Human Activity Is Actually Making Earth’s Days Longer

We call a day 24 hours because it’s a clean, reliable number. But Earth isn’t a perfect clock.

When our planet formed 4.6 billion years ago, a day lasted a mere six hours. By the time the dinosaurs took over, it had stretched to 23. Usually, this slowdown is the Moon’s fault, with its gravity acting like a long-term brake on our rotation. But new research suggests a new player is making an impact: us.

By studying foraminifera, tiny single-celled marine organisms that leave behind fossil shells, a team of researchers from the University of Vienna found that climate change is affecting the length of the days. The effect is tiny (around 1.3 milliseconds per century), but significant. And with man-made climate change, it’s getting even bigger.

The Earth As a Figure Skater

To understand why the planet is slowing down, think of a figure skater mid-spin. When she tucks her arms in, she spins fast. When she extends them, the rotation slows down. This is called the conservation of angular momentum.

This law states that if the way the mass is distributed changes, the spin rate will also adjust to compensate. The skater with arms pulled in has more mass closer to the rotation axis, which means faster spinning. If the Earth could magically extend its arms outwards, it would spin slower.

Earth works the same way. When mass stays locked in heavy ice sheets at the poles, it’s like the skater’s arms are tucked in. But as we melt that ice, the water flows toward the equator, redistributing mass further from the planet’s axis. It’s as if the Earth is extending its arms, and the result is a measurable slowdown in its spin.

But how do you measure this? To find out, the researchers had to go back to the Pliocene, a time about 3.6 million years ago when the world was a very different place.

Tiny Creatures

Tiny, single-celled organisms called benthic foraminifera have been living and dying in the sea for eons. When they die, they leave behind shells. Those shells are like chemical diary entries. They record the temperature and the volume of the sea at the moment the organism lived. By analyzing these fossils, scientists can reconstruct exactly how much water was in the ocean and how much was locked up in ice.

But these records are incomplete and messy. Data from three million years ago is noisy and inconsistent. To clean it up, the team built a digital brain. They called it a Physics-Informed Diffusion Model (PIDM).

In essence, this PIDM is an AI that has been taught the laws of physics and has been given information on geological processes. By feeding the fossil data into this model, they could reconstruct the Earth’s rotation history with a precision that was previously impossible.

That matters because many machine-learning systems are talented mimics. They can fit data beautifully while drifting away from the real rules governing the world. A physics-informed system is an attempt to keep the algorithm honest.

What the researchers found was staggering.

Yet More Proof of Climate Change

During the Quaternary ice ages that started 2.6 million years ago, the Earth’s spin changed quite a lot. Ice grew, ice melted. The day got longer, then shorter.

But the headline result lies at the sharp end of the graph, near the present.

Human-caused climate change is now lengthening the day by slowing Earth’s rotation. The mechanism is sea-level rise: as warming melts polar ice sheets and mountain glaciers, water moves from land into the oceans, redistributing mass farther from Earth’s spin axis. The recent rate cited in the material is about 1.33 milliseconds per century.

This appears to be unprecedented in the past 3.6 million years. The study looks at many past climate changes, most of them natural, but concludes that today’s rapid rise in day length stands out and is primarily attributable to human influence.

But Does This Matter?

The result matters on three levels: as a signal of climate disruption, as a test of planetary physics, and as a practical issue for precision technology.

Start with climate. We think of sea-level rise in human terms, including problems like floods, storm surges, saltwater intrusion, and so on. Those are the stakes that matter to us. But sea-level rise is also a planetary rearrangement of mass. We are literally changing how the whole planet spins. That gives the public a fresh way to grasp the scale of the disturbance.

Next, the result matters because it ties together systems that are often discussed separately. Climate scientists track ice loss, while oceanographers track sea level and paleoclimatologists decode fossil proxies. This work braids those threads into one story. It says, in effect, that the cryosphere, ocean, and rotating solid Earth are parts of a single machine, and offers a framework to look at how things are interconnected.

Then there is the practical side. Milliseconds don’t matter when you’re setting up a date, but they do matter in systems that must know Earth’s orientation exactly. Satellite navigation, space missions, Earth-observing satellites, geodetic reference frames, and some aspects of telecommunications all depend on precise timing and precise knowledge of the planet’s spin.

This creates a massive headache for GPS. Your phone knows where you are because it communicates with satellites. Those satellites are moving at incredible speeds. To tell you that you need to turn left in 50 feet, the system has to know exactly where the satellite is and exactly where the Earth is underneath it. If the Earth’s rotation slows down, the ground moves a few inches or feet away from where the satellite thinks it should be.

Ultimately, there’s something disturbing in this study . Human beings have burned enough fossil carbon, melted enough land ice, and raised enough sea water to nudge the rotation of an entire planet. Not by much, but measurably, and uniquely across millions of years.

We are, ever so slightly, rewriting the rhythm of the planet.

The study was published in JGR Solid Earth.