Extensive liquid meltwater storage in firn within the Greenland ice sheet

Abstract

Mass loss from the Greenland ice sheet contributes significantly to present sea level rise1. High meltwater runoff is responsible for half of Greenland’s mass loss2. Surface melt has been spreading and intensifying in Greenland, with the highest ever surface area melt and runoff recorded in 20123. However, how surface melt water reaches the ocean, and how fast it does so, is poorly understood. Firn—partially compacted snow from previous years—potentially has the capacity to store significant amounts of melt water in liquid or frozen form4, and thus delay its contribution to sea level. Here we present direct observations from ground and airborne radar, as well as ice cores, of liquid water within firn in the southern Greenland ice sheet. We find a substantial amount of water in this firn aquifer that persists throughout the winter, when snow accumulation and melt rates are high. This represents a previously unknown storage mode for water within the ice sheet. We estimate, using a regional climate model, aquifer area at about 70,000 km2 and the depth to the top of the water table as 5–50 m. The perennial firn aquifer could be important for estimates of ice sheet mass and energy budget.

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Figure 1: Perennial firn aquifer (PFA) locations on the Greenland ice sheet detected by radar and firn cores in April 2011.
Figure 2: Profile of the top of the PFA from GPR along the ACT-11 traverse including PFA firn-core locations (ACT11-A and ACT11-A2).
Figure 3: Annual snow accumulation (1958–2008) from regional climate model with output calibrated by ice core values10 (colour).
Figure 4: Modelled liquid water content (LWC) in the firn and detected PFA from airborne radar.

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Acknowledgements

This work was supported by National Science Foundation Office of Polar Programs Award ARC-0909499, ARC-0909469, and the Polar Program of the Netherlands Organization for Scientific Research (NWO/ALW). We acknowledge the use of data and/or data products from Center for Remote Sensing of the Ice Sheets generated with support from NSF grant ANT-0424589 and NASA grant NNX10AT68G. CH2MHill Polar Field Services provided vital logistical support. The NASA airborne radar data can be obtained free of charge from the National Snow and Ice Data Center (IceBridge Accumulation Radar L1B Geolocated Radar Echo Strength Profiles, April–May 2010).

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R.R.F. and J.E.B conceived the idea of the analysis. M.R.v.d.B., J.H.v.A. and J.T.M.L. conducted the modelling. C.M. processed and analysed the GPR data. E.W.B and C.M. collected the field data. L.S.K., J.P. and S.P.G. assisted with airborne radar data processing and identification of melt features. C.L., S.P.G. and C.L. developed the airborne radar and assisted in its interpretation. J.R.M. dated and analysed the firn cores. R.R.F. analysed the airborne radar. R.R.F., J.E.B. and M.R.v.d.B. wrote the manuscript. All authors commented on the data and the manuscript.

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Correspondence to Richard R. Forster.

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The authors declare no competing financial interests.

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Forster, R., Box, J., van den Broeke, M. et al. Extensive liquid meltwater storage in firn within the Greenland ice sheet. Nature Geosci 7, 95–98 (2014). https://doi.org/10.1038/ngeo2043

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