Melting polar ice caps due to climate change are redistributing Earth’s mass, slowing its rotation and slightly lengthening the day, suggesting a greater human influence on Earth’s rotation dynamics than previously thought, according to a NASA-funded ETH Zurich study.

Climate change is causing ice in Greenland and Antarctica to melt. As a result, water from these polar regions flows into the world’s oceans, especially in the equatorial region. “This means there is a change in mass, and this affects the Earth’s rotation,” explains Benedikt Soja, Professor of Space Geodesy at the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich.

“It’s like a skater spinning around by first keeping their arms close to their body and then extending them,” Soya says. The initial, fast rotation slows down as the masses move away from the axis of rotation, and physical inertia increases. In physics, we talk about the law of conservation of angular momentum, and the same law governs the rotation of the Earth. If the Earth spins more slowly, the days get longer. So climate change is also changing the length of the day on Earth, albeit minimally.

With the support of the American space agency NASA, ETH researchers from the Soja group published two new studies in the journals Nature Geology And Proceedings of the National Academy of Sciences (PNAS) explained how climate change affects polar motion and day length.

Climate change surpasses the moon’s influence

In the PNAS study, ETH Zurich researchers show that climate change has also increased the length of the day by a few milliseconds from the current 86,400 seconds. This is because water is flowing from the poles to lower latitudes and thus slowing down the rotation rate.

Another reason for this slowdown is the tidal friction caused by the Moon. However, a new study has reached a surprising conclusion: if humans continue to emit more greenhouse gases and the Earth warms accordingly, this will eventually have a greater effect on the speed of the Earth’s rotation than the Moon’s effect, which determines the increase in the length of the day for billions of years. Soja concludes: “We humans have a greater impact on our planet than we realize, and this naturally gives us a great responsibility for the future of our planet.”

The Earth’s axis of rotation is changing

However, the mass changes occurring on the Earth’s surface and in its interior due to melting ice do not only change the speed of the Earth’s rotation and the length of the day: as researchers have shown Nature Geology they also change the axis of rotation. This means that the points where the axis of rotation actually meets the Earth’s surface are moving. Researchers can observe this polar movement over a longer period of time, reaching about ten meters per hundred years. It is not just the melting of ice sheets that plays a role here, but also movements in the bowels of the Earth. Displacements occur over long periods deep in the Earth’s mantle, where the rock becomes viscous due to high pressure. There are also heat flows in the liquid metal in the Earth’s outer core, which are responsible for both the creation of the Earth’s magnetic field and the mass displacement.

In the most comprehensive simulation to date, Soja and his team showed how polar motion is driven by separate processes in the core, mantle and surface climate. Their research was recently published in the journal Nature Geology“For the first time, we provide a complete explanation for the long-term movement of the poles,” says Mostafa Kiani Shahwandi, one of Soja’s postdoctoral researchers and lead author of the study. “In other words, we now know why and how the Earth’s rotation axis moves relative to the Earth’s crust.”

One result in particular stands out in their work: Nature Geology : The processes within and on the Earth are interconnected and affect each other. Soya explains: “Climate change is causing the Earth’s rotation axis to shift, and feedback from conservation of angular momentum also appears to be changing the dynamics of the Earth’s core.” Kiani Shahwandi adds: “Continuing climate change could even affect processes deep within the Earth and have a greater impact than previously thought.” However, these effects are small and unlikely to pose a risk, so they are not a cause for concern.

Physical laws combined with artificial intelligence

For their study of the motion of the poles, the researchers used so-called neural networks that incorporate physical knowledge. These are cutting-edge artificial intelligence (AI) methods in which researchers apply the laws and principles of physics to develop particularly powerful and reliable machine learning algorithms. Kiani Shahwandi received support from ETH Zurich Professor of Mathematics Siddhartha Mishra, an expert in the field who received the 2023 Rössler Prize from ETH Zurich, the university’s highest research award.

The algorithms developed by Kiani Shahwandi made it possible for the first time to record all the different effects on the Earth’s surface, mantle and core, and to model their possible interactions. The result of the calculations shows how the poles of the Earth’s rotation have changed since 1900. These model values ​​​​are in excellent agreement with past astronomical observations and real data provided by satellites over the last three decades, which means that they also allow predictions for the future.

Important for space travel

“Even though the Earth’s rotation changes slowly, this effect has to be taken into account when navigating through space, for example when sending a space probe to another planet,” Soja says. Even a slight deviation of just one centimeter on Earth can grow to hundreds of meters over very large distances. “Otherwise, landing in a specific crater on Mars would be impossible,” he says.