| Summary: | Ice-sheet mass balance is commonly estimated by the input output method, the altimetry method, the gravimetry method, or their combination. All of them are subject to uncertainties, e.g. due to limitations in sampling and resolution, residual calibration issues, and corrections from geophysical models involved. Error budgets have been built for each of them. Intercomparison exercises have been conducted to analyze residual uncertainties, to learn about the implications this may have on our understanding of physical processes controlling the mass balance, and to generate compound estimates. Here we review approaches and discussions on uncertainty characterization related to the Ice-sheet Mass Balance Intercomparison Exercise (IMBIE). We discuss the following problems: First, the target mass balance estimates embrace different temporal scales from monthly to multi-decadal, and embrace different mass balance representations, either as rates of mass change or as mass anomalies w.r.t. a reference state or as volume change converted into mass change. The intercomparison needs to accommodate these representations in a consistent uncertainty characterization framework, and the organizers need to consider what compromises are needed and acceptable to simplify and unify the uncertainty characterization tasks posed to the contributors to the intercomparison. Second, compound estimates need to weight the different contributions. The intercomparison needs to decide how assessed uncertainties should affect the definition of weights and hence of the compound estimates. Discussions on these and other aspects are ongoing within the IMBIE Working Group on Error Budget. Soliciting feedback from the wider community is one aim of this presentation.
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