Breakdown: Why Earth’s rotation is slowing down & its impacts

MEMPHIS, Tenn. (WMC) - Ever since its formation around 4.5 billion years ago (Neoproterozoic days), Earth’s rotation has been gradually slowing down. It’s a process that continues to this day, and estimates suggest that the length of a day currently increases by about 1.8 milliseconds every century.

But why is the Earth’s spin slowing down? It’s because the Moon exerts a gravitational pull on the planet, which causes a rotational deceleration since the Moon is gradually pulling away.

For billions of years, the moon has been ever so gently tugging at the Earth and slowing down its rotation. The moon’s gravity is the reason those Neoproterozoic days were shorter than ours today; it is also why the days millions of years from now will be longer still.

The mechanism boils down to an exchange of energy between the Earth and moon. The moon’s gravitational pull creates a slight bump in the solid surface of the Earth, near to, but not exactly underneath where the moon is. The disparity between the bump’s position and the moon’s pull creates a torque on both the Earth and moon with the end result that the Earth slows down gradually. That rotational energy is transferred to the moon, which is moving away from the Earth ever so slowly, at a rate of about an inch and a half every year.

It’s a process that has been going on ever since the moon began circling the Earth. Some studies have attempted to look even further back in time, and one group of researchers estimates that 1.4 billion years ago a day was just 18.7 hours. At that time, the moon was likely some 27,000 miles closer to Earth than it is now, they say.

While Earth’s slowdown isn’t noticeable on human timescales, it’s enough to create significant changes over thousands and thousands of years.

One of those changes, new research suggests, is perhaps the most significant of all, at least to us: lengthening days have now been linked to the oxygenation of Earth’s atmosphere.

Roughly 2.4 billions years ago, there was the something known as the Great Oxidation Event: when cyanobacteria (Blue-green algae) emerged in such great quantities that Earth’s atmosphere experienced a sharp, significant rise in oxygen because Earth’s days grew longer.

Without this oxidation, scientists think life as we know it could not have emerged; so, although cyanobacteria may cop a bit of side-eye today, the fact is we probably wouldn’t be here without them.

These results were incorporated into global models of oxygen levels, and the team found that lengthening days were linked to the increase in Earth’s oxygen - not just the Great Oxidation Event, but another, second atmospheric oxygenation called the Neoproterozoic Oxygenation Event around 550 to 800 million years ago.

Both these Oxidation events brought atmospheric oxygen up to the present-day level of about 21%. The researchers say that Earth’s atmosphere will maintain high levels of oxygen as the moon continues to move further away, causing the slowing of Earth’s rotation — resulting, of course, in more oxygen over time.

The results of this study have been published in the journal Nature Geoscience and can be accessed here.

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