The melting of ice sheets and global glaciers results in sea-level rise, a pole-to-equator mass transport increasing Earth’s oblateness resulting in an increase in the Length of Day (LOD). Here, we use observations and reconstructions of mass variations at the Earth’s surface since 1900 to show that the climate-induced LOD trend hovered between 0.3 and 1.0 milliseconds per century (ms/cy) in the 20th century, but has accelerated to 1.33±0.03 ms/cy since 2000. We further show that surface mass transport fully explains the accelerating trend in the Earth oblateness observed in the past three decades. We derive an independent measure of the decreasing LOD trend induced by Glacial Isostatic Adjustment (GIA) of −0.80±0.10 ms/cy, which provides a constraint for the mantle viscosity. The sum of this GIA rate and lunar tidal friction fully explains the secular LOD trend that is inferred from the eclipse record in the past three millennia prior to the onset of contemporary climate change. Projections of future climate warming under high emission scenarios suggest that the climate-induced LOD rate may reach 2.62±0.79 ms/cy by 2100, overtaking lunar tidal friction as the single most important contributor to the long term LOD variations.