Earth's Rotation Is Slowing at an Unprecedented Rate Because of Rising Oceans

Longer Days Ahead

A new study has found that Earth's rotation is slowing at a pace not seen in millions of years, and the primary driver is one that humanity has created: the redistribution of mass from polar ice sheets and glaciers to the world's oceans as the climate warms. As water moves from land to sea, it changes the planet's moment of inertia in ways that measurably reduce the speed at which Earth spins—making each day very slightly longer than the last.

The effect is tiny in absolute terms—we are talking about milliseconds per day per century—but it is detectable with precision timekeeping instruments and is now being measured at rates the researchers describe as "almost unprecedented" in the geological record. The finding adds a new dimension to the catalog of climate change's physical consequences, connecting the burning of fossil fuels to the mechanics of Earth's rotation in ways that previous generations could not have measured.

The Physics of Spinning Mass

The connection between melting ice and Earth's rotation rate is a consequence of conservation of angular momentum—the same principle that causes a spinning figure skater to accelerate when they pull their arms in and slow when they extend them. The distribution of mass relative to an axis of rotation determines how fast an object spins at a given angular momentum.

When ice melts on Greenland or Antarctica, mass that was concentrated at high latitudes—closer to the poles, relatively close to Earth's rotational axis—moves to the oceans, where it distributes globally but on average ends up at lower latitudes, farther from the rotational axis. This redistribution is equivalent to the figure skater extending their arms: it increases the moment of inertia, which reduces the rotation rate.

How Unprecedented Is Unprecedented?

The study's use of "almost unprecedented" requires unpacking. Earth's rotation rate has varied significantly over geological timescales, driven by factors including the gradual recession of the Moon (which slows Earth's rotation through tidal friction), post-glacial rebound (the slow spring-back of land crust after the weight of ice age glaciers was removed), and the movement of mass in the mantle and core.

The current rate of slowing, driven by ice melt and sea level rise, is described as comparable in magnitude to what planetary scientists infer from geological proxies during periods of dramatic glacial change. Finding a comparable rate in the record requires looking back millions of years, when the configuration of continents and glaciers was substantially different. In the context of the modern Holocene epoch—the period since the last ice age—the current slowing is anomalous.

Practical Implications

Precise timekeeping—GPS, financial systems, telecommunications networks, scientific experiments—depends on detailed knowledge of Earth's rotation rate. International timekeeping authorities regularly add or subtract "leap seconds" to reconcile atomic time standards with Earth's actual rotation. In recent years, Earth had actually been speeding up slightly (before the current slowing trend), leading to discussion of introducing a negative leap second for the first time in history.

The trend toward longer days may reduce the immediate pressure for a negative leap second, but the long-term management of timekeeping in the context of a changing rotation rate will require careful attention from the organizations responsible for maintaining global time standards. The Coordinated Universal Time (UTC) system is designed to accommodate gradual rotation rate changes, but large or rapid changes in trend are operationally inconvenient for the infrastructure built on precise time.

A Quantifiable Climate Signal

What makes this finding scientifically notable is not primarily the practical importance of slightly longer days, but the demonstration that climate change's physical reach extends to the rotation of the planet itself. Researchers can now point to a measurable geophysical quantity—Earth's rotation rate—and trace its recent anomalous change to the anthropogenic emissions that are melting ice and raising sea levels.

This kind of quantitative connection between human activity and planetary physics is a powerful illustration of the scale of the changes underway. Melting ice does not merely flood low-lying coasts or contribute to more intense storms; it measurably changes the rate at which our planet rotates—a consequence that would have seemed like science fiction to atmospheric scientists of the mid-20th century and is now a measurable reality for contemporary geophysicists.

What Can Be Done

There is no practical intervention that could address the rotation rate change specifically—it is a physical consequence of mass redistribution that follows automatically from ice melt. Slowing the rate of change requires slowing the rate of ice melt, which requires slowing global warming, which requires reducing greenhouse gas emissions.

The study adds one more item to the list of concrete, measurable consequences that climate scientists can point to when describing the urgency of emissions reduction. Days are getting longer because of human activity, at rates not seen in millions of years. That is a simple, factual statement with implications that extend well beyond the length of our afternoons.

This article is based on reporting by Gizmodo. Read the original article.