Human activities slowed down Earth’s rotation

Earth’s rotation has slowed down because of human activity. This could have been caused by the melting of polar ice due to human-induced global warming and subsequent movement of water towards the equator, a paper published in the journal Nature on March 27, 2024 showed.

Due to the movement of water from the pole to the equator, Earth has become slightly less spherical and more flattened. As a result, the moment of inertia of Earth, which is a measure of how spread its mass is, has increased. 

The angular momentum of a spinning body, which is the product of its moment of inertia and angular velocity, is conserved unless acted upon by an external torque. Earth’s angular velocity, which is how fast it is spinning, must therefore decrease if the moment of inertia increases. This is the same natural law due to which when an ice skater stretches out their arms while spinning, their spinning velocity decreases.

Normally, Earth’s rotational speed decreases due to the gravitational pull of the moon over millions of years. The Earth was spinning much faster earlier. Analysis of sediments suggested that 1.4 billion years ago Earth was completing a full rotation around its axis in only 19 hours as opposed to the 24 hours that it takes today. Even around 370 million years ago a day was only 22 hours long. A leap second is added from time to time to the Coordinated Universal Time to account for this slowing down. The last time a leap second was added, was on December 31, 2016.

However, from 1970 onwards Earth’s rotational speed has been increasing due to some movement of fluids in its outer core. This increase in speed is superseding the slowing down of Earth’s rotational speed due to the Moon’s gravity. To account for that, it was calculated that a leap second was to be subtracted in either 2025 or 2026.

But human-induced slowing down has postponed this subtraction. The article in Nature showed that now a leap second has to be subtracted later, probably, in 2028 or 2029.

The addition or subtraction of a leap second can be problematic for telecommunications and computing. However, it doesn’t significantly impact our daily lives otherwise.

The major concern is how our actions are impacting significant changes in the movement of our planet. Last year, a paper published by the Seoul National University showed that pumping out water by human beings from aquifers and its redistribution to the oceans resulted in a drift of Earth’s rotational pole. It is becoming clear that our actions may have more far-reaching consequences than we have previously realised.

Dibyendu Chaudhuri is associated with Professional Assistance for Development Action (PRADAN). Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth.

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