Climate change is not only reshaping coastlines and weather patterns. It is also nudging the length of the day itself, and that is creating a new headache for the people who keep the world’s clocks in sync. A new study in the journal Nature finds that human-driven warming has delayed, but not removed, the need for a risky adjustment known as a negative leap second.
In the work, geophysicist Duncan Carr Agnew of the Scripps Institution of Oceanography combined decades of measurements of Earth’s rotation with satellite gravity data that track how ice is melting from Greenland and Antarctica. His calculations show that under today’s Coordinated Universal Time, timekeepers will probably have to subtract one second from the global clock in the late 2020s, most likely around 2028 or 2029. If the recent acceleration of polar ice loss had not happened, that awkward moment would arrive about three years sooner.
Since 1972, international timekeepers have occasionally added leap seconds so that atomic clocks stay aligned with the actual spin of the planet. In practice that has meant inserting an extra tick at the end of a day, and it has happened 27 times so far, most recently in 2016. A negative leap second would do the opposite by deleting one second from a day, something that has never been tried.
Melting ice and the length of a day
Earth does not spin like a perfectly rigid ball. Winds, ocean currents, earthquakes and even slow churning in the liquid iron core all speed up or slow down the rotation by tiny amounts. Agnew’s analysis separates the influence of the core from changes at the surface, and finds that the recent surge of ice melt is shifting water toward the equator and slightly slowing down the spin, much like a figure skater stretching out their arms.
Satellite missions that sense subtle changes in Earth’s gravity field have already shown that Greenland and Antarctica are losing ice at an increasing rate, which is raising sea level and altering the planet’s shape. The new work shows that this shift in mass is now large enough to affect timekeeping at the level of microseconds, the scale that matters when you try to keep navigation satellites, power grids and high-speed financial trading platforms perfectly in step.
Why a missing second worries engineers
One second may sound like nothing. For computer networks it is plenty. Global timekeeping is what lets your phone, your bank, streaming services and air traffic control agree on when something happens. Past leap seconds have already triggered crashes in major websites and airline systems when software was not prepared for a minute that lasts sixty one seconds. A second that suddenly vanishes instead of appearing would be even trickier, because very little code has ever been tested for that scenario.
Metrologists, the scientists who define time standards, already dislike leap seconds for that reason. In 2022 an international consortium voted to phase them out by 2035 and to replace them with rarer, larger corrections, perhaps a leap minute every few decades. Agnew’s result suggests that the first ever negative leap second is likely to fall right into this transition period, so standards bodies may need to decide soon whether to change the rules or let civil time drift a little farther from the position of the Sun in the sky.
From a climate perspective, the irony is hard to miss. The same melting that threatens coastal communities is also buying engineers a bit more time to prepare for a risky step in global timekeeping. Agnew describes the delay as a “trivial benefit” compared with the far larger dangers of global warming, and stresses that the main lesson is how powerful human influence on the planet has become.
If a negative leap second eventually happens, most people will not feel anything. Your smartphone will quietly keep up. Your electric bill will not magically shrink when one second disappears. Yet the control rooms that balance electricity supply and demand, and the systems that route trains, planes and data packets, all depend on precise time stamps, so they have to take this seriously.
What this tells us about climate change
For scientists the study highlights something more fundamental. Climate change is no longer just a story about local temperatures or shrinking glaciers. It is altering the distribution of mass across the whole planet in ways that careful measurements of day length can track. As Agnew told colleagues, humankind “has done something that affects, measurably, the rotation rate of the entire Earth”.
At the end of the day, a missing second in some year around 2028 or 2029 will be a curiosity for most of us, not a crisis. The deeper message is that global warming is now reaching into places we rarely think about, from the stability of ice sheets to the quiet beat of atomic clocks. If our emissions can tug on the planet’s spin and on the world’s clocks, it is one more reminder that the window for limiting that influence is finite.The study was published in Nature.
