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.
