Meltwater runoff from Greenland's firn area – what we have learned during five years of research focused on the Greenland Ice Sheet runoff limit

Streams and lakes develop each summer over the marginal regions of the Greenland ice sheet. These hydrological features reach into the accumulation area and confirm that surface runoff of meltwater from above the ice sheet’s equilibrium line contributes to Greenland’s mass loss.

The CASSANDRA project (2019 to 2024) united a team of four researchers to (i) study the physical processes at the visible runoff limit of the Greenland Ice Sheet, (ii) quantify how the runoff limit changed over time and (iii) assess the impact of a rising runoff limit on the ice sheet’s surface mass balance.  To this end, we carried out six field campaigns on the ice sheet, we developed algorithms for runoff limit mapping from Landsat and MODIS, we quantified changing firn properties from Operation Ice Bridge (OIB) radar data and we modelled lateral meltwater flow and superimposed ice formation.

We found that the area of the ice sheet experiencing visible surface runoff has expanded by about 30 % since the late 1980s. The visible runoff area peaked in 2012 and thereafter fluctuated around relatively high extents. By comparing the extent of the runoff area with firn structure mapped from OIB, we found a clear agreement between visible runoff and areas where near-surface firn pore space is depleted. These areas contain metres-thick near-surface ice slabs, which are substantially thicker directly underneath supraglacial streams and lakes.

In our field area close to the visible runoff limit we measured and modelled that up to roughly 80 % of the meltwater refreezes as superimposed ice on top of existing ice slabs, thickening the slabs by between 0.2 to 1 m per year. Ice-sheet-wide estimates show that due to intense refreezing, current ice slab areas contribute only modest amounts of runoff.

While we shed light on the previously understudied area of the Greenland Ice Sheet around the runoff limit, we also revealed that this area is the source of substantial uncertainties in RCM-modelled Greenland surface mass balance. RCM-simulated runoff limits differ strongly between models, either placing them lower or higher than our measurements indicate. The differences between RCM-simulated runoff limits also substantially impact simulated total runoff. Addressing these uncertainties requires improved simulation of meltwater hydrology and refreezing processes near the runoff limit. This is crucial, as firn areas newly affected by surface runoff are projected to continue to expand.