Approximately half of Greenland’s current annual mass loss is attributed to runoff from surface melt1. At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn2. Two recent studies suggest that all3 or most3,4 of Greenland’s firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come3. Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012 (refs 5,6), revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than ∼1,900 m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient7 surface discharge system and intensifying ice sheet mass loss earlier than previously suggested3.
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This work is supported by the US National Aeronautics and Space Administration (NASA) Grant no. NNX10AR76G, ‘Comprehensive Assessment of Ice Sheet Contributions to Sea Level Based on Integrated Remote Sensing Observations’, by the Greenland Analogue Project (GAP), funded by Svensk Kärnbränslehantering AB, Sweden, Posiva Oy, Finland, and NWMO, Canada, the Refreeze Project funded by GEUS, the RETAIN project, funded by the Danish Council for Independent research (Grant no. 4002-00234) and the Programme for Monitoring of the Greenland Ice Sheet (PROMICE), funded by The Danish Energy Agency DANCEA programme. Collection and analyses of the legacy cores was supported by NASA’s PARCA Program. The K-transect programme has been funded by Utrecht University, the Netherlands Polar Program of NWO/ALW and a Spinoza grant. This publication is contribution number 62 of the Nordic Centre of Excellence SVALI, ‘Stability and Variations of Arctic Land Ice’, funded by the Nordic Top-level Research Initiative (TRI). The authors acknowledge field assistance by K. Alley, A. Crawford, S. Doyle, M. Eijkelboom, S. Grigsby, D. Hill, A. Heilig, A. Hubbard, K. Lindbäck, R. Petterson and M. Stevens as well as logistical contributions from W. Abdalati, R. Bauer, A. Hubbard and T. Scambos. Satellite imagery in Supplementary Fig. 1 is subject to copyright by European Space Imaging/DigitalGlobe.
The authors declare no competing financial interests.
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Machguth, H., MacFerrin, M., van As, D. et al. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Clim Change 6, 390–393 (2016). https://doi.org/10.1038/nclimate2899
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