It may sound surprising, but climate change is slowly affecting how long a day lasts on Earth. According to a new study, rising sea levels are gradually slowing down the planet’s rotation, which means the length of an average day is increasing. Scientists say the length of a day is currently growing by about 1.33 milliseconds every century. While that difference is extremely small, researchers say the current rate is unprecedented in at least the last 3.6 million years. A day isn’t always exactly 24 hours Most people assume a day always lasts exactly 24 hours, but that’s not entirely true. The length of a day can change slightly due to several natural factors. These include: However, scientists now believe that climate change could become a stronger influence than the Moon when it comes to affecting Earth’s rotation. According to Benedikt Soja, a geophysicist at the Swiss Federal Institute of Technology Zurich and senior author of the study, climate change could dominate this effect by the end of the century. Even tiny changes matter. Soja explains that shifts of just a few milliseconds can create issues for systems that require extreme accuracy, such as space navigation. How melting ice slows the planet The slowdown is linked to melting glaciers and shrinking polar ice sheets, which are raising sea levels worldwide. When ice at the poles melts, that mass moves toward the middle of the planet. This redistribution changes how Earth spins. Lead study author Mostafa Kiani Shahvandi, a geoscientist at the University of Vienna, compared this to a figure skater. When a skater stretches their arms outward, they spin more slowly. When they pull their arms inward, they spin faster. Earth reacts in a similar way when mass shifts across the planet. Evidence from 3.6 million years of data The research, published in the Journal of Geophysical Research: Solid Earth, examined the chemical makeup of fossils to estimate sea-level changes over the past 3.6 million years. Scientists then calculated how those changes affected the length of a day. Using a deep-learning algorithm, they discovered that day length has historically changed alongside the growth and melting of global ice. But today’s changes stand out. The only somewhat similar period occurred around 2 million years ago, though even then the changes were slower than those recorded between 2000 and 2020. Why this matters for global timekeeping The slowdown is already influencing how scientists manage global time. Because days are getting slightly longer, the need for a “negative leap second” a rare time adjustment has been pushed back from 2026 to at least 2029. Precise timekeeping is essential for technologies like GPS, satellites, and computing systems, which rely on extremely accurate atomic clocks. Fortunately, these tiny changes are unlikely to affect human biology, such as sleep cycles or circadian rhythms. However, if global warming continues, scientists predict that the length of a day could increase by about 2.62 milliseconds by the end of this century.