"It's like when a figure skater does a pirouette, first holding her arms close to her body and then stretching them out," Soja says. The initially fast rotation slows down as the masses move away from the axis of rotation, increasing physical inertia. In physics, this is known as the conservation of angular momentum, and it governs the Earth's rotation as well. As the Earth's rotation slows, days become longer, though the change is minimal.
Supported by NASA, researchers from Soja's group at ETH Zurich have published studies in *Nature Geoscience* and *Proceedings of the National Academy of Sciences (PNAS)* on how climate change affects the polar motion and the length of the day.
Climate Change Outweighs Lunar Influence
The PNAS study indicates that climate change is increasing the length of the day by a few milliseconds from its current 86,400 seconds. This occurs as water from the poles moves to lower latitudes, slowing the Earth's rotation.
Another factor causing this slowdown is tidal friction triggered by the moon. However, the study concludes that if greenhouse gas emissions continue to rise and the Earth warms accordingly, this will have a greater impact on the Earth's rotational speed than the moon's influence, which has dominated for billions of years. "We humans have a greater impact on our planet than we realise," Soja concludes, "and this naturally places great responsibility on us for the future of our planet."
Earth's Axis of Rotation is Shifting
Mass shifts on Earth's surface and interior due to melting ice not only alter the rotational speed and length of the day but also shift the axis of rotation. This means that the points where the axis intersects the Earth's surface move, a phenomenon known as polar motion, which can reach up to ten meters over a hundred years. Alongside melting ice sheets, movements within the Earth's mantle and heat flows in the outer core also play a role.
In their most comprehensive modeling to date, Soja and his team have demonstrated how polar motion results from processes in the core, mantle, and climate at the surface. Their study, published in Nature Geoscience, provides a complete explanation for the causes of long-period polar motion.
"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," Soja explains. Kiani Shahvandi, a doctoral student and lead author of the study, adds, "Ongoing climate change could therefore even be affecting processes deep inside the Earth and have a greater reach than previously assumed." However, these effects are minor and unlikely to pose a risk.
Combining Physical Laws with AI
For their study on polar motion, the researchers used physics-informed neural networks, which incorporate the laws and principles of physics into AI algorithms. Kiani Shahvandi received support from Siddhartha Mishra, Professor of Mathematics at ETH Zurich, who received ETH Zurich's Rossler Prize in 2023 for his work in this field.
These advanced algorithms have enabled the recording of all different effects on the Earth's surface, mantle, and core, and their interactions. The results show how the Earth's rotational poles have moved since 1900, aligning well with data from astronomical observations and satellites, enabling future forecasts.
Implications for Space Travel
"Even if the Earth's rotation is changing only slowly, this effect has to be taken into account when navigating in space - for example, when sending a space probe to land on another planet," Soja says. Even a slight deviation on Earth can result in a significant error over long distances, affecting precise landings such as in specific craters on Mars.
Research Report:The increasingly dominant role of climate change on length of day variations
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