Earth's Spin Slows as Ice Melts

When geoscientist Mostafa Kiani Shahvandi needed to explain how melting ice caps affect Earth's spin, he reached for the figure skating analogy. A skater pulls in their arms to spin faster, extends them to slow down. Now picture Earth as that skater, with water from melting polar ice flowing toward the equator—arms stretching outward, rotation slowing, days growing longer.

The numbers tell a story that spans millions of years. Days are lengthening by 1.33 milliseconds per century because of climate change. That might sound trivial until you realize this rate hasn't occurred in the past 3.6 million years of Earth's history. Only once, around 2 million years ago, did the planet experience anything close to this pace of change. "Never before or after that has the planetary 'figure skater' raised her arms and sea-levels so quickly as in 2000 to 2020," Kiani Shahvandi said.

Reading Earth's Ancient Clocks

To understand how unusual our current moment is, Kiani Shahvandi and senior author Benedikt Soja from ETH Zurich turned to an unlikely source: the fossilized shells of benthic foraminifera. These single-celled marine organisms lived on ocean floors for millions of years, and their chemical composition reveals how sea levels changed over time.

The team fed this fossil data into a probabilistic deep-learning algorithm that modeled the physics of sea-level change across 3.6 million years. What emerged was a clear pattern: day length has fluctuated in lockstep with the planet's ice formation and melting cycles throughout history. When ice sheets grew, pulling water toward the poles, days shortened slightly. When ice melted and water spread toward the equator, days lengthened.

But the period from 2000 to 2020 stands out as an outlier in this ancient record.

Why Mass Distribution Matters

The mechanism behind this slowdown comes down to basic physics. Earth spins on its axis once every 24 hours—or so we assume. In reality, that duration constantly shifts based on how the planet's mass is distributed.

When glaciers and polar ice sheets melt, they release water that was concentrated near Earth's rotational axis. This water flows into the oceans and spreads toward the equator, moving mass away from the axis of rotation. The result is like adding weight to the outer edge of a spinning wheel: it takes more time to complete each rotation.

This redistribution of mass once held at the poles toward the planet's midsection creates measurable changes in rotational speed. The effect compounds as melting accelerates. More meltwater means more mass redistribution, which means slower rotation, which means longer days.

The Moon's Waning Influence

For billions of years, the moon has been the primary force lengthening Earth's days. Its gravitational pull creates tidal friction that gradually slows our planet's spin. This process has been remarkably consistent, adding roughly 2.3 milliseconds to each day per century over recent millennia.

Climate change is now competing with this ancient cosmic dance. The researchers found that by the end of this century, climate-driven changes will surpass the moon as the dominant influence on day length variations. A force that has shaped Earth's rotation since its formation will be overtaken by human-altered ice sheets within decades.

This shift represents more than a scientific curiosity. It marks the point at which human activity becomes the primary driver of a planetary-scale physical process that has operated independently for geological ages.

Milliseconds That Matter

A day that's 1.33 milliseconds longer per century sounds like a rounding error. Most of us wouldn't notice if tomorrow lasted a few milliseconds longer than today. But precision systems certainly would.

"Even though the changes are only milliseconds, they can cause problems in many areas, for example in precise space navigation, which requires accurate information on Earth's rotation," Soja explained. GPS satellites, deep-space probes, and telecommunications networks all depend on knowing exactly where Earth is in its rotation at any given moment.

Navigation systems assume a standard day length to calculate positions. When that assumption drifts by even milliseconds, errors accumulate. A spacecraft traveling to Mars might miss its target. A GPS coordinate could shift meters from its true location. These systems require constant updates to account for variations in Earth's rotation—variations that are now changing faster than at any point in millions of years.

The Spinning Planet's New Rhythm

The study, published in the Journal of Geophysical Research: Solid Earth, reveals something profound about our current moment. We often think of climate change in terms of rising temperatures or extreme weather. But the effects reach down to the fundamental physics of how our planet moves through space.

Day length varies for many reasons: the moon's gravity, geophysical processes in Earth's interior and surface, atmospheric conditions. These factors have always created small fluctuations. What makes the current change significant isn't just its magnitude but its source and trajectory.

For 3.6 million years, natural cycles of ice growth and melting drove changes in rotation speed. Those cycles had rhythms, patterns, predictable bounds. The current acceleration breaks from those patterns. It's driven not by orbital variations or volcanic activity but by the cumulative effect of greenhouse gases trapping heat and melting ice at rates the planet hasn't experienced in millions of years.