The perfect day should have 86,400 seconds: 24 hours for Earth to spin around its axis, 60 minutes in each hour, and 60 seconds in each minute. However, the apparent precision of these calculations is disrupted by the complex reality of planetary bodies. Tidal forces, coupled with the churning currents in Earth’s core and the shifting distribution of ice sheets on its surface, cause our planet’s rate of spin to subtly fluctuate from year to year.
This inconsistency was addressed in 1967 with the introduction of a new second, derived from the vibrations of cesium atoms within ultra-precise atomic clocks. The two seconds, solar and atomic, are nearly identical, but not quite. Take the leap year 1972, for instance. It should have had 31,622,400 seconds. Measured by atomic seconds, though, Earth’s full journey around the sun took 31,622,401.14 seconds. Consequently, two additional seconds were added, marking the first ‘leap seconds.’ One, on June 30th of that year, compensated for the lag; the other anticipated a subsequent one. It was added to the very last minute of the year’s last day.
For a period, leap seconds were a regular occurrence. Between 1972 and 2016, there were 27 of them. However, due to a gradual acceleration in Earth’s spin, allowing solar seconds to catch up with atomic ones, there have been none since. In fact, within the next few years, the time experts at the International Earth Rotation Service (IERS), the organization responsible for deciding when leap seconds are implemented, might need to introduce an entirely new ‘negative leap second.’ In other words, on some future December 31st, midnight would follow a 59-second minute.
Such adjustments pose a significant challenge for organizations that rely on flawless timekeeping, from stock markets to power grids. But a new study suggests that climate change will buy them some much-needed time. In a paper published in Nature last month, Duncan Agnew, a geophysicist at the University of California in San Diego with a keen interest in timekeeping, meticulously analyzed the various factors contributing to Earth’s accelerating spin. To achieve this, he utilized a diverse range of data sources, including laser measurements of the distance between Earth and the Moon, disturbances to Earth’s gravity, and historical records of eclipses.
His findings revealed that the currents swirling through Earth’s molten core are partly responsible for the recent speed-up. Additionally, the melting of the polar ice sheet since the end of the last ice age, 12,000 years ago, has also accelerated Earth’s spin. The weight of these ice sheets pressed down on the poles; their subsequent disappearance allowed Earth’s crust to rebound and become more spherical. This caused an acceleration in the planet’s spin, a phenomenon familiar to skaters who tuck their arms in to spin faster.
Dr. Agnew also uncovered effects working in the opposite direction. In recent decades, climate change has been shrinking the Greenland and Antarctic ice sheets, causing a shift of water mass from land to oceans where it can be redistributed. This reduction in the mass of these regions lessens their gravitational pull, effectively ‘pushing’ water away from their shores. Water released from the Greenland ice sheet accumulates most noticeably around the equator and in the southern hemisphere. The opposite, more or less, occurs with water released from the Antarctic ice sheet.
Glaciologists who have tracked the movement of this water mass from land to oceans have observed a shift away from the poles and towards the equator. Consequently, Earth’s waistline is expanding, says Jonathan Bamber, a glaciologist at the University of Bristol. The effect is subtle, measured in millimeters per year, but nonetheless sufficient to exert a braking effect on Earth’s spin.
Not a Second Too Soon
This braking effect is also delaying the need for a negative leap second. Without climate change, current trends suggest that IERS would need to implement one within just two years. However, Dr. Agnew’s calculations indicate that they have until 2029. This extra time will allow software engineers who maintain systems reliant on the precision of atomic clocks to develop new programs capable of handling the negative leap second.
Alternatively, as some have proposed, IERS could use this time to eliminate the concept entirely. Solar seconds and atomic seconds are already permitted to differ by one second. Expanding this tolerance to one minute would likely eliminate the need for leap seconds of all kinds for decades to come. For timekeepers worldwide, that day might be as close to perfect as it is ever possible to achieve.
