The Greenland Ice Sheet loses mass and is currently the largest contributor to global sea level rise. Meltwater running off the flanks of the ice sheet contributes roughly 60% to its mass loss, the rest being due to calving. Only meltwater originating from below the elevation of the runoff limit leaves the ice sheet, contributing to mass loss; melt at higher elevations refreezes in the porous subsurface and does not drive mass loss. Therefore, any shift in the runoff limit modifies mass loss and subsequent sea level rise.
The CASSANDRA project created a comprehensive understanding of how melting progresses to higher elevations on the Greenland Ice Sheet, how increased melting has driven the runoff limit to higher elevations and thereby expanded the area experiencing meltwater runoff. These changes were studied using large amounts of satellite imagery and radar data, by performing several field campaigns and through computer modelling.
The CASSANDRA project has shown that meltwater nowadays runs off from higher elevations on the ice sheet than it did a few decades ago. The reason is substantial and regular melting that now occurs at high elevations where melting was rare in the past. Runoff from these areas is visible as streams of meltwater running across the ice sheet surface in summer. However, our work has also shown that those streams evacuate only a relatively modest fraction of total melt. The majority of the melt occurring in the newly formed runoff areas refreezes before reaching a stream. The refreezing, however, forms thick layers of ice which coat the ice sheet surface and promote runoff and mass loss, in particular during extraordinarily warm summers.
Melting is projected to further increase in the future and consequently Greenland’s runoff area will continue to grow. The CASSANDRA project has created a thorough understanding of the processes and the mass balance of new runoff areas. This knowledge will be essential to improved estimates of Greenland’s future mass balance.
