How Humans Became Earth's Rotional Engineers
Beneath the waves, in the fossilized shells of organisms smaller than a grain of sand, lies evidence of an unprecedented moment in planetary history. Researchers from the University of Vienna and ETH Zurich have found that human-driven climate change is slowing Earth's spin — lengthening our days at a rate unmatched in nearly four million years.
The number is tiny: just 1.33 milliseconds per century. But add it up across geological time, and this represents a fundamental reshaping of our planet's motion — one that the researchers say will soon surpass even the Moon's gravitational influence on our length of day.
The Mechanism
The process is elegantly simple. Melting glaciers and polar ice sheets transfer mass from the poles toward the equator, causing Earth to bulge slightly at its middle. This redistribution acts like a a figure skater spreading their arms — the body slows without shedding angular momentum.
The researchers described it this way: the planetary figure skater has raised her arms, and the sea has risen in response.
Reading Ancient Climates
To reconstruct how dramatically this effect has accelerated, the team turned to foraminifera — single-celled marine organisms that have built calcium carbonate shells from seawater for more than 500 million years.
These tiny architects left behind a chemical record wherever oceans existed, making their fossils natural climate trackers. The scientists could infer sea-level fluctuations from foraminifera fossil composition and mathematically derive corresponding changes in day length.
The researchers paired this fossil data with a physics-informed deep learning model designed to handle the enormous uncertainties inherent in paleoclimate records. The algorithm accounts for how mass redistribution changes Earth's oblateness — how much our planet bulges at the equator compared to the poles — and translates those shifts into precise calculations of ancient day length.
Unprecedented in Geological Time
The current day-lengthening effect hasn't occurred at this speed in roughly 3.6 million years, save for brief, violent climate swings when ice sheets rapidly grew or collapsed.
The scientists noted that the rate of modern climate change has been unprecedented at least since the late Pliocene, and the current rapid rise in day length can be attributed primarily to human influences.
The trend shows no sign of plateauing. Under more pessimistic projections, day length could stretch by 2.62 milliseconds per century by the century's final decades — a rate that would eclipse the Moon's own braking effect on Earth's rotation. Only once before, around two million years ago, did a comparable shift occur, and it never sustained itself the way the current trajectory threatens to.
Why It Matters
On its own, a couple of milliseconds seems trivial against the 86,400 seconds that already make up a day. But precision technologies don't tolerate imprecision. The cumulative drift disrupts satellite communications, navigation systems, and the coordination between atomic clocks that modern infrastructure depends on.
There's a grim irony embedded in the finding. To reshape the rotational mechanics of an entire planet is, from a purely physical standpoint, a remarkable feat. That it happens unintentionally — as an unwanted side effect of burning fossil fuels — offers little comfort.
Among the consequences of a warming world, a slightly longer workday ranks somewhere near the bottom. But it joins a list that grows longer with each passing year.
Based on: Slowdown of Earth's Rotation Linked to Anthropogenic Climate Change; University of Vienna and ETH Zurich; Journal of Geophysical Research: Solid Earth.