Climate change may alter humanity's way of keeping time. An analysis published on March 27 in Nature predicts that melting ice caps are slowing down Earth's rotation so much that the next leap second will be postponed by 3 years.
Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography in the United States and one of the authors of the paper, stated, "Melting ice is sufficient to raise sea levels to the point where we can see an effect on the Earth's rotation."
Leap seconds have been artificially adjusted to accommodate Earth's rotation since 1972. According to Agnew's analysis, global warming will postpone the need for another leap second adjustment from 2026 to 2029. Leap seconds cause significant disruptions to computer systems, leading scientists to vote in 2022 to eliminate them before 2035. Researchers are particularly concerned about the next leap second adjustment, as it might be the first negative leap second rather than a positive one.
"We don't know how to deal with a loss of one second, which is why timekeepers are concerned," said Felicitas Arias, former head of the Time Department at the International Bureau of Weights and Measures. From a metrological perspective, postponing it by 3 years is "good news," as even if negative leap seconds are still necessary, they will occur later, and the frequency of negative leap seconds occurring before 2035 may be less than expected.
However, Agnew emphasized that this should not be seen as supporting the viewpoint of global warming.
Currently, there are two commonly used time measurement systems: Coordinated Universal Time (UTC), which is based on the rotation of the Earth, and International Atomic Time (TAI), which is based on atomic oscillations. Because the two measurement methods differ, the two timing systems will diverge over time, leading to the concept of Coordinated Universal Time (UTC).
UTC is based on the length of the second of International Atomic Time, aiming to stay as close to Universal Time as possible. The International Conference on Weights and Measures in 1972 decided that when the difference between International Atomic Time and Universal Time reaches 0.9 seconds, UTC will add or subtract 1 second to stay as close to Universal Time as possible, a correction known as a leap second.
Atomic clocks are better timekeepers than Earth's rotation because they are stable over millions of years, whereas Earth's rotation speed varies. In this study, Agnew used a mathematical model to analyze the known geophysical phenomena affecting rotation and predicted their impact on future leap seconds.
Many metrologists expect leap seconds to only increase because, on the scale of millions of years, Earth's rotation speed is slowing down. This means that sometimes, a minute of UTC needs to be 61 seconds to approximate Universal Time.
However, Agnew said that geophysical phenomena cause fluctuations in rotation speed on shorter time scales. Currently, Earth's rotation speed is influenced by flows in the Earth's core, which have accelerated Earth's rotation speed since the 1970s, meaning the frequency of adding leap seconds will decrease. If this trend continues, leap seconds may need to be removed from UTC.
The study found that, due to climate change, the timing of this occurrence may be later than previously thought. Satellite data plotting Earth's gravitational field shows that since the early 1990s, melting ice from Greenland and Antarctica has caused a significant redistribution of mass from the poles to the equator, flattening the Earth. Just as a spinning ice skater slows down when extending their arms outward and speeds up when pulling them in, water flows away from Earth's axis of rotation will slow down the planet's rotation.
The ultimate result of flows in the Earth's core and climate change is still an acceleration of Earth's rotation. However, the research found that without the influence of ice melting, the time for adjusting negative leap seconds would be 3 years earlier than currently predicted.
"Human activities have far-reaching effects on climate change. Postponing leap seconds is just one example," said Jianli Chen, a geophysicist at the Hong Kong Polytechnic University.
The postponement of leap seconds will be welcomed by metrologists. Elizabeth Donley, head of the Time and Frequency Division at the National Institute of Standards and Technology in the United States, said leap seconds are already a "big problem" because in a society increasingly reliant on precise timing, leap seconds cause significant disruptions to computing systems. The possibility of unprecedented negative leap seconds could be even worse because "existing computer code hasn't taken this into account."
Agnew hopes that the impact of climate change on timing will prompt action. While Agnew's paper is helpful for predictions, experts still believe there is significant uncertainty about when negative leap seconds will be needed.
Related Paper Information: https://doi.org/10.1038/s41586-024-07170-0 China Science News (2024-03-29, 2nd Edition, International)
