Quantification of climate forcing of glacial hydrological systems at the decadal scale is rare because most measurement stations are too far downstream for glacier impacts to be clearly detected. ...Here we apply a measure of daily hydrograph entropy to a unique set of reliable, high‐altitude gauging stations, dating from the late 1960s. We find a progressive shift to a greater number of days with diurnal discharge variation as well as more pronounced diurnal discharge amplitude. These changes were associated with the onset of rapid warming in the 1980s as well as declining end of winter snow depths as inferred from climate data. In glaciated catchments, lower winter snow depths reduce the magnitude and duration of snowpack buffering and encourage the earlier onset of glacier ice exposure, with associated lower surface albedo and more rapid melt. Together, these processes explain the increase in the observed intensity of diurnal discharge fluctuations.
Plain Language Summary
River basins that have a high proportion of ice cover are particularly sensitive to climate warming. Daily variations in insolation and temperature typically lead to fluctuations in snow and/or ice melt and thus a daily rise and fall in river flow. Snow, and the glaciers themselves, can buffer this rise and fall. For six high mountain Alpine basins, we show that daily discharge fluctuations are changing due to climate warming at the decadal scale, with both increasing daily discharge maxima and reducing daily discharge minima. These changes reflect decreased snow accumulation at the end of winter, reducing the buffering and increasing the onset of rapid glacier melt.
Key Points
A Shannon Entropy Index is used to quantify changing daily hydrograph shape for six high‐altitude Alpine basins, with a range of degrees of glacier cover, between 1969 and 2014
For the five most glaciated basins, there has been an increase in the frequency and amplitude of diurnal discharge fluctuations since the onset of more rapid warming in the 1980s
These changes in diurnal discharge are driven by reduced snow buffering of ice melt and runoff resulting from declining end of winter snow depths and greater mean annual temperatures
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Recent acceleration of Greenland's ocean‐terminating glaciers has substantially amplified the ice sheet's contribution to global sea level. Increased oceanic melting of these tidewater glaciers is ...widely cited as the likely trigger, and is thought to be highest within vigorous plumes driven by freshwater drainage from beneath glaciers. Yet melting of the larger part of calving fronts outside of plumes remains largely unstudied. Here we combine ocean observations collected within 100 m of a tidewater glacier with a numerical model to show that unlike previously assumed, plumes drive an energetic fjord‐wide circulation which enhances melting along the entire calving front. Compared to estimates of melting within plumes alone, this fjord‐wide circulation effectively doubles the glacier‐wide melt rate, and through shaping the calving front has a potential dynamic impact on calving. Our results suggest that melting driven by fjord‐scale circulation should be considered in process‐based projections of Greenland's sea level contribution.
Plain Language Summary
As the world warms, loss of ice from the Greenland Ice Sheet will be a significant source of sea level rise. Greenland loses ice partly through the flow of huge rivers of ice called tidewater glaciers that dump solid ice directly into the ocean. Over the past two decades, tidewater glaciers around Greenland have accelerated dramatically, increasing Greenland's contribution to global mean sea level. There is mounting evidence that these accelerations have been driven by ocean warming, and a resulting increase in the rate at which the ocean melts the front of tidewater glaciers (called submarine melting). Yet submarine melting is at present poorly understood, in part due to the danger and difficulty of collecting data close to tidewater glaciers. We present observations of the ocean in front of a tidewater glacier that are unprecedented in their proximity to the glacier. These data reveal an ocean circulation which flushes warm water along the front of the glacier, driving high rates of submarine melting. We then use a numerical model to identify what drives this circulation. Our results are an important step toward understanding a key process which will modulate future sea level contribution from the Greenland ice sheet.
