Multi-technique estimation of Ice Mass Balance in Greenland: impact of the uncertainties on firn densification and GIA models

We conduct a comprehensive comparison of Ice Mass Balance (IMB) estimates for Greenland derived from satellite observations of ice Surface Elevation Changes (SEC), gravity and GNSS observations. Our analysis integrates data from the ICESat and CryoSat-2 satellite altimetry missions, augmented by optical stereo-imagery for peripheral glaciers, and GRACE satellite gravimetry mission, spanning the 2003-2008 and 2011-2015 periods. We also consider three firn densification models (FDM) and five Glacial Isostatic Adjustment (GIA) models for correcting the datasets for these effects when necessary. Our results reveal significant differences among FDM corrections applied to SEC observations, with particularly large variations in IMB estimates reaching up to 90 Gt/yr. To address this, we develop an innovative method for estimating equivalent firn corrections to the ice elevation observations, based on a least- squares fit of filtered ice SEC observations to GRACE mass- change estimates. This approach is both simple and independent from climate models assumptions and shows minimal sensitivity to GIA model differences. Using this method, we estimate IMBs for Greenland at -217.6 \ensuremath± 15.7 Gt/yr for 2003-2008 and -253.2 \ensuremath± 18.8 Gt/yr for 2011-2015. Importantly, these values indicate an acceleration of the thinning rate, not consistently captured by the IMB estimates inferred from the ice SEC observations corrected by FDMs. Finally, we compute elastic ground deformation induced by ice mass change during 2011-2015, using the four proposed mass- variation distributions and compare the predicted vertical velocities with GNSS observations in Greenland, accounting for all GIA models. While all models are consistent with most of the GNSS-derived uplift rates, they cannot fully explain the observed vertical velocities, especially in the South-East Greenland, which confirms the need to refine our understanding of GIA contributions in this region.