Summary
One of the principal factors influencing mass loss and sea level contribution from the Antarctic Ice Sheet currently and in future centuries is the melting of ice shelves by the ocean. These floating extensions of the ice sheet buttress against ice-sheet movement, and melt-driven ice-shelf thinning can potentially lead to rapid ice-sheet loss. In addition, the meltwater from the ice shelves influences sea ice formation as well as ocean density stratification and circulation, and can trigger critical feedbacks in the global carbon and energy cycles. However, ice shelf-ocean interactions are not well understood or well quantified. Melt (and refreeze) rates depend on processes that cover a broad range of scales, from the centimeter-scale ice-shelf boundary layer processes to the large-scale circulation within ice-shelf cavities and beyond. Both remain incredibly challenging to ob- serve, limiting our ability to derive and validate model parameterizations of, and constrain simulations of, submarine melt. A range of methodologies are em- ployed to measure ice-shelf melt rates, broadly grouped as: (1) Satellite-based – using satellite altimetry and velocity together with estimates of surface melt and compaction to estimate broad melt patterns; (2) Oceanographic – quantifying spatially-integrated (bulk) melt via tracer and heat balance; and (3) Geophys- ical – high fidelity melt estimation through direct observations of the ice-shelf base at discrete locations. Each method differs in spatial and temporal extent and resolution, and each has limitations. Where melt estimates overlap, there is potential to independently validate estimates. However, this requires both a careful consideration of the methods used in data collection as well as a standard basis for comparison – considerations which, to date, have been lacking. Thus opportunities exist in terms of planned and future campaigns to ensure that mea- surements are as comparable as possible. Here we propose an outline framework for ensuring such intercomparability. We make two "levels" of recommendation. In the first (low-cost), we make a set of recommendations by which to evalu- ate and plan proposed field campaigns to maximise the degree to which melt measurements can be compared. In the second (high cost), we propose a bold "ice-shelf laboratory" concept in which a single ice shelf can be instrumented as much as possible and biases in large-scale and low-cost measurement platforms can be better understood. We further propose several candidate ice shelves for such "supersites". We intend that this framework will inform the community in the planning stages for future internationally-led field programmes.