Using more than 120 years of data, researchers worked out how melting ice, declining groundwater, and rising sea levels are shifting the Earth’s axis and lengthening the days.
Earth’s days are slowly getting longer, and the change is accelerating. The reason is linked to the same mechanism that caused Earth’s axis to meander by about 30 feet (10 meters) over the past 120 years. The findings come from two recent NASA-funded studies that focus on how climate-related redistribution of ice and water has affected Earth’s rotation.
This redistribution occurs when ice sheets and glaciers melt more than they snow, and aquifers lose more groundwater than they receive precipitation. The resulting change in mass causes the Earth to wobble as it spins, changing its position on its axis, a phenomenon known as polar motion. It is also measured by the Earth’s rotation slowing and increasing hours of daylight, both of which have been recorded since 1900.
Scientists analyzed 120 years of polar motion and concluded that the periodic oscillations in the axis’ position are almost entirely due to changes in groundwater, ice sheets, glaciers and sea levels, according to a recently published paper. Nature ChemistryMass fluctuations in the 20th century were mainly caused by natural climate cycles.
The same researchers teamed up for a follow-up study focusing on day length. They found that since 2000, day length has increased by about 1.33 milliseconds per century, faster than at any time in the last century. The culprit is the rapid melting of glaciers and the Antarctic and Greenland ice sheets due to human greenhouse gas emissions. Their findings were published on July 15. Proceedings of the National Academy of Sciences.
“What the two papers have in common is that climate-related changes to the Earth’s surface, whether human-induced or not, are strong drivers of the changes we see in the Earth’s rotation,” said Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory in Southern California and co-author of both papers.
Early scientists tracked the polar motion by measuring the apparent motion of the stars. Very Long Baseline InterferometryAnalyzing radio signals from quasars, or Satellite Laser RangingThis is a device that shines a laser on a satellite.
Researchers have long speculated that polar motion is caused by a combination of processes within the Earth and at its surface, but it has been less clear how much each process shifts the axis and what their effect is (whether it is a cyclical motion that repeats over weeks to decades, or a sustained drift over centuries to millennia).
In their paper, the researchers used machine learning algorithms to analyze 120 years of records and found that changes in groundwater, ice sheets, glaciers, and sea levels could explain 90% of the repeated fluctuations between 1900 and 2018. The rest was mostly due to dynamics within the Earth, including the wobble caused by the tilt of the inner core relative to the bulk of the planet.
The pattern of polar motion associated with surface mass transfer repeated several times during the 20th century, roughly every 25 years, leading researchers to suggest that it was primarily due to natural climate variability. Previous papers have linked more recent polar motion to human activity. Includes Adhikari’s works The study concluded that the sudden eastward shift of the axis (starting around 2000) was caused by the rapid melting of the Greenland and Antarctic ice sheets and the depletion of groundwater in Eurasia.
The study focused on the past two decades and found that losses in groundwater and ice mass, as well as sea-level rise over this period – all measured by satellite – are strongly linked to human-induced climate change.
“It’s true to some extent that human activities influence the polar motion,” said Mostafa Kiani Shavandi, lead author of both papers and a doctoral student at the Swiss Federal Institute of Technology in Zurich, “but there are natural modes in the climate system that have a major influence on the polar oscillations.”
In the second paper, the authors: Grace Mission (short for Gravity Recovery and Climate Experiment) and subsequent Grace FourThey also drew on previous mass balance studies that analysed the impact of 20th century changes in groundwater, ice sheets and glaciers on sea level rise, to reconstruct changes in day length due to these factors from 1900 to 2018.
Scientists know from historical records of solar eclipses that the length of day has been increasing for thousands of years. Although nearly imperceptible to humans, the delay explained That’s because many modern technologies, including GPS, rely on accurate time measurement.
In recent years, the melting of ice sheets has accelerated, shifting mass from the poles towards equatorial waters. This flattening causes the Earth to slow down and its days to get longer, similar to an ice skater lowering and spreading their arms to slow down their rotation.
The authors found that the rate of day length increase increased immediately after the year 2000, a change that correlates closely with independent observations of flattening. From 2000 to 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 past 100 years, and ranged from 0.3 to 1.0 milliseconds per century.
The researchers note that under climate scenarios in which emissions are significantly reduced, the lengthening of glacial periods due to changes in ice and groundwater could slow by 2100. (Even if emissions stopped today, previously released gases, especially carbon dioxide, would stick around for decades.)
If emissions continue to rise, the daylight extension due to climate change could reach 2.62 milliseconds per century, surpassing the influence of the Moon’s tidal forces, which continue to extend Earth’s daylight by an average of 2.4 milliseconds per century. This effect, called tidal friction, has been the main cause of Earth’s daylight extension for billions of years.
“In just 100 years, we’re seeing humans dramatically altering the climate system and affecting the very rotation of the Earth,” Adhikari said.
Andrew Wang / Jane J. Lee
Jet Propulsion Laboratory, Pasadena, California
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2024-101
