Extraordinary runoff from the Greenland ice sheet in 2012 amplified by hypsometry and depleted firn retention

Abstract

Abstract. It has been argued that the infiltration and retention of meltwater within firn across the percolation zone of the Greenland ice sheet has the potential to buffer up to ∼ 3.6 mm of global sea-level rise (Harper et al., 2012). Despite evidence confirming active refreezing processes above the equilibrium line, their impact on runoff and proglacial discharge has yet to be assessed. Here, we compare meteorological, melt, firn stratigraphy and discharge data from the extreme 2010 and 2012 summers to determine the relationship between atmospheric forcing and melt runoff at the land-terminating Kangerlussuaq sector of the Greenland ice sheet, which drains into the Watson River. The 6.8 km3 bulk discharge in 2012 exceeded that in 2010 by 28 %, despite only a 3 % difference in net incoming melt energy between the two years. This large disparity can be explained by a 10 % contribution of runoff originating from above the long-term equilibrium line in 2012 caused by diminished firn retention. The amplified 2012 response was compounded by catchment hypsometry; the disproportionate increase in area contributing to runoff as the melt-level rose high into the accumulation area.Satellite imagery and aerial photographs reveal an extensive supraglacial network extending 140 km from the ice margin that confirms active meltwater runoff originating well above the equilibrium line. This runoff culminated in three days with record discharge of 3100 m3 s−1 (0.27 Gt d−1) that peaked on 11 July and washed out the Watson River Bridge. Our findings corroborate melt infiltration processes in the percolation zone, though the resulting patterns of refreezing are complex and can lead to spatially extensive, perched superimposed ice layers within the firn. In 2012, such layers extended to an elevation of at least 1840 m and provided a semi-impermeable barrier to further meltwater storage, thereby promoting widespread runoff from the accumulation area of the Greenland ice sheet that contributed directly to proglacial discharge and global sea-level rise.

21st-century climate change around Kangerlussuaq, west Greenland: From the ice sheet to the shores of Davis Strait

“…These small-area totals agree well with the Greenland-wide total of 30 percent°C −1 found by Fettweis et al (2008). With average melt and runoff rates doubled or tripled in the RCP8.5 scenario, massive melt and flooding events such as those affecting the Watson River in 2012 (Mikkelsen et al 2016) will become quite common. Even in the RCP4.5 scenario, several spikes comparable to the ERA-I-driven 2012 value occur when taking into account the 2.4 Gt yr −1 offset between the two present-day references.…”

“…Because the runoff of from the ice sheet is important for the local infrastructure (Mikkelsen et al 2016), we estimate the amount of glacial freshwater input to the Watson River. For this calculation we use the drainage basin definition (see Figure 1) from Lindbäck et al (2014Lindbäck et al ( , 2015, who used a single-direction flow algorithm and surface analysis in order to derive a drainage catchment on the hydraulic potential surface.…”

“…Locally, impacts from increasing amounts of ice-sheet mass loss, as well as regional climate change with increased extreme weather events and changes in long-term climate, have implications for infrastructure, industry, and agriculture. Recent work by Christensen et al (2016) used a combination of global climate models from CMIP5 and high-resolution regional downscaling to determine what the future climate of Greenland will be like and how local inhabitants can best prepare for and adapt to, for example, permafrost degradation, enhanced flooding related to extreme weather events, and high melt rates (Mikkelsen et al 2016), as well as the implications for fisheries, hunting, and agriculture. Climate change can have unpredictable consequences as, for example, in Greenland, where the length of the growing season is expected to increase during the 21st century, but there are also indications that periods of drought will increase.…”

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…A series of warm summers, including the exceptional melt years of 2010 and 2012, have modified the firn structure such that at higher elevations (above~1900 m), the firn has experienced substantial densification, while at lower elevations, the formation of laterally extensive near-surface ice layers has almost certainly reduced the potential for percolation and storage in the deep firn. As a result, meltwater is instead draining more rapidly from the ice sheet surface [20], thereby promoting earlier ice sheet mass loss from higher elevations in response to rising temperatures.…”

“…The volume of meltwater generated at the GrIS surface has increased dramatically in recent years due to an expansion of ice sheet melt extent [12] and enhanced local melt rates [13] the result of: warmer air temperatures [14]; decreasing albedo [15] due to a darkening ice surface caused by increased aeolian dust [16], exposure of dirty Holocene ice [17], increased biota and cryoconite [18] and changing snowpack structure and moisture content [19]; decreased meltwater retention capacity in the firn [20]; and changing radiation budgets associated with synoptically driven changes in cloud cover [21,22]. The impact of this enhanced melt is reflected in the GrIS's increasingly negative balance, contributing 0.5 mm year to sea level between 1991 and 2015, at an accelerating rate of loss of~17 Gt year −2 [23•].…”