Melting ice makes Earth heavier to rotate, causing longer days

For ages, human actions had little effect on the Earth’s movements i.e., its rotation speed and axis. 

These motions were primarily governed by the moon’s gravitational pull and internal processes in the core and mantle. 

But now, melting ice sheets caused by human activity are messing with these natural movements. 

A new study reveals that Earth’s spin axis is “shifting” due to climate change and the planet’s internal dynamics. 

Researchers at ETH Zurich have used cutting-edge AI models to understand better polar motion, which is the movement of the Earth’s rotating axis relative to the crust.

The study predicts that if greenhouse gas emissions continue to rise, they will ultimately outweigh the long-standing influence of the moon’s tidal force. For billions of years, the moon has determined the steady lengthening of our days. 

“We humans have a greater impact on our planet than we realize. And this naturally places great responsibility on us for the future of our planet,” said Benedikt Soja, Professor of Space Geodesy at the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich.

Lengthening of days

Two recent studies have examined a surprising link between melting ice caps, the planet’s wobble, and the length of our days.

Ice melt at the poles redistributes mass towards the equator, slowing Earth’s rotation. Water travels from the poles to the equator, disrupting the Earth’s equilibrium. 

As ice melts on the Earth’s surface, it slows the planet’s rotation and increases the day. ETH Zurich research shows that the slowdown slightly lengthens our days—just a few milliseconds compared to the typical 86,400 seconds.

In physics, there’s a law called conservation of angular momentum. It basically says a spinning object likes to keep spinning at the same rate unless something forces it to change. Earth’s rotation follows this rule too. Melting ice redistributes mass, and this shift, according to the law, slows Earth’s spin down.

“This means that a shift in mass is taking place, and this is affecting the Earth’s rotation,” explained Soja.

Impact on Earth’s core

The researchers also studied why and how the Earth’s axis moves gradually over extended periods. The team used physics-informed neural networks to create the most comprehensive model to date, explaining how movements within the Earth’s core, mantle, and climate at the surface all contribute to polar motion.

The two studies highlight the complex web of interactions within Earth. Events on the surface, like melting ice, can have consequences deep within the core, and vice versa.

“Climate change is causing the Earth’s axis of rotation to move, and it appears that the feedback from the conservation of angular momentum is also changing the dynamics of the Earth’s core,” explained Soja. 

As the polar ice melts, the resulting water shifts from the poles toward the equator, causing the Earth to rotate more slowly as it becomes bulkier. Benedikt Soja explained that this phenomenon is similar to a figure skater who spins slower when her arms are extended compared to when they are tucked in. The same principle applies to Earth’s rotation.

“Ongoing climate change could therefore even be affecting processes deep inside the Earth and have a greater reach than previously assumed,” added Kiani Shahvandi, lead author of the study and doctoral student, in the press release.