Every day, without you noticing, Earth is taking slightly longer to complete its rotation. The shift is only measured in fractions of a millisecond, but the forces required to produce it are almost incomprehensibly large.
And according to a new study, the rate at which our days are lengthening is now “unprecedented” in 3.6 million years of geological history.
The findings show that as polar ice sheets and glaciers melt as a consequence of climate change, water that was once locked up at high latitudes flows into the oceans and spreads toward the equator.
This shifts mass away from Earth’s poles, and – like a figure skater extending their arms mid-spin – slows the rotation.
Previous studies had already shown that climate change was having strange effects on our planet’s spin. Now, the team from the University of Vienna and ETH Zurich have looked back through geological time to ask whether anything like today’s rate of change has happened before.
The answer, unequivocally, is no.
Reading time in ancient shells
The length of a day has never been perfectly fixed. Earth’s rotation is constantly being nudged by the gravitational pull of the Moon, by processes deep within the planet’s interior and by shifts in the atmosphere.
These forces push and pull in different directions, and, over geological time, they’ve produced swings in how long a day lasts. What scientists have now established is that climate change has become a force powerful enough to stand alongside, and, in time, dwarf all of them.
To look back millions of years, the researchers turned to the fossilised remains of a single-celled marine organism that lived on the sea floor, known as benthic foraminifera.
The chemistry of their shells records ancient shifts in sea level. By looking at these shifts, the researchers can calculate how Earth’s rotation must have changed.
A specially developed machine learning algorithm, designed to handle the uncertainties in data this old, allowed them to draw robust conclusions from material stretching back to the Late Pliocene, 3.6 million years ago.
Across that entire sweep of time, one data point stands out: today.
The current rate of climate-driven day lengthening – 1.33 milliseconds per century – sounds trivial. But considering the forces involved, the mass redistribution is gigantic.
“Such a shift in the length of day requires a staggering redistribution of mass: on the order of 1,000 gigatonnes moving from the poles to the oceans,” says Prof Benedikt Soja of ETH Zurich, co-author of the study. “To visualise this amount, imagine a solid cube of ice placed over New York City. It would be 10km high, taller than Mount Everest.”
In terms of the energy involved in creating such a shift, Dr Mostafa Kiani Shahvandi of the University of Vienna, the study’s lead author, puts it like this: “The change in the Earth’s rotational energy is equivalent to a magnitude 9.0 earthquake.” Not in terms of destruction, but sheer planetary-scale force.
Bigger than the Moon by 2100
The researchers did find one moment in the geological record, roughly two million years ago, where the rate of change came close to today’s. But it was exceptional.
“A ’perfect storm’ of fragile ice sheets and a natural CO₂ spike triggered a massive melting of polar ice sheets,” Soja says. “This rare event has not been repeated naturally since, yet human activity is now matching that same planetary-scale force in just over a century.”
So, where do we go from here? Under a high emissions future with continued heavy reliance on fossil fuels, climate change is projected to become the biggest driver of day-length change by the end of the century, outpacing even the gravitational tug of the Moon.
The practical consequences of this are subtle but real. “While a millisecond may seem tiny, this effect is critical for the ultraprecise timing required for GPS navigation on Earth and spacecraft navigation across the solar system,” Soja says.
Moreover, the planetary-scale shifts we’re inflicting on the Earth’s spin are reflective of the sweeping impacts we’re having on Earth’s ecosystem. The profound mass redistribution goes hand in hand with more extreme weather events and rising sea levels, both of which are set to have vast impacts on where people can safely live in the future.
“The most important takeaway is that human influence on the Earth system has become so profound that we are now changing the very way our Earth spins,” Soja says.
As for what comes next, the team is now turning their attention to other ways human activity is shifting mass around the planet, particularly groundwater depletion and changes in the water cycle driven by climate change.
Early calculations, Soja says, suggest these effects are considerably smaller than those from melting ice and glaciers, but understanding them in full will sharpen the picture of exactly how, and how fast, we are altering Earth’s rotation.
Read more:
