Continuity of the GRACE/GRACE-FO measurements over land ice and impacts for climate models.

Abstract:

The GRACE missions have changed the way that we measure mass changes of ice sheets, glaciers and ice caps, with quantified uncertainties that factor processing errors, atmospheric and oceanic corrections, and removal of glacial isostatic adjustment (GIA). We present GRACE/GRACE-FO estimates of mass balance over the Greenland and Antarctic Ice Sheets and the World’s glaciers and ice caps (GIC). The data gap between missions is filled with GRACE-calibrated results from the input-output method for ice sheets and surface mass balance (SMB) reconstructions from regional atmospheric climate models and MERRA-2 reanalysis data for the glaciers and ice caps. Over Greenland, we report low losses during the cold years of 2017-2018 followed by record melt in 2019 and an onset of rapid melt for 2020. As warm air and ocean masses get blocked over Greenland more frequently because of interactions between the wobbling jet stream and topography, we observe more high melt events in this decade than recorded in prior centuries. In Antarctica, the ongoing rapid loss in West Antarctica dominates the mass balance, but we observe a steady increase in snowfall in the Atlantic sector of East Antarctica. Overall, we do find discrepancies between GRACE and SMB on the East Antarctic plateau which reveal shortcomings in the GIA. Combining altimetry, GRACE, GPS, SMB and forward modeling of the GIA, we derive a better GIA constrained by contemporaneous observations, which in turn yields a smaller mass balance for East Antarctica. Overall the exercise provides a mass balance record that can be continuously improved with better corrections and improved processing, errors are reduced, so that we can provide better constraints for ice sheet models in charge of sea level projections and improve the validation of various Earth system models and global climate models.