Earth’s day is lengthening by about 1.33 milliseconds per century from climate-driven mass shifts, a rate researchers say stands out against at least 3.6 million years of reconstructed history. That number is too small for any person to feel, yet it reaches into systems built on extreme precision. Modern navigation, satellite tracking, telecommunications, and global timekeeping all depend on knowing exactly how fast the planet is turning. What sounds like an abstract geophysics problem is also an engineering one: when Earth’s spin changes, the machinery that synchronizes the modern world has to keep up.
The mechanism is simple in outline and profound in consequence. As glaciers and the great ice sheets of Greenland and Antarctica lose mass, that water is redistributed across the oceans. The planet’s mass shifts outward, away from the rotation axis, and the spin eases slightly much like a skater slowing after extending their arms. NASA-backed work found that days have been getting longer by about 1.33 milliseconds per 100 years since 2000, with the faster pace linked to accelerated melting and other water-mass changes. In the same research, scientists also tied much of the planet’s polar motion to groundwater loss, glaciers, ice sheets, and sea-level change. The picture that emerges is not of a perfectly rigid spinning sphere, but of a living planet whose water and ice are constantly redistributing its momentum.
To place the present trend in deep time, researchers turned to fossilized benthic foraminifera, single-celled marine organisms that preserve chemical clues about ancient oceans. From those chemical signatures, the team reconstructed past sea-level changes and then estimated how those shifts would have altered Earth’s rotation. A deep-learning model helped them handle the large uncertainties that come with paleoclimate records while preserving the physics of sea-level change.
The result was stark. According to the reconstruction, today’s climate-related increase in day length is unmatched since the late Pliocene. Benedikt Soja said, “This rapid increase in day length implies that the rate of modern climate change has been unprecedented at least since the late Pliocene, 3.6 million years ago.” Mostafa Kiani Shahvandi framed the image even more vividly: “Never before or after that has the planetary ‘figure skater’ raised her arms and sea levels so quickly as in 2000 to 2020.”
The story becomes even more striking because Earth is never driven by one force alone. Over long spans, lunar tidal friction adds about 2.3 milliseconds per century to the length of a day. Over shorter intervals, the atmosphere, ocean currents, tides, and the liquid core can push rotation in either direction by fractions of a millisecond. That is why some recent individual days have actually come in slightly short, even while the broader climate-linked trend points toward a longer average day. Researchers tracking timekeeping have also noted a trend toward slightly faster days since 1972, along with irregular swings that complicate prediction.
Those swings matter because official time is not set by Earth alone. Coordinated Universal Time is maintained by atomic clocks, and when astronomical time drifts too far, timekeepers have historically inserted leap seconds. Since 1972, 27 leap seconds have been added. The possibility of a future negative leap second has drawn attention in engineering circles because software, networks, and timing infrastructure have little real-world experience with subtracting a second rather than adding one.
For now, the shift remains measured in milliseconds. But it is also a reminder that climate change is not only altering coastlines, glaciers, and weather patterns. It is subtly changing the rotational behavior of the planet itself, and the instruments that guide spacecraft, synchronize servers, and anchor GPS have to measure that change with extraordinary care.
