening, a change closely correlated with independent observations of the flattening. For the period from 2000 to 2018, the rate of length-of-day increase due to movement of ice and groundwater was 1.33 milliseconds per century - faster than at any period in the prior 100 years, when it varied from 0.3 to 1.0 milliseconds per century.
The lengthening due to ice and groundwater changes could decelerate by 2100 under a climate scenario of severely reduced emissions, the researchers note. (Even if emissions were to stop today, previously released gases - particularly carbon dioxide - would linger for decades longer.)
If emissions continue to rise, lengthening of day from climate change could reach as high as 2.62 milliseconds per century, overtaking the effect of the Moon's pull on tides, which has been increasing Earth's length of day by 2.4 milliseconds per century, on average. Called lunar tidal friction, the effect has been the primary cause of Earth's day-length increase for billions for years.
"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.
