In a new study published in Nature Geoscience, a team of researchers have uncovered the causes of Earth's spin axis motion.
Published in Earth & Environment and Computational Sciences
Jul 12, 2024
Mostafa Kiani Shahvandi
PhD student, ETH Zurich
Read the paper
Contributions of core, mantle and climatological processes to Earth’s polar motion - Nature Geoscience
Core processes, dynamically linked to mantle and climate-related surface processes, contribute to both the long-term trend and shorter-term fluctuations observed in Earth’s polar motion, according to predictions from physics-informed neural networks.
It has long been known that the Earth's spin axis shows movement relative to the crust, which is commonly referred to as polar motion (see Figure 1 below).
The causes of polar motion have not yet been known precisely, but they are rooted in the exchange of angular momentum between different components of the Earth system. Observations of polar motion since 1900 exhibit various signals, which can be categorized in four different components: (1) the annual wobble that is thought to be mainly caused by atmospheric forcing, (2) the Chandler wobble, which is a natural mode of oscillation with period of around 14 months and believed to be caused by a combination of atmospheric and oceanic processes, (3) the decadal oscillations, and (4) the secular trend. In this paper, the focus has been on the decadal oscillations and the secular trend, which could be referred to as long-period polar motion. The understanding of these components is important because they provide constraints on many geophysical processes that happen on decadal and longer time scales.
Figure 1. The long-period polar motion observations in the range 1900-2018. By definition, polar motion represents a two-dimensional motion, with coordinates denoted as and , which are positive towards central Greenwich meridian and W longitude. The figure is taken from the recent paper https://www.nature.com/articles/s41561-024-01478-2.
In this new study, considerable efforts have been put into analyzing the influence of all the geophysical processes on polar motion. The idea behind the paper is to incorporate all the processes in a unified modelling framework so as to disentangle the contribution of each individual process and take into account the possible feedback between processes. The processes considered include: (1) core processes, which include the effect of torque and pressure at core-mantle and inner-core boundaries, as well as the tilt of the oblate inner core figure, (2) mantle processes, which include the effect of seismic activities, mantle convection, and the rebound of the solid Earth after the termination of the last ice age (a process called glacial isostatic adjustment), and (3) climatological processes, namely the melting of polar ice sheets, global glaciers, and changes in terrestrial water storage, together with the associated rise in sea levels.
The underlying method behind the paper is based on machine learning. Specifically, the so-called physics-informed neural networks (PINNs) have been used. PINNs are powerful mathematical tools that can accurately model and predict a dynamic system. They take advantage of the capabilities of neural networks (including the analysis of linear and nonlinear relationships between different processes) as well as the prior physical information. The result is an algorithm that obeys the physical laws and has exceptional prediction capability. Based on this, the authors of the study have been able to unravel the causes of long-period polar motion (see Figure 2 below).
It has long been known that the Earth's spin axis shows movement relative to the crust, which is commonly referred to as polar motion (see Figure 1 below).
The causes of polar motion have not yet been known precisely, but they are rooted in the exchange of angular momentum between different components of the Earth system. Observations of polar motion since 1900 exhibit various signals, which can be categorized in four different components: (1) the annual wobble that is thought to be mainly caused by atmospheric forcing, (2) the Chandler wobble, which is a natural mode of oscillation with period of around 14 months and believed to be caused by a combination of atmospheric and oceanic processes, (3) the decadal oscillations, and (4) the secular trend. In this paper, the focus has been on the decadal oscillations and the secular trend, which could be referred to as long-period polar motion. The understanding of these components is important because they provide constraints on many geophysical processes that happen on decadal and longer time scales.
Figure 1. The long-period polar motion observations in the range 1900-2018. By definition, polar motion represents a two-dimensional motion, with coordinates denoted as and , which are positive towards central Greenwich meridian and W longitude. The figure is taken from the recent paper https://www.nature.com/articles/s41561-024-01478-2.
In this new study, considerable efforts have been put into analyzing the influence of all the geophysical processes on polar motion. The idea behind the paper is to incorporate all the processes in a unified modelling framework so as to disentangle the contribution of each individual process and take into account the possible feedback between processes. The processes considered include: (1) core processes, which include the effect of torque and pressure at core-mantle and inner-core boundaries, as well as the tilt of the oblate inner core figure, (2) mantle processes, which include the effect of seismic activities, mantle convection, and the rebound of the solid Earth after the termination of the last ice age (a process called glacial isostatic adjustment), and (3) climatological processes, namely the melting of polar ice sheets, global glaciers, and changes in terrestrial water storage, together with the associated rise in sea levels.
The underlying method behind the paper is based on machine learning. Specifically, the so-called physics-informed neural networks (PINNs) have been used. PINNs are powerful mathematical tools that can accurately model and predict a dynamic system. They take advantage of the capabilities of neural networks (including the analysis of linear and nonlinear relationships between different processes) as well as the prior physical information. The result is an algorithm that obeys the physical laws and has exceptional prediction capability. Based on this, the authors of the study have been able to unravel the causes of long-period polar motion (see Figure 2 below).
Figure 2. The algorithm used to analyze the long-period polar motion, considering all the geophysical processes. The figure is adapted from the recent paper https://www.nature.com/articles/s41561-024-01478-2.
The new study sheds light upon the contribution of the aforementioned geophysical processes to polar motion. First, the climatological processes are the main cause of the decadal oscillations observed in the polar motion record. The quasi-decadal variations in terrestrial water are likely to be the most important contributor to these oscillations. However, a small part of these oscillations is caused by core processes, which are usually anti-correlated with those from climatological processes and arise from the torque at core-mantle boundary. Second, the secular trend is primarily caused by mantle convection and glacial isostatic adjustment, although a small contribution comes from the core processes. Third, seismic processes contribute only negligibly to long-period polar motion, although their inclusion to the analyses provides improvement to the agreement with the observed polar motion record. These results highlight the climatological processes as the most important contributors to polar motion. The ongoing climate change will have considerable influence on displacing the Earth's spin axis. It may also impact the dynamics of Earth's core, since the results derived by PINNs suggest weak feedback between climatological and core processes, although the exact mechanism of this feedback is not known. Furthermore, the majority of the secular trend is caused by glacial isostatic adjustment, which is a remnant of the last glacial cycle, thus being ultimately related to climate.
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