Global Warming Is Influencing Global Timekeeping

A problem is coming for global timekeeping, according to a paper published in the March 27, 2024 issue of Nature by Duncan Agnew, a geophysicist at Scripps Institution of Oceanography at UC San Diego, and global warming is influencing when that problem might arrive.

Worldwide coordination of timekeeping is how all smartphones and computers can keep the same time. This timekeeping includes, every so often, an extra second, called a leap second, which makes a particular minute last for 61 seconds. Anyone who has forgotten the change to or from daylight saving time and found themselves an hour early (or late) the next morning knows how confusing it is to be out of sync with everyone else’s time. 

The same thing can happen with leap seconds: if a computer network doesn’t know about them, it will become out of sync with everything else. For example, if an online store’s computer and your bank’s computer keep different times, clicking the “order” button will appear to create two actions at different times, not just one.

Leap seconds were first used in 1972, when computer networks were in their infancy. Code that keeps track of time for each computer has usually been designed to handle an occasional extra second. But at some point in the next 10 years, according to Agnew, computers might need to allow for a negative leap second, that is a minute only 59 seconds long. This has never happened before, so it maybe difficult, if not impossible, to make sure that all the world’s interconnected computers can stay synchronized when it does. If they can’t, it is unknown what could happen.

"Even a few years ago, the expectation was that leap seconds would always be positive, and happen more and more often,” said Agnew. "But if you look at changes in the Earth's rotation, which is the reason for leap seconds, and break down what causes these changes, it looks like a negative one is quite likely. One second doesn't sound like much, but in today's interconnected world, getting the time wrong could lead to huge problems."

The paper also finds that this negative leap second would have come even sooner if it were not for global warming. The connection between timekeeping and climate change comes partly from the history of timekeeping, and partly from the geophysics of the Earth’s rotation.

Today, timekeeping is based on atomic clocks, which measure time by counting a particular type of vibration in cesium atoms. Because one second of time is defined by the frequency of this vibration, all these clocks produce the same “true time.’’ Adding up seconds gives minutes, hours, days, and years. But the time society uses, called Coordinated Universal Time (abbreviated UTC), is a mix of time from atomic clocks and time defined as it was before they were invented. 

The older definition was based on the number of seconds it took Earth to rotate once on its axis. If the earth always rotated at the same speed, the atomic-clock second could always match the Earth-rotation second, but in fact Earth’s speed of rotation varies. In 1972, a day defined by the Earth’s rotation was 0.0025 seconds (2,500 microseconds) longer than the one defined by atomic clocks. In a year, the accumulated difference was almost a second, so a leap second was used to keep the difference from getting any larger. But in 2023, the two kinds of days differed by only 80 microseconds, so the difference in time over a year added up to only 0.03 seconds.

Why did this happen? Several things cause the rotation rate to vary from year to year. Some change the rate by the same amount every year, but two phenomena are changing Earth’s rotation rate by different amounts each year.

The first phenomenon is the melting of landice at high latitudes. Much more has been melting as the polar regions have gotten warmer, one sign of global warming. The meltwater goes into the ocean and raises sea level over most of the ocean. This amounts to a transfer of mass away from the poles towards the equator, which slows down the Earth’s rotation rate. Measurements of Earth’s gravity show that before 1990, gravity changed in a way that would make the Earth closer to a sphere, and made it rotate faster. But since 1990, this trend has reversed and made the Earth rotate more slowly. Global warming, by moving water from the poles to the equator, has changed how fast the whole of the solid earth is spinning.

The second cause of rotation changes comes from something scientists know much less about, Earth’s liquid core: a mass of molten iron inside the solid part of the planet. Very slow motions of different parts of the core interact to produce the Earth’s magnetic field. These changing motions also cause the solid part of Earth to spin faster or slower, something first suggested 70 years ago by Scripps scientists Roger Revelle and Walter Munk.

Since 1972, the observed rotation of the solid part, after subtracting out all the other causes of change, shows that the rotation of the core has steadily slowed down. This slowing speeds up the rotation of the solid earth, so a day now is 0.0025 seconds shorter than it was 50 years ago. If this trend towards faster rotation keeps going, then, despite the effects of Earth’s global warming, a day as defined by planetary rotation would become shorter, enough so that the atomic-time day shown on smart phone screens would require a negative leap second.

Including all the causes of changes, Agnew suggests that a negative leap second might be needed some time in 2028. Without the slowing of Earth’s rotation caused by melting ice, it would have been expected three years earlier. This postponement, while giving more time to prepare (perhaps by simply not having a negative leap second), is only a trivial benefit compared to massive problems from global warming, said Agnew. 

That climate change has been able to change how fast the whole Earth spins is yet another indication that we are having an effect on the world unlike anything seen before, he added.