Leap Second: How Chinese Dam Affects Planet Rotation Speed
Researchers predict that this summer, the planet may once again approach its maximum rotation speed. The causes of this acceleration are not fully understood, and there is still no definitive scientific explanation. Leonid Zotov, Professor at the School of Applied Mathematics, MIEM HSE, notes that if, in the past, the so-called ‘leap second’ was added on New Year’s Eve, then for the first time in history, the question has arisen about whether it is time to subtract one.
Scientists estimate that Earth’s maximum rotation speed may occur on July 9, July 22, and August 5 of the year 2025. They expect that the maximum reduction in the length of the day could be 1.51 milliseconds.
Of course, for humans, millisecond-level differences in the length of day are undetectable. However, these fluctuations are crucial for satellite navigation, global time synchronization, and precise scientific measurements. This is why scientists monitor them in real-time and even adjust the UTC (Coordinated Universal Time) atomic time scale to accommodate them.
‘In the past, on New Year’s Eve, a so-called “leap second” was always added when the need arose. Now, due to the acceleration of Earth’s rotation, there has been a question about whether it is time to subtract one. This may be the first time in history,’ explained Leonid Zotov, Professor at the School of Applied Mathematics, MIEM HSE.
Recently, Earth has started rotating faster. The reasons for this acceleration are still not fully understood. There are many theories: it could be related to the behaviour of the Earth’s core, atmospheric and ocean currents, or even the characteristics of the Moon’s orbit. However, there is no definitive scientific explanation for this phenomenon yet.
At that, some scientists believe that Earth's rotation can both speed up and slow down due to human activity. Recently, there has been news in the media and on social networks about how the Chinese Three Gorges Dam has slowed down Earth's rotation, causing an increase in the length of the day. According to calculations by Dr. Benjamin Fong Chao from the Shandong University of Science and Technology in Qingdao, 40 cubic kilometres of water from the Three Gorges Dam slightly changed Earth's moment of inertia. This has caused the day to lengthen by 0.06 microseconds and the axis of rotation to shift by two centimetres, according to calculations.
According to Leonid Zotov, this is possible, but such small changes are not yet detectable by modern instruments. Even though we are talking about the melting of glaciers in Greenland (270 gigatonnes per year) and Antarctica (150 gigatonnes per year), which leads to an increase in world ocean levels by several millimetres per year, if it could slow down Earth's rotation, these changes are still within the limits of current measurement accuracy. Additionally, there are natural processes that can accelerate Earth's rotation. For example, the level of the Caspian Sea has been steadily declining by about 10 centimetres annually since the early 2000s. This decrease in water level is similar to the amount of water held by the largest Chinese dam. However, while this water has accumulated in a dam once, it has been decreasing in the Caspian for more than two decades.
If we assume that this water evaporates and does not return to the same latitude, then Earth's rotation really accelerates. The effect of the shallowing of the Caspian Sea is stronger than that of the dam and is counter-directional, meaning it accelerates the rotation of Earth. The reasons for the decrease in water levels in the Caspian Sea are still unknown. Scientists are exploring several possible causes, including a reduction in the flow of the Volga River, increased evaporation, tectonic activity, and subsidence of the sea floor. They are also actively studying atmospheric, climatic, and geological factors, but have not yet reached any definitive conclusions. However, the observed increase in Earth’s rotational speed is too sinificant to be explained only by the shrinking of the Caspian Sea or other surface processes. Since 2020, scientists have recorded unprecedented values, indicating that Earth is rotating faster than at any time since accurate measurements started in the 1950s. The shortest day in history was recorded on July 5, 2024. It was 1.66 milliseconds shorter than the usual length of a day.
‘Most likely, the processes occurring inside the planet, such as the complex movement of the core, are likely responsible for this unusual event. However, oceanic and atmospheric models are not able to fully explain this sharp acceleration. Scientists continue to investigate the phenomenon in order to better understand it,’ says Leonid Zotov.
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