CORRECTING ICE SHEET MASS BALANCE FOR REFREEZING OF INFILTRATED SURFACE MELT: THE FORGOTTEN GRAND PROBLEM

Recent images showing vastly expanded regions of surface melt on the Greenland Ice Sheet are testimony to the climatic anomalies now occurring, with ever greater frequency and duration, in the Arctic. What these images tell about the overall mass balance of the Greenland Ice Sheet, on the other hand, is another question, not directly addressed by determination of surface melt rates. The processes of heterogeneous infiltration into initially sub-freezing permeable firn, the establishment of vertical and down-slope hydrologic pathways, and the eventual escape of surface melt to the ocean, are highly non-linear, highly complex, possibly of first-order significance in calculations of environmentally-forced surface mass balance in Greenland, and essentially unrepresented in present state-of-the-art mass balance models. No tractable models of heterogeneous piping infiltration yet exist, even in prototype form, and even parameterized estimates of the errors incurred by neglecting piping have not yet been made. The problem and its potential influence on overall ice sheet mass balance has been known, however, since the earliest days of quantitative modeling of cryospheric land ice contributions to sea level rise, and at one time was assigned as high a priority for sea level rise projections as ice sheet instability and ocean-ice heat transfer. The earliest work on heterogeneous infiltration was conducted in the 1950s and 1960s by such figures as Petr Shumskii, Fritz Müller, Carl Benson, and Ed LaChapelle, but Mark Meier was among the first to consider it in the context of global modeling of glacier and ice sheet mass balance. I review the fundamental physics of the problem, trace its history as a research issue, make an initial evaluation of the effect of its present neglect, and suggest some options for the future.