Climate change is slowing the Earth’s rotation

THE PERFECT a day should have 86,400 seconds: 24 hours during which the Earth rotates on its axis, 60 minutes per hour and 60 seconds per minute. But the apparent accuracy of these simple calculations ignores the messy reality of planetary bodies. Tidal forces, combined with the swirling currents in the Earth’s core and the redistribution of ice sheets on the surface, cause the planet’s rotation rate to vary slightly from year to year.

This irritant was dropped in 1967 with the definition of a new second, derived from the vibrations of cesium atoms in high-precision atomic clocks. The two seconds are almost exactly equivalent. But not completely. For example, the leap year 1972 should have had 31,622,400 seconds. However, measured in atomic seconds, the entire journey of the Earth around the Sun took 31,622,401.14. As a result, two extra seconds were added: the very first ‘leap seconds’. One, on June 30 of that year, made up for the delay; the second anticipated an approaching extra. It was added to the very last minute of the year.

For a while, leap seconds were a regular feature. There were 27 between 1972 and 2016. Due to a gradual acceleration of the Earth’s rotation, which allowed solar seconds to catch up with atomic seconds, there have been none since. In fact, within a few years the time nerds at the International Earth Rotation Service (IRISH), the body that decides when leap seconds should occur may have to implement an entirely new “negative leap second.” In other words, on a future December 31, the stroke of midnight will follow one minute of 59 seconds later. Such adjustments are an unpleasant prospect for organizations that depend on perfect timekeeping, from stock markets to electricity grids. But a new study suggests climate change will buy them some welcome extra time.

In an article published in Nature Last month, Duncan Agnew, a geophysicist with an interest in timekeeping at the University of California San Diego, unraveled the various factors causing the Earth’s rotation to accelerate. To do this he used a range of data sources, including laser measurements of the distance between the Earth and the moon, perturbations of Earth’s gravity, and data from ancient eclipses. Partly responsible for the recent acceleration, he concluded, are the currents rippling through Earth’s molten core. The melting of the polar ice cap since the end of the last ice age 12,000 years ago has also made the Earth spin faster. Their weight crushed the posts; their disappearance allowed the Earth’s crust to bounce back and become more rounded. This caused an acceleration, as if skaters were tucking their arms in to turn faster.

Dr. Agnew also discovered effects that worked in the other direction. In recent decades, climate change has shrunk the ice sheets in Greenland and Antarctica, shifting the water mass from land to the oceans, where it can be redistributed. By reducing the mass of both regions, the melt reduces their gravity, with the net effect of “pushing” water away from their coasts. The water lost through the Greenland ice sheet collects most prominently around the equator and in the Southern Hemisphere. The opposite is more or less true for water from the Antarctic ice sheet. Glaciologists who have tracked how all this mass of water moves from land to the oceans have discovered a shift from the poles to the equator as a result. That means the Earth’s waist is getting thicker, says Jonathan Bamber, a glaciologist at the University of Bristol. The effect is not huge – it is measured in millimeters per year – but is enough to exert a braking effect on the Earth’s rotation.

Not a second too soon

It also delays the need for a negative leap second. Without climate change, current trends suggest IRISH will have to implement one in just two years. Dr Agnew’s calculations suggest they have until 2029. That time will allow software engineers who run systems that rely on the precision of atomic clocks to come up with new programs capable of handling the negative leap second. Alternatively, as some have suggested, IRISH could use that time to eliminate it completely. Solar seconds and atomic seconds may differ by as much as a second. Expanding that tolerance to one minute would likely eliminate the need for leap seconds of all kinds for decades to come. For timekeepers around the world, that day can be as perfect as possible. ■