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Deceleration in the Earth's oblateness

For over three decades, satellite laser ranging (SLR) has recorded the global nature of the long‐wavelength mass change within the Earth system. Analysis of the most recent time series of 30 day SLR‐based estimates of Earth's dynamical oblateness, characterized by the gravitational degree‐2 zonal spherical harmonic J2, indicates that the long‐term variation of J2 appears to be more quadratic than linear in nature. The superposition of a quadratic and an 18.6 year variation leads to the “unknown decadal variation” reported by Cheng and Tapley (2004). Although the primary trend is expected to be linear due to global isostatic adjustment, there is an evident deceleration ( J¨2=18±1×10−13/yr2 ) in the rate of the decrease in J2 during the last few decades, likely due to changes in the rate of the global mass redistribution from melting of the glaciers and ice sheets as well as mass changes in the atmosphere and ocean.