Researchers used more than 120 years of data to determine how melting glaciers, shrinking groundwater and rising seas are changing the planet’s orbit and lengthening days.
Earth Days is getting a little longer, and that change is increasing. The reason is related to the same processes that caused the planet’s crust to move about 10 meters in the last 120 years. The findings come from two recent NASA-funded studies that focus on how the climate-related redistribution of ice and water has affected Earth’s circulation.
This redistribution occurs when glaciers melt more than they grow when snow falls and when aquifers lose more groundwater than rainfall returns. These changes in mass cause the planet to wobble as it spins and its axis shifts in space – a phenomenon called polar motion. They also cause the Earth’s rotation to slow down, measured by the lengthening of the sun. Both are dated from 1900.
Analyzing the polar movement over 12 decades, scientists say that the almost periodic movement in the axis is caused by changes in the ground water, ice, ice and sea water. According to a paper recently published in Nature Geoscience, the large variability in the 20th century is mostly due to natural climate cycles.
The same researchers came together in a follow-up study looking at the length of the day. They found that, since 2000, the days have been increasing by about 1.33 milliseconds per 100 years, a faster rate than at any time in the last century. The cause: rapid melting of the Antarctic and Greenland ice sheets due to human-caused emissions. Their results were published July 15 in the Proceedings of the National Academy of Sciences.
“The common thread between the two papers is that changes in Earth’s climate, whether caused by humans or not, are driving the changes we see in the planet’s rotation,” said Surendra. Adhikari, co-author. both papers and a physicist at NASA’s Jet Propulsion Laboratory in Southern California.
In ancient times, scientists tracked the motion of the earth by measuring the apparent motion of the stars. Later they switched to very long baseline interferometry, which analyzes radio signals from quasars, or satellite laser range, which points lasers at satellites.
Researchers have long speculated that the polar motion is caused by a combination of processes within the Earth’s surface. It is not clear how much each movement changes the axis and what kind of effect each has – whether it is repetitive circular movements from weeks to to decades, or a constant drift over centuries or millennia.
For their paper, the researchers used machine learning techniques to analyze the 120-year record. They found that 90% of the changes in frequency between 1900 and 2018 can be explained by changes in groundwater levels. earth, ice, glaciers and sea level. The rest is due to conditions inside the Earth, such as motion from the tilt of the interior relative to the planet’s mass.
Patterns of polar motion associated with changes in the climate have repeated several times about every 25 years in the 20th century, suggesting to researchers that it is mainly caused by natural weather conditions. Previous papers have made connections between recent polar movement and human activity, including one by Adhikari that linked the sudden eastward drift of the axis (starting in 2000) to the rapid melting of the Greenland and Antarctic ice sheets and the depletion of groundwater in Eurasia.
The research focused on the past two decades, during which the loss of groundwater and ice as well as sea level rise – all measured by satellites – had strong links with climate change. caused by humans.
“It is true to some extent” that human activities contribute to polar movement, said Mostafa Kiani Shahvandi, lead author of both papers and a doctoral student at the Swiss university ETH Zurich. But there are natural processes in the climate system that have a major impact on the polar oscillation.
For the second paper, the authors used satellite observations of mass change from the GRACE (short for Gravity Recovery and Climate Experiment) mission and its follow-up to GRACE-FO, as well as Previous weighted studies that analyzed the contributions of changes in groundwater, ice and glaciers to sea level in the 20th century reconstructed changes in day length due to those factors in from 1900 to 2018.
Scientists have known from historical records of solar eclipses that the length of the sun has been increasing for thousands of years. Although it is almost invisible to humans, the time must be recorded because many modern systems, including GPS, rely on accurate timekeeping.
In recent decades, the rapid melting of ice has moved mass from the poles towards the equatorial ocean. This flattening causes the Earth to shrink and the sun to lengthen, much like an ice skater going down and stretching their arms to slow down the spin.
The authors noted a rise after 2000 in how long the sun is getting, a change that is closely related to the subjective perception of flatness. From 2000 to 2018, the rate of increase in the length of the sun due to movement of ice and groundwater was 1.33 milliseconds per century – faster than at any time in the past 100 years ago, when it varied from 0.3 to 1.0 milliseconds each. a century.
Glacial extension and changes in groundwater levels could slow by 2100 under a climate of greatly reduced temperatures, the researchers note. (Even if emissions were to stop today, the gases released earlier — especially carbon dioxide — would last decades longer.)
If emissions continue to rise, the lengthening of the sun from climate change could reach as high as 2.62 milliseconds per century, more than the effect of the Moon’s drag on the tides, which is increasing the length of the Earth’s day by 2.4 milliseconds per cent, on average. Called the lunar collision, the effect has been the main cause of the increase in the length of the sun on Earth for billions of years.
“In just about 100 years, humans have changed the climate to such an extent that we are seeing an impact on the way the planet rotates,” Adhikari said.
Andrew Wang / Jane J. Lee
Jet Propulsion Laboratory, Pasadena, Calif.
626-379-6874 / 818-354-0307
andrew.wang@jpl.nasa.gov / jane.j.lee@jpl.nasa.gov
2024-101
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