On the fundamental impact of ice melt at grounding lines.

Abstract:

Glaciers in Antarctica and Greenland responded sooner and more strongly than anticipated to climate warming because of ice-ocean interaction at glacier fronts. Winds and ocean circulation started to push more subsurface, warm, salty waters (of Atlantic origin in Greenland, of circumpolar deep water origin in Antarctica) toward the glacier floating sections (Antarctica and north Greenland only) and grounding lines. Seawater melts ice because of the salinity depresses the melting point and seawater contains vast quantities of heat. Ice melt applies to ice shelves and calving faces, but is mostly efficient near grounding lines. Melt away from grounding lines only affect pinning points or lateral drag (if thinning removes a large fraction of the thickness) or sea ice melange (inc. surface forcing). Ice melt at the grounding line, however, has a direct, dominant impact on the glacier: it reduces basal resistance. Is ice melting at the grounding line? We will show evidence that the answer is yes, which calls for a major revision of the way models treat ice-ocean interactions at grounding lines. First, the grounding line does not exist, it is a grounding zone where seawater moves at tidal frequencies not over 100m's but over km, and melt is effective in these narrow zones. Second, melt in the grounding zone is large, we can measure it from altimetry, ice is near the melting point, seawater is at high pressure, and tidal currents renew the water column regularly, despite a narrow water column. Third, models have gone as far recently as stating that it is impossible to match the current retreat rates if they do not include melt at the grounding line. Models in West Antarctica struggle to match 1 km/yr retreat rates, but we will show observations of grounding lines retreat of 3 km/yr on Smith glacier, West Antarctica. Fourth, it is essential to appreciate that the impact of ice-ocean interactions is not an average melt but to force grounding line retreat by removing grounded ice. We will show evidence in this presentation using the latest time series of CSK and TDX observations at the grounding lines of north Greenland and West Antarctica.