Researchers used more than 120 years of data to decipher how melting ice, declining groundwater and rising seas are shifting the planet’s axis of rotation and making days longer.
Days on Earth are getting slightly longer, and the change is accelerating. The reason is related to the same mechanisms that have also caused the planet’s axis to shift by about 10 meters over the past 120 years. The findings come from two recent NASA-funded studies that focused on how climate-related ice and water redistribution has affected Earth’s rotation.
This redistribution occurs when ice caps and glaciers melt more than they grow from snowfall, and when aquifers lose more groundwater than precipitation replenishes it. These resulting mass changes cause the planet to wobble as it spins and shift its axis of rotation—a phenomenon called polar motion. They also cause the Earth’s rotation to slow, as measured by the lengthening of the day. Both of these phenomena have been recorded since 1900.
By analyzing the movement of the poles over 12 decades, scientists have attributed almost all of the periodic oscillations in the position of the axis to changes in groundwater, ice sheets, glaciers and sea level. According to a recent paper published in Nature Geoscience, the mass variations during the 20th century are mainly the result of natural climate cycles.
The same researchers then conducted a study of day length. They found that since 2000, days have been lengthening by about 1.33 milliseconds per century, a faster rate than at any time in the previous century. The cause: the accelerated melting of glaciers and ice sheets in Antarctica and Greenland, due to human-caused greenhouse gas emissions. Their findings were published July 15 in the 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 powerful 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 in Southern California.
At first, scientists tracked the movement 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 to satellite laser ranging, which points lasers at satellites.
Researchers have long assumed that the movement of the poles results from a combination of processes inside and on the Earth’s surface. It was unclear how much each process shifts the axis and what kind of effect each has—whether cyclical movements that repeat over periods of weeks to decades, or a sustained drift over centuries or millennia.
For their study, the researchers used machine learning algorithms to dissect 120 years of observations. They found that 90% of the recurring fluctuations between 1900 and 2018 could be explained by changes in groundwater, ice sheets, glaciers, and sea level. The rest was mainly due to the Earth’s internal dynamics, such as the wobble caused by the tilt of the inner core relative to the planet’s mass.
Patterns of pole motion related to surface mass changes repeated themselves several times about every 25 years during the 20th century, suggesting to researchers that they were largely due to natural climate variations. Earlier papers have linked more recent pole motion to human activities, including one by Adhikari that attributed a 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.
The research focused on the past two decades, during which groundwater and ice mass loss and sea level rise – all measured by satellite – have been closely linked to human-induced climate change.
“It is true to some extent” that human activities play a role in the pole movement, said Mostafa Kiani Shahvandi, lead author of both studies and a doctoral student at ETH Zurich in Switzerland. “But there are natural modes in the climate system that have the main effect on the oscillations of the pole movement.”
For the second paper, the authors used satellite observations of mass change from the Gravity Recovery and Climate Experiment (GRACE) mission and its follow-on mission GRACE-FO, as well as previous mass balance studies that analyzed the contributions of changes in groundwater, ice sheets, and glaciers to 20th-century sea-level rise, to reconstruct changes in day length due to these factors from 1900 to 2018.
Scientists know from historical records of eclipses that the length of the day has been increasing for millennia. Although almost imperceptible to humans, this shift must be taken into account because many modern technologies, including GPS, rely on precise timing.
In recent decades, faster melting of the ice caps has shifted mass from the poles to the equatorial ocean. This flattening is causing the Earth to slow down and the days to lengthen, much like when an ice skater lowers and spreads his arms to slow a pirouette.
The authors observed an increase in the rate of day lengthening just after 2000, a change closely related to independent observations of flattening. For the period 2000 to 2018, the rate of increase in day length due to ice and groundwater movement was 1.33 milliseconds per century, faster than any period in the previous 100 years, when it ranged from 0.3 to 1.0 milliseconds per century.
The elongation due to changes in ice and groundwater could slow by 2100 under a climate scenario of drastic emissions reductions, the researchers note. (Even if emissions stopped today, the gases previously released — especially carbon dioxide — would persist for decades longer.)
If emissions continue to rise, the increase in day length due to climate change could reach 2.62 milliseconds per century, exceeding the effect of the Moon’s pull on tides, which increases Earth’s day length by an average of 2.4 milliseconds per century. Called lunar tidal drag, this effect has been the main cause of Earth’s increasing day length for billions of years.
“In just 100 years, humans have changed the climate system to such an extent that we are seeing the impact on how the planet rotates,” Adhikari said.
Andrew Wang / Jane J. Lee
Jet Propulsion Laboratory, Pasadena, California.
626-379-6874 / 818-354-0307
andrew.wang@jpl.nasa.gov / jane.j.lee@jpl.nasa.gov
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