The melting of ice sheets and global glaciers has led to a rise in sea levels, increasing Earth's oblateness and affecting the length of day (LOD). The study uses observations and reconstructions of mass variations at the Earth's surface since 1900 to show that the climate-induced LOD trend was between 0.3 and 1.0 ms/cy in the 20th century but has accelerated to 1.33 ± 0.03 ms/cy since 2000. Surface mass transport fully explains the accelerating trend in Earth's oblateness observed in the past three decades. An independent measure of the decreasing LOD trend induced by Glacial Isostatic Adjustment (GIA) is −0.80 ± 0.10 ms/cy, providing a constraint for mantle viscosity. The sum of this GIA rate and lunar tidal friction explains the secular LOD trend inferred from eclipse records over the past three millennia. Projections under high emission scenarios suggest that the climate-induced LOD rate may reach 2.62 ± 0.79 ms/cy by 2100, surpassing the impact of lunar tidal friction. This highlights the unprecedented effect of climate change on Earth's rotation and its implications for precise timekeeping and space navigation.The melting of ice sheets and global glaciers has led to a rise in sea levels, increasing Earth's oblateness and affecting the length of day (LOD). The study uses observations and reconstructions of mass variations at the Earth's surface since 1900 to show that the climate-induced LOD trend was between 0.3 and 1.0 ms/cy in the 20th century but has accelerated to 1.33 ± 0.03 ms/cy since 2000. Surface mass transport fully explains the accelerating trend in Earth's oblateness observed in the past three decades. An independent measure of the decreasing LOD trend induced by Glacial Isostatic Adjustment (GIA) is −0.80 ± 0.10 ms/cy, providing a constraint for mantle viscosity. The sum of this GIA rate and lunar tidal friction explains the secular LOD trend inferred from eclipse records over the past three millennia. Projections under high emission scenarios suggest that the climate-induced LOD rate may reach 2.62 ± 0.79 ms/cy by 2100, surpassing the impact of lunar tidal friction. This highlights the unprecedented effect of climate change on Earth's rotation and its implications for precise timekeeping and space navigation.