| Summary: | The Greenland ice sheet (GrIS) is the world’s second largest ice mass, storing about one tenth of the Earth’s freshwater. If totally melted, global sea level would rise by 7.4 m, affecting low-lying regions worldwide. Since the mid-1990s, increased atmospheric and oceanic temperatures have accelerated GrIS mass loss through increased meltwater runoff and ice discharge from marine-terminating glaciers. To understand the causes of recent GrIS surface mass loss, we use the Regional Atmospheric Climate Model RACMO2. This meteorological model simulates the GrIS surface mass balance (SMB), i.e. the difference between snowfall accumulation and ablation from meltwater runoff. To cover a large domain at reasonable computational cost, RACMO2 is run at a relatively coarse horizontal resolution of 11 km (1958-2016). At this spatial resolution, the model does not well resolve small glaciated bodies, such as narrow glaciers and small peripheral ice caps (GICs), detached from the main ice sheet. Therefore, we developed a statistical downscaling algorithm that reprojects the RACMO2 output on a 1 km grid. This downscaled product allows to quantify mass changes of small ice masses in unprecedented detail. Using the downscaled data set, we identify 1997 as a tipping point for the mass balance of Greenland’s GICs. The GICs are located in relatively dry regions where summer melt nominally exceeds winter snowfall. To sustain these ice caps, the refreezing of meltwater in the snow is a key process. The snow acts as a ”sponge” that buffers a large fraction of meltwater, which subsequently refreezes in winter. The remaining meltwater runs off to the ocean and directly contributes to mass loss. Until 1997, the snow layer in the interior of these GICs could compensate for increased melt by refreezing more meltwater. Around 1997, following decades of increased melt, the snow became saturated with refrozen meltwater, so that any additional summer melt was forced to run off to the ocean, tripling the mass loss. We call this a tipping point, ...
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