Influence of GIA Uncertainty on Climate Model Evaluation With GRACE/GRACE‐FO Satellite Gravimetry Data.

Global coupled climate models are in continuous need for evaluation against independent observations to reveal systematic model deficits and uncertainties. Changes in terrestrial water storage (TWS) as measured by satellite gravimetry missions GRACE and GRACE‐FO provide valuable information on wetting and drying trends over the continents. Challenges arising from a comparison of observed and modelled water storage trends are related to gravity observations including non‐water related variations such as, for example, glacial isostatic adjustment (GIA). Therefore, correcting secular changes in the Earth's gravity field caused by ongoing GIA is important for the monitoring of long‐term changes in terrestrial water from GRACE in particular in former ice‐covered regions. By utilizing a new ensemble of 56 individual realizations of GIA signals based on perturbations of mantle viscosities and ice history, we find that many of those alternative GIA corrections change the direction of GRACE‐derived water storage trends, for example, from gaining mass into drying conditions, in particular in Eastern Canada. The change in the sign of the TWS trends subsequently impacts the conclusions drawn from using GRACE as observational basis for the evaluation of climate models as it influences the dis‐/agreement between observed and modelled wetting/drying trends. A modified GIA correction, a combined GRACE/GRACE‐FO data record extending over two decades, and a new generation of climate model experiments leads to substantially larger continental areas where wetting/drying trends currently observed by satellite missions coincide with long‐term predictions obtained from climate model experiments. Plain Language Summary: The satellite gravimetry missions GRACE and GRACE‐FO measure changes in the lateral distribution of water stored on Earth. These data give important insight into climate‐driven wetting and drying on the continents and can thus be applied to evaluate the output of climate models designed to simulate current and future changes in the Earth system. However, GRACE/‐FO are not only sensitive to changes in water storage, but also to other mass change processes such as glacial isostatic adjustment (GIA). GIA refers to the uplift of the land surface, which is still ongoing after the melting of the heavy ice cover from the last ice age. To isolate climate‐induced water storage trends, the GIA effect has to subtracted from the GRACE/‐FO time series. With 56 different GIA models, we show that the choice of this correction affects the direction of the GRACE‐derived water storage trend from wetting to drying (or vice versa). Therefore, it is crucial for the dis‐/agreement of the observed trend direction and climate models, particularly in the previously ice‐covered regions of Canada. With the revised data processing scheme, the satellite observations document long‐term changes in terrestrial water storage in various regions that are in line with climate model projections. Key Points: A glacial isostatic adjustment (GIA) model ensemble characterizes the uncertainty of GIA corrections on GRACE‐derived water storage trendsThe GIA correction critically impacts the dis‐/agreement between GRACE and climate model wetting/drying trends in Eastern CanadaSpatially coherent regions of agreement between observed trends and climate model predictions have grown substantially over earlier studies [ABSTRACT FROM AUTHOR]