Spin cycle: Why Earth’s rotational speed is changing
A gigantic dam, our extraction of groundwater, melting ice sheets and rising sea levels are all altering Earth’s spin. The thing they all have in common: water.
Talk about making the world go round.
Earth’s rotational speed is changing, and we’re more than partly responsible.
A gigantic dam, our extraction of groundwater, melting ice sheets and rising sea levels are among the human-linked factors altering Earth’s spin.
The thing all these factors have in common, is water. Where it stands, how it flows and where it settles has always shaped Earth’s rotation.
During the last Ice Age, which ended about 20,000 years ago, for instance, the weight of the ice was so great that it depressed the surface of the planet. As it melted, large parts of the planet returned to their original form, leading to shifts in the orbit then too.
Giant earthquakes have also traditionally altered Earth’s rotation, in tiny but still real ways.
Anything that redistributes Earth’s mass can cause such a change. One of the things now estimated to do this is the world’s most powerful dam.
China’s Three Gorges Dam across the Yangtze River generates more hydroelectric power than certain small countries. Completed in 2006, it is over 7,500 ft long and 600 ft tall. At capacity, it holds back (and draws power from) 10 trillion gallons of water.
Because this water has been moved from a lower elevation to a higher one, at a certain distance from the equator, it has likely caused a measurable shift in the Earth’s spin.
According to research by Benjamin Fong Chao, a geophysicist with the US National Aeronautics and Space Administration (NASA), released in 2005, the dam has likely increased the length of a day by 0.06 microseconds (for context, there are a million microseconds in a second).
Incidentally, other dams have been said to cause infinitesimal shifts in spin too; few have been studied as closely as the Three Gorges.
Elsewhere, “contemporary mass loss” and its impact on Earth’s rotation are being studied on two primary fronts: the cracking and collapsing of ice sheets at the poles, and the extraction of massive volumes of groundwater over time.
Through the 20th century, the Greenland ice sheet (the largest ice mass in the northern hemisphere) has had an estimated 7,500 billion tonnes of ice melt into the ocean. The rate of melting has since sped up. NASA estimates that about 270 billion tonnes are now being lost a year in Greenland, and another 135 billion tonnes are being lost in Antarctica.
Meanwhile, humans pumped out more than 2,100 billion tonnes of groundwater between 1993 and 2010 alone, according to a study by Korean geophysicist Ki-Weon Seo, published in Geophysical Research Letters in 2023. Most of it eventually ended up in the sea.
This shifting of mass directly affects Earth’s rotation too.
The direct implications of such shifts are felt in areas that rely on extreme precision, such as satellite orbits, GPS accuracy and the synchronisation of atomic clocks.
The impacts of the factors causing the shifts, of course, are a matter of survival.
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