The Earth’s rotation, which dictates the length of our day, fluctuates slightly over time. Recent observations have shown that the planet has experienced periods of acceleration, leading to some of the shortest days recorded since precise measurements began. This variability is tiny, measured in milliseconds, and the question of whether this speed-up is concerning depends entirely on the context of the measurement. While the change is imperceptible in daily life, it presents a unique challenge for the world’s most precise timekeeping systems.
Defining Earth’s Rotational Variability
The planet’s rotation is subject to a complex interplay of forces that cause the length of a day to change irregularly. A long-term trend, driven by the Moon’s gravitational pull, causes the Earth’s spin to slow down, lengthening the day slightly each century. Superimposed on this deceleration are short-term fluctuations that cause the Earth to speed up or slow down.
These short-term changes are primarily driven by the dynamic movement of mass both inside and on the surface of the planet. For instance, the churning of the Earth’s molten outer core and its interaction with the solid mantle can transfer angular momentum, causing decade-scale changes in rotation speed. Atmospheric winds and ocean currents, particularly large-scale phenomena like El Niño, also contribute to seasonal and inter-annual variations by exchanging momentum with the solid Earth.
Precise measurement of these variations is accomplished through techniques such as Very Long Baseline Interferometry (VLBI). This method uses a global network of radio telescopes to measure the Earth’s orientation relative to distant, fixed quasars with millisecond accuracy. This data is used by the International Earth Rotation and Reference Systems Service (IERS) to track the Earth’s time, known as Universal Time 1 (UT1).
Immediate Impact on Global Timekeeping
The recent periods of acceleration have created a growing divergence between the Earth’s rotation-based time (UT1) and the highly stable time kept by atomic clocks, known as Coordinated Universal Time (UTC). UTC is based on the average of thousands of atomic clocks worldwide and is the foundation for modern digital infrastructure. Timekeepers historically added a “leap second” to UTC to prevent it from drifting more than 0.9 seconds away from UT1.
The recent speed-up, however, has reversed this trend, meaning that the Earth’s day is temporarily shorter than 86,400 atomic seconds. If the Earth continues to spin faster, this will necessitate the unprecedented implementation of a “negative leap second,” where one second would be subtracted from the global clock. This subtraction is technically complex for computer systems, which are largely designed to handle an occasional addition, not the sudden removal of a second.
Modern digital systems, including global navigation satellite systems (GNSS), financial trading platforms, and telecommunications networks, rely on a continuous, uninterrupted time flow. The potential for a negative leap second poses a significant technical risk, as a sudden time discontinuity can cause software crashes, data corruption, and system outages, similar to issues seen with past positive leap seconds. The disruption is severe because a negative leap second violates the fundamental assumption of continuous time built into many networking protocols.
Geophysical and Environmental Effects
Despite timekeeping concerns, the millisecond-scale variations in Earth’s rotation speed have a negligible effect on the planet’s physical stability. These fluctuations are far too small to trigger major geophysical events, such as tectonic plate movement, earthquakes, or volcanic eruptions. The forces required for such large-scale phenomena are immensely greater than the subtle shifts in angular momentum being observed.
While mass redistribution, such as the melting of ice sheets, affects the planet’s rotation, this effect is primarily on the long-term trend. The short-term speed-ups are more closely related to internal fluid dynamics and atmospheric shifts, which do not translate into immediate, catastrophic geological consequences. Climate and weather systems, while contributing to rotational changes, are not significantly altered by the resulting millisecond adjustments in the length of the day. The influence on ocean tides also remains minimal, as the Moon’s gravitational pull is overwhelmingly the dominant factor in tidal cycles.
Managing the Speed Change
The technical risks associated with the leap second system have prompted a global effort to redefine the future of UTC. International bodies, including the International Telecommunication Union (ITU), have debated proposals to abandon inserting or subtracting a leap second. The current proposal, which has strong international backing, suggests eliminating the leap second entirely by 2035.
This change would allow UTC to run continuously based solely on atomic time, breaking its direct link with the Earth’s rotational time (UT1). The time difference between the two scales would be allowed to grow to a larger threshold, possibly up to one minute, before correction is considered. This shift eliminates disruptive, unpredictable adjustments that pose a threat to digital infrastructure, prioritizing the stability of global timekeeping over precise alignment with the Earth’s variable rotation.