Why Earth's Rotation Rate Matters for Clocks
UTC is kept within 0.9 seconds of UT1 (solar time) by inserting or removing leap seconds. Earth's rotation is gradually slowing due to tidal friction, which has required 27 positive leap seconds since 1972. But Earth's rotation is also affected by other factors -- including the redistribution of mass as ice sheets melt and reform. Since the 1990s, the Earth has been rotating slightly faster than expected, which means UTC has been running slightly behind, and at some point a negative leap second (removing a second) would be needed to realign them.
The 2024 Nature Study
A study published in Nature in March 2024 by Duncan Agnew (UC San Diego) found that the accelerated melting of Greenland and Antarctic ice sheets since the 1990s has redistributed enough mass away from the poles to slow Earth's spin relative to projections. The change is small -- on the order of milliseconds per day -- but cumulative. Without the ice melt effect, a negative leap second would likely have been needed around 2026. With it, the need is now projected to be pushed to 2029 or later.
Why a Negative Leap Second Worries Engineers
All 27 historical leap seconds have been positive: clocks showed 23:59:60 UTC before rolling to 00:00:00. Positive leap seconds already cause software bugs (the 2012 Reddit/LinkedIn outage, the 2017 Cloudflare crash). A negative leap second -- where 23:59:58 is followed immediately by 00:00:00, skipping 23:59:59 -- has never happened. Most NTP implementations, databases, and distributed systems have never been tested for a backward time jump. POSIX and most OS kernels would step the clock backward, which is explicitly prohibited by many consistency algorithms.
Context: Leap Seconds Are Already Scheduled to End
The 2022 BIPM vote to eliminate leap seconds by 2035 means that even if a negative leap second becomes necessary before 2035, the conventions would need to be revisited. After 2035, UTC will be allowed to drift from UT1 by up to one minute before any correction is applied -- and the mechanism for that future correction has not yet been defined. The Nature study adds a new variable: climate change is now measurably affecting the world's timekeeping infrastructure.