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 candidate 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.
