Why are Earth's days getting longer?
Rising seas are subtly slowing Earth's spin
Scientists report that human-driven climate change is adding measurable time to the length of a day by shifting mass on the planet. As ice sheets and glaciers melt, large volumes of water move from high latitudes toward the equator. That redistribution increases Earth's moment of inertia — the same physical principle that makes a spinning skater slow when they extend their arms — and causes the planet to rotate a bit more slowly.
Researchers say the effect is already detectable and unusual in the geological record: the rate of change is the fastest seen in millions of years. Quantitatively, the long-term trend amounts to an additional fraction of a millisecond added to the average day on human timescales; the studies cite an increase on the order of about 1.33 milliseconds per century.
Why this matters
- Timekeeping and navigation: Precise knowledge of Earth's rotation underpins technologies such as GPS and the systems that keep coordinated universal time. Small, cumulative changes can require occasional adjustments (for example, leap seconds or other timing corrections) to keep civil time aligned with solar time.
- Climate feedbacks as a global signal: The change links a familiar climate impact — sea-level rise — to a fundamental planetary property, making the effect another metric scientists can use to track how the Earth system responds to warming.
- Long-term geophysics: A faster-than-typical change in rotational speed is notable to geophysicists because it can interact with other processes in the solid Earth and fluid envelopes (oceans and atmosphere).
What remains uncertain
It is still unclear exactly how short-term contributors (seasonal water storage, atmospheric circulation, large earthquakes) will interact with long-term sea-level rise to produce year-to-year variability. But the bottom line is clear: melting ice and redistributed water are doing more than raise seas — they are measurably lengthening our days.