Climate change is now also affecting timekeeping

The rapidly melting ice in Antarctica and Greenland has pushed a timekeeping problem a few years further into the future. The first negative leap second in the history of modern time measurement would actually have been due in 2026. Due to the increasing melting of ice in the polar regions, this will probably not happen until 2029, reports Duncan Carr Agnew from the University of California in San Diego. In a recent publication in the scientific journal "Nature", he analyses how the difference between time and the Earth's rotation changes over time. According to the study, the Earth's rotation is currently beginning to accelerate after a long phase of decline - but more slowly than the long-term trend would suggest. This may be due to the fact that the Earth is being slowed down more than expected by rising sea levels.
The reason why there have to be additional leap seconds is that the Earth's rotation is not constant. Modern time measurement is so precise that even very small differences between the time - measured with a caesium atomic clock - and the Earth's rotation are recognisable. Gradually, tidal friction slows down the Earth's rotation so that the day becomes longer and a leap second has to be inserted every now and then. But that's only half the story. The Earth can also rotate faster.

The most important reason for this is that the Earth's liquid core is currently rotating more and more slowly; and as the Earth's angular momentum is maintained, the mantle and Earth's crust rotate faster over time. At first, this meant that fewer leap seconds had to be inserted to compensate for tidal friction. Until the turn of the millennium, this happened on average almost every year, but in the past 23 years only four times. And now the first negative leap second is imminent: the day is getting shorter. This is actually a major problem. After all, it is not trivial to synchronise normal leap seconds with UTC world time. For example, different internet services handle leap seconds differently. This is why the leap second will be abolished in 2035 - but probably too late to avoid a negative leap second.
Most computer systems now have the option of inserting leap seconds as required. But hardly any system is designed to remove a second. "Therefore, a negative leap second is expected to cause many difficulties," writes Agnew. Because of the melting glaciers near the Arctic, this day X is now a little further away. This is because when ice melts at the poles, the water spreads across the entire oceans - and its mass moves away from the Earth's axis as a result. This gives the water additional angular momentum, which is subtracted from that of the Earth as a whole. This counteracts the additional rotation from the Earth's core. And the faster the ice melts, the further it pushes back the negative leap second.

Spectrum of Science

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