Earth’s Rotation Axis Shift: Ice Melt and the Figure-Skater Effect

Earth spins around an imaginary line called its axis. People often think this line is fixed. It is not. The spot where the axis meets the surface moves a little every year. Scientists call this polar motion.

1.1 How far has the axis moved?

NASA’s Jet Propulsion Laboratory (JPL) reports that the spin axis moved about 10 metres between 1900 and 2023. That is about 30 feet. The movement is tiny next to Earth’s size. It still matters for precise timekeeping and navigation.

Think of a spinning top. Its tip wobbles a little while it spins. Earth’s axis wobbles in a similar way, but far more slowly.

How do scientists measure this? Early workers tracked how stars seemed to move. Later they used radio signals from distant quasars. They also used lasers aimed at satellites.

1.2 What are the different wobbles?

Polar motion has four parts, according to the authors of a 2024 study:

• Annual wobble: mainly caused by the atmosphere.
• Chandler wobble: a natural wobble of about 14 months. The authors link it to the atmosphere and oceans.
• Decadal swings: slower shifts over ten years or more.
• Long-term trend: a steady drift over many decades.

Common mistake: the Chandler wobble is not caused by melting ice. It is a natural cycle.

1.3 What is the difference between the spin axis and the geographic poles?

The geographic axis is the imaginary line between the geographic North and South poles. It is the line we draw on maps. The spin axis is the line Earth actually spins around.

The two do not sit in exactly the same place. JPL’s animation shows the spin axis drifting away from the geographic axis. That gap is what scientists call polar motion.

Think of a wheel with a lump of clay stuck on one side. The wheel no longer spins around its drawn centre. The spin settles around a slightly different line. Move the clay, and the line moves too. (This analogy is ours, not JPL’s.)

1.4 How small is a milliarcsecond?

Scientists measure the pole’s position in milliarcseconds, or mas. A milliarcsecond is a tiny angle. On Earth’s surface it is about 3 centimetres. That figure is our calculation, using an Earth radius of about 6,371 km.

The Nature Geoscience paper reports two numbers. The long-term trend is about 3 mas a year. The swings on top of it are 20 to 40 mas.

Now check the sum. Three mas a year is about 9 cm a year. Over 120 years that is about 11 metres. This matches JPL’s figure of about 10 metres. The sum is our own check.

2.1 What did the 2024 Nature Geoscience study find?

A team studied 120 years of polar motion data. It used neural networks trained on the measurements. The main findings were:

• The long-term trend: it comes mainly from glacial isostatic adjustment and mantle convection. Glacial adjustment means the land is still rising after the last ice age.
• The shorter swings: surface changes explain about 90% of the yearly and multi-decade swings. These changes are melting ice and shifts in water storage.
• The core: Earth’s liquid core also adds to both the trend and the swings.

What does “mass redistribution” mean? It means water and ice moving from one place on Earth to another. That changes where Earth’s weight sits.

JPL adds one point about the 20th century. The surface patterns repeated a few times about every 25 years. The researchers think they were mostly natural climate cycles.

2.2 Did the drift change direction?

Yes. An AGU press release on a 2021 study explains it. In 1995, the direction of the pole’s drift shifted from southward to eastward. The average speed from 1995 to 2020 was about 17 times the speed from 1981 to 1995.

The study blamed the loss of water stored on land. Most of that water was ice from glaciers. AGU says the melt in Alaska, Greenland, the Southern Andes, Antarctica, the Caucasus and the Middle East sped up in the mid-1990s.

JPL notes a separate paper. It linked an eastward drift from around the year 2000 to faster melting of the Greenland and Antarctic ice sheets. It also named groundwater loss in Eurasia.

Common mistake: the axis did not “tip over”. It moved by metres, not by degrees of tilt.

2.3 What are the three groups of causes?

The authors of the 2024 study sorted the causes into three groups. They traced each one inside one model.

• Core processes: torque and pressure at the boundary between the core and the mantle. Also the tilt of the inner core.
• Mantle processes: earthquakes, slow mantle convection, and the rebound of the solid Earth after the last ice age. That rebound is called glacial isostatic adjustment.
• Climate processes: melting polar ice sheets, melting glaciers, changes in water stored on land, and the resulting rise in sea level.

Why does the core matter? The authors say the causes are rooted in the exchange of angular momentum between different parts of Earth. Angular momentum is the “amount of spin”. If one part gains some, another part must lose some.

2.4 How do groundwater, glaciers and sea level move the axis?

JPL explains the surface part in simple steps. Ice sheets and glaciers sometimes melt more than they grow from snowfall. Aquifers sometimes lose more groundwater than rain refills. Both cases move mass from one place to another.

