Insights of multiple sensors remote sensing techniques for the mapping of subglacial valleys beneath glaciers and ice shelves

Accurate mapping of subglacial bedrock topography is of prime importance to correctly simulate the past and future evolution of glaciers and ice sheets. As ocean warming is a major driver of recent changes in Greenland and Antarctica, mapping the bathymetry of the ocean seafloor in fjords and underneath ice shelves is crucial to accurately model warm water pathways up to the ice margins and grounding lines. A good knowledge of this bedrock topography also allows to better understand the past extent of the ice sheets and identify vulnerable regions that are sitting on retrograde bed slopes, hence that might be prone to the marine ice sheet instability. For mountain glaciers, accurately mapping the bedrock topography is mandatory to estimate ice thicknesses, which are used to simulate the contribution of glaciers to sea level rise, but also to quantify the amount of freshwater resources stored in glaciers. Because of their large number, remote locations, and difficult access conditions, only scarce in-situ data exists for bedrock topography. Hence, while being a fundamental variable for glacier modeling, it remains poorly constrained at the time. Here, we present how the use of multiple sensors remote sensing techniques has helped us to unravel the hidden relief beneath glaciers and ice sheets. In Greenland and Antarctica, we use airborne gravimetry measurements along with multibeam and radar echoe sounder to map the bathymetry in fjords and below ice shelves. We show that the use of these new bathymetric products help us to understand the retreat history of glaciers, revealing pathways for warm water, and contributes to better modeling ocean circulation up to the grounding lines of glaciers. For mountain glaciers, we mapped the ice velocity worldwide at an enhanced sampling resolution of 50 m, using massive cross correlation techniques on image pairs from both optical (ESA’s Sentinel-2; USGS/NASA’s Landsat-7/8) and radar imagery (ESA’s Sentinel-1a/b). Finally, we combine this mapping with airborne and ground penetrating radar to recover the ice thickness of all glaciers on Earth. These estimations reveal a different picture of the bedrock topography beneath glaciers, with a modified ice thickness distribution. Using these new estimations as initial state in the Open Global Glacier Model, we show the important impact on the evolution of freshwater resources, and specifically on the timing of the peak water.