Periodic Reporting for period 4 - CASSANDRA (Accelerating mass loss of Greenland: firn and the shifting runoff limit)
Periodo di rendicontazione: 2023-11-01 al 2024-10-31
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.
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.
The CASSANDRA team carried out a total of nine field campaigns, thereof six to the Greenland Ice Sheet. On the ice sheet we observed the meltwater hydrology at the runoff limit, we measured the refreezing of meltwater and we established a network of 10 automatic measuring stations. In an associated project, we also measured the surface velocity of ice flow.
We worked intensively with satellite data and developed computer models. By means of satellite imagery we measured Greenland-wide changes in surface meltwater occurrence. The computer models simulate how meltwater interacts with the snow and firn.
Our research has shown that nowadays surface meltwater occurs over a 30 % larger area than in he late 1980s. The area of surface meltwater occurrence peaked in 2012 and has remained at high levels since. Our research has also demonstrated an important feedback mechanism where increased melting renders the originally porous near-surface snow more icy, which reduces its meltwater permeability and promotes surface water ponding or runoff.
The growing extent of surface meltwater means that more meltwater runs off into the oceans and ice sheet mass loss increases. Our research, however, has shown that current models need improvement to accurately simulate the observed increase of the area subject to meltwater runoff. Our fieldwork demonstrated that surface meltwater occurrence does not mean that all of this water runs off into the oceans. We measured that a considerable part of the water refreezes instead of leaving the ice sheet. We have also shown that models disagree on the fraction of meltwater that refreezes.
Our results have been published in numerous scientific publications and presented at several conferences. We also organized conferences and workshops and gave numerous presentations to the general public. Our work is already used to improve the computer models that simulate the current and future Greenland Ice Sheet.
We worked intensively with satellite data and developed computer models. By means of satellite imagery we measured Greenland-wide changes in surface meltwater occurrence. The computer models simulate how meltwater interacts with the snow and firn.
Our research has shown that nowadays surface meltwater occurs over a 30 % larger area than in he late 1980s. The area of surface meltwater occurrence peaked in 2012 and has remained at high levels since. Our research has also demonstrated an important feedback mechanism where increased melting renders the originally porous near-surface snow more icy, which reduces its meltwater permeability and promotes surface water ponding or runoff.
The growing extent of surface meltwater means that more meltwater runs off into the oceans and ice sheet mass loss increases. Our research, however, has shown that current models need improvement to accurately simulate the observed increase of the area subject to meltwater runoff. Our fieldwork demonstrated that surface meltwater occurrence does not mean that all of this water runs off into the oceans. We measured that a considerable part of the water refreezes instead of leaving the ice sheet. We have also shown that models disagree on the fraction of meltwater that refreezes.
Our results have been published in numerous scientific publications and presented at several conferences. We also organized conferences and workshops and gave numerous presentations to the general public. Our work is already used to improve the computer models that simulate the current and future Greenland Ice Sheet.
The CASSANDRA project has moved beyond state-of-the-art by focussing on those areas of the Greenland Ice Sheet that only recently started to experience regular and substantial melting. We worked for extended periods in these regions, which are difficult to access due to their remoteness from Greenland’s logistic bases and where research is challenging, especially in summer when the snow is soaked with water. We developed and applied novel methods to measure firn hydrology, firn permeability and refreezing within these areas.
We further progressed beyond current state-of-the-art by combining our new field measurements with newly developed remote sensing approaches and models. Together, field measurements, remote sensing and modelling allowed us to gain a novel and detailed understanding of the physical processes that take place within newly formed melt areas. We were also able to quantify the mass budget of these new melt areas which, together with the understanding of the physical processes, paves the ground for improved estimates of Greenland’s future mass balance.
We further progressed beyond current state-of-the-art by combining our new field measurements with newly developed remote sensing approaches and models. Together, field measurements, remote sensing and modelling allowed us to gain a novel and detailed understanding of the physical processes that take place within newly formed melt areas. We were also able to quantify the mass budget of these new melt areas which, together with the understanding of the physical processes, paves the ground for improved estimates of Greenland’s future mass balance.