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We present a new approach to estimate time variations in J(2). Those variations are represented as the sum of contributions from individual sources. This approach uses solely Gravity Recovery And Climate Experiment (GRACE) data and the geoid fingerprints of mass redistributions that take place both at the surface and in the interior of the solid Earth. The results agree remarkably well with those based on satellite laser ranging, while estimates of the sources explain the observed variations in J(2). Seasonal variations are dominated by terrestrial water storage and by mass redistribution in the atmosphere and ocean. Trends, however, are primarily controlled by the Greenland and Antarctic ice sheets and by glacial isostatic adjustment. The positive trend from surface mass variations is larger than the negative trend due to glacial isostatic adjustment and leads to an overall rising trend during the GRACE period (2002-2017).

Plain Language SummaryJ(2) variations indicate changes in the flattening of the Earth, which are mainly due to the Earth's response to large-scale mass redistribution at its surface and related to ongoing climate change. Though monitored over four decades by Earth observations satellites, the contributing sources to J(2) variations have not yet been accurately constrained, mostly due to deficiencies in geophysical models. In this study, we propose an approach to simultaneously estimate and interpret J(2) temporal variations based on gravity observations from the Gravity Recovery And Climate Experiment satellite mission. We reconstruct the observed gravity changes by a superimposition of spatial patterns characteristic of individual sources. We find that the seasonal and interannual variations are well explained by mass changes in atmosphere, oceans, and land water storage. The secular trend, on the other hand, is mainly caused by ice sheet melt, which has a positive effect (causing the Earth to be flatter), and by the ongoing solid Earth response to past glaciations, which has a negative trend (causing the Earth to be rounder). The trend due to ice sheet melting during 2002-2017 has a larger absolute value, so that the overall trend is rising, and the Earth is currently becoming flatter.