Earth slows down, but everything stays (almost) the same.

Jan 29, 2023 | Current affairs, Featured, Post, Thursday Daily Bulletin, Tradition

According to a new study, the inner core of our planet has slowed down to the same rotational speed as the outer layers, or even slightly slower. From the surface, we would see it as if it had stopped, or in the opposite direction. This is explained by researchers at the Institute of Geosciences (CSIC-UCM), who point out that these small changes are unlikely to have any appreciable effect.

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Earth slows down

A study has recently been published that states that the Earth’s core has suffered a standstill and that this event has an influence on different aspects of the planet, such as the magnetic field or the climate.

As a result of this publication, some voices have emerged suggesting exaggerated and catastrophic consequences. It is important to analyse this news with a critical eye and, without underestimating the importance of the great impact of scientific discoveries on the functioning of our planet (even with so many mysteries), not to fall into simplicity or dramatism.

Earth’s internal structure
To understand this news, we must first understand the internal structure of our planet. The Earth is made up of different layers. At the centre is the inner core, a solid sphere of iron and nickel with a radius of 1220 km. It is surrounded by a 2260 km thick layer of similar composition, but in a molten state, the outer core.

The first thing to make clear is that the core has not stopped: the Earth with all its layers is rotating in a way that takes about 24 hours for a complete revolution.

Convection movements in this fluid layer, coupled with the Earth’s rotation, generate the magnetic field that protects our planet from particles arriving from the Sun and from space. Surrounding the core is the mantle, which is about 2,900 km thick, and above this is the Earth’s crust, on which we live, which is only 10 to 50 km thick.

Until now, it was thought that the inner core rotated a little faster than the mantle and crust (this is called superrotation), so that it moved forward by about a tenth of a degree each year.

According to this new study, the core would have slowed down to the same rotational speed as the outermost layers or even slightly slower. These relative velocity differences are very small.

The example of the car on the motorway
Let’s think, as an example, that we are driving along the motorway at 120 km/h and are overtaken by another car at 121 km/h. Through the window, you see that it is slowly overtaking you. If the other car now slows down and goes to 120 km/h, we will see it “stationary” next to our car, although it is still moving, just like us.

In the same way, the core would have slowed down and now, rotating at the same speed as the mantle and crust, we would see it from the Earth’s surface as stationary.

The core would have slowed down and now, rotating at the same speed as the mantle and crust, we would see it from the Earth’s surface as stationary.

To reach the conclusion that the core is now rotating more slowly, the researchers selected earthquakes originating in the South Sandwich Islands, in the southern Atlantic Ocean, and studied the signal recorded at an observatory in Alaska, almost on the other side of the planet.

In this way they were able to analyse how long it took for the waves that had passed through the Earth’s core to arrive, always following the same trajectories. They found that the waves took different times to cross the core at different times.

Different areas of the core can have different properties, which means that waves take longer to cross some areas than others, so they concluded that if the travel time of the waves changed over the years, it is because the inner core was getting ahead of the crust.

In other words, if waves are emitted and recorded at the same points on the surface we get different results depending on the time, it means that the waves are crossing different areas of the core, i.e. it is rotating at a different speed than the Earth’s surface.

Since 2009, however, the waves have been taking the same time to cross the core. This means that the core is now stationary with respect to the surface (spinning at the same speed). The same results were obtained when the study was extended to earthquakes generated elsewhere on the planet, supporting the above conclusions.

This slight change in core rotation is not the first time it has happened. The data show another similar event in the 1970s. This suggests that the phenomenon repeats itself with a periodicity of about seven decades.

Association with geophysical phenomena
Interestingly, this same frequency also appears in other geophysical observables, such as the geomagnetic field, day length or weather, suggesting that they may be related.

It is currently thought that this phenomenon of periodic variation of the core rotation is due, on the one hand, to the electromagnetic interaction between the inner and outer core, which tends to accelerate the inner core, and, on the other hand, to the gravitational coupling with the mantle, which forces it back into step.

In short, does the study say that the core stopped dead in 2009 and is going to start spinning in the opposite direction? No, it has only changed its speed relative to the crust.

Throughout history, the magnetic field has already reversed many times, does this slowing down of the core suggest a new imminent pole reversal or will the magnetic field disappear? No, the core is still spinning and the magnetic field will continue to be generated.

Will this phenomenon have implications for the climate? The paper proposes that there may be some relationship, but that the origin of multidecadal variations in climate is not yet fully understood. Furthermore, it seems unlikely that such small changes in the rotation of the core could have any really appreciable effects.

It seems unlikely that such small changes in the rotation of the core could have really appreciable effects.

As we can see, the dynamics of the Earth is a highly complex system and a multitude of interconnected factors determine the characteristics and evolution of our planet.

The long history of the Earth compared to our study of it makes understanding its evolution a great challenge. Discoveries like this are a sign of science’s efforts to understand more and more about how the planet we live on works.