These shifts in mass make Earth wobble as it spins. They also slow the spin a little. Scientists have recorded both effects since 1900.

• Share explained: about 90% of the recurring fluctuations between 1900 and 2018 came from groundwater, ice sheets, glaciers and sea level.
• The rest: it mostly came from Earth’s interior. One example is the wobble from the tilt of the inner core.
• Groundwater: a separate paper linked an eastward drift to groundwater loss in Eurasia.

Is it all caused by humans? No. The lead author, Mostafa Kiani Shahvandi, says human activity plays a part. He adds that natural modes in the climate system have the main effect on the oscillations.

3.1 How does a skater explain the spin?

A skater spins fast with arms pulled in. The spin slows when the arms spread out. The mass moves outward, so the spin slows.

JPL uses the same idea for Earth. Faster melting of ice sheets moves mass from the poles toward the equatorial ocean. This flattens Earth a little. The spin slows and the day gets longer.

Why the equator? Meltwater flows into the oceans. JPL says the mass shifts from the poles toward the equatorial ocean.

3.2 How much longer is the day getting?

JPL reports a study published on 15 July 2024 in PNAS. For 2000 to 2018, ice and groundwater changes made the day longer by 1.33 milliseconds per century. That is faster than at any time in the previous 100 years, when it ranged from 0.3 to 1.0.

The Moon matters more for now. Lunar tidal friction adds about 2.4 milliseconds per century. JPL says the climate effect could reach 2.62 if emissions keep rising.

3.3 What does this mean for clocks and leap seconds?

Duncan Agnew published a paper in Nature on 27 March 2024. He found that faster ice melt in Greenland and Antarctica has slowed Earth’s spin more quickly.

His forecast says Coordinated Universal Time (UTC) will need a negative leap second by 2029. Without the recent melt, the need would come 3 years earlier.

So far, every leap second added to UTC has been positive. A negative one would be the first.

3.4 Why does a few milliseconds matter?

One millisecond is one-thousandth of a second. You cannot feel it. JPL still says the lag must be counted. Many modern technologies, including GPS, rely on precise timekeeping.

The day has been growing longer for a very long time. Historical eclipse records show it has lengthened for thousands of years. The main reason is the lunar tidal friction from the Moon’s pull on the tides. JPL says it has been the main cause for billions of years.

The newer climate effect is small but growing. JPL says it could slow down by 2100 if emissions fall sharply. If emissions keep rising, it could pass the Moon’s effect.

4.0 What tools track the axis?

JPL lists three generations of tools:

• Stars: the earliest scientists tracked how stars seemed to move.
• Radio signals: later they used very long baseline interferometry. It analyses radio signals from distant quasars.
• Lasers: they also used satellite laser ranging, which points lasers at satellites.

These tools find where the axis is. A different set of tools finds why it moves. Those tools weigh ice and water from space. GRACE and GRACE-FO are the best-known examples.

4.1 How does GRACE-FO weigh the ice?

GRACE was a pair of NASA satellites that flew from 2002 to 2017. GRACE-FO launched on 22 May 2018 and continues the work. The two satellites fly about 220 km apart. They send microwave signals to measure the gap.

A region with more mass pulls the lead satellite first. The gap then changes. Scientists turn these changes into maps of ice, groundwater and sea level.

Two more tools keep the numbers clean. An accelerometer on each satellite measures forces that are not gravity, such as air drag. GPS receivers fix each satellite’s position to a centimetre or less. NASA says the data become monthly maps of Earth’s gravity field.

4.2 Do solar flares move the axis?

No source opened says they do. NOAA lists what flares and CMEs affect.

• Solar flares: X-rays and ultraviolet light ionise the ionosphere. This causes radio blackouts on the sunlit side.
• CMEs (coronal mass ejections): they can cause geomagnetic storms.

None of these effects is a shift of Earth’s spin axis. The axis depends on how Earth’s own mass is spread. That part is our reasoning, not a quote from NOAA.

Common mistake: solar activity changes radio and space weather. Melting ice and water storage change polar motion.

4.3 What should you remember?

• Polar motion: the spin axis moved about 10 metres from 1900 to 2023.
• Causes: mainly rebound and mantle flow for the trend, and surface water and ice for the swings.
• Direction change: in 1995 the drift turned eastward.
• Day length: up by 1.33 ms per century (2000 to 2018).
• Leap second: a negative one is forecast by 2029 (Agnew, 2024).
• Flares: they affect radio and space weather, not the axis.