Key Points
Ocean observations that are unprecedented in their spatial detail and proximity to a Greenlandic tidewater glacier are reported
Despite being highly localized, plumes drive fjord‐wide circulation and hence glacier‐wide submarine melting at tidewater glaciers
Fjord‐scale submarine melting drives significant mass loss and may promote calving, hence is a key process determining glacier stability
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Predicting the retreat of tidewater outlet glaciers forms a major obstacle to forecasting the rate of mass loss from the Greenland Ice Sheet. This reflects the challenges of modeling the highly ...dynamic, topographically complex, and data-poor environment of the glacier–fjord systems that link the ice sheet to the ocean. To avoid these difficulties, we investigate the extent to which tidewater glacier retreat can be explained by simple variables: air temperature, meltwater runoff, ocean temperature, and two simple parameterizations of “ocean/atmosphere” forcing based on the combined influence of runoff and ocean temperature. Over a 20-y period at 10 large tidewater outlet glaciers along the east coast of Greenland, we find that ocean/atmosphere forcing can explain up to 76% of the variability in terminus position at individual glaciers and 54% of variation in terminus position across all 10 glaciers. Our findings indicate that (i) the retreat of east Greenland’s tidewater glaciers is best explained as a product of both oceanic and atmospheric warming and (ii) despite the complexity of tidewater glacier behavior, over multiyear timescales a significant proportion of terminus position change can be explained as a simple function of this forcing. These findings thus demonstrate that simple parameterizations can play an important role in predicting the response of the ice sheet to future climate warming.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
The Northern and Southern Patagonian Ice Fields (NPI and SPI) in South America are the largest bodies of ice in the Southern hemisphere outside of Antarctica and the largest contributors to eustatic ...sea level rise (SLR) in the world, per unit area. Here we exploit swath processed CryoSat-2 interferometric data to produce maps of surface elevation change at sub-kilometer spatial resolution over the Ice Fields for six glaciological years between April 2011 and March 2017. Mass balance is calculated independently for nine sub-regions, including six individual glaciers larger than 300 km2. Overall, between 2011 and 2017 the Patagonian Ice Fields have lost mass at a combined rate of 21.29 ± 1.98 Gt a−1, contributing 0.059 ± 0.005 mm a−1 to SLR. We observe widespread thinning on the Ice Fields, particularly north of 49° S. However the pattern of surface elevation change is highly heterogeneous, partly reflecting the importance of dynamic processes on the Ice Fields. The Jorge Montt glacier (SPI), whose tidewater terminus is approaching floatation, retreated ~2.5 km during our study period and lost mass at the rate of 2.20 ± 0.38 Gt a−1 (4.64 ± 0.80 mwe a−1). In contrast with the general pattern of retreat and mass loss, Pio XI, the largest glacier in South America, is advancing and gaining mass at 0.67 ± 0.29 Gt a−1 rate.
•CS2 swath altimetry captures elevation change patterns over complex topography.•Ice dynamics plays important role in driving strong mass losses on the ice fields.•Pio XI, the only glacier with positive mass balance, advanced at both its termini.•Between 2011 and 2017 the Patagonian ice fields lost 21.29 ± 1.98 Gt a−1.•Mass loss is 24% (42%) higher compared to the period 2000–2012/14 (1975–2000).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Arctic glaciers and ice caps (GIC) are losing mass rapidly, and this process is expected to continue during the 21st century owing to polar amplification of climate warming. Here, we use seven years ...of CryoSat-2 swath interferometric altimetry to track changes in the volume of Arctic GIC. From these data, we produce a pan-Arctic assessment of GIC mass imbalance, and we partition their losses into signals associated with atmospheric processes and glacier dynamics. Between 2010 and 2017, Arctic GIC lost 609 ± 7 Gt of ice, contributing 0.240 ± 0.007 mm per year to global sea level rise. While surface ablation is responsible for 87% of losses across the Arctic, dynamic imbalance is increasing in the Barents and Kara Sea region where it now accounts for 43% of total ice loss. Arctic GIC's dynamic imbalance is associated with a northward shift of Atlantic climate, and this effect should be considered in global sea level projections.
•CryoSat-2 interferometric altimetry tracking pan-arctic changes in land ice volume.•High-resolution swath processing to estimate surface ablation and ice discharge.•Arctic glaciers and ice caps display a sustained rate of loss since the early 2000s.•Ice dynamics account for 43% of total mass loss in the Barents and Kara Sea region.•Dynamic imbalance is driven by polar amplification of warming and sea ice decline.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Iceberg discharge is estimated to account for up to 50% of the freshwater flux delivered to glacial fjords. The amount, timing, and location of iceberg melting impacts fjord‐water circulation and ...heat budget, with implications for glacier dynamics, nutrient cycling, and fjord productivity. We use Sentinel‐2 imagery to examine seasonal variations in freshwater flux from open‐water icebergs in Sermilik Fjord, Greenland during summer and fall of 2017–2018. Using iceberg velocities derived from visual‐tracking and changes in total iceberg volume with distance down‐fjord from Helheim Glacier, we estimate maximum average two‐month full‐fjord iceberg‐derived freshwater fluxes of ~1,060 ± 615, 1,270 ± 735, 1,200 ± 700, 3,410 ± 1,975, and 1,150 ± 670 m3/s for May–June, June–July, July–August, August–September, and September–November, respectively. Fluxes decrease with distance down‐fjord, and on average, 86–91% of iceberg volume is lost before reaching the fjord mouth. This method provides a simple, invaluable tool for monitoring seasonal and interannual iceberg freshwater fluxes across a range of Greenlandic fjords.
Plain Language Summary
Recent studies have shown that the freshwater produced via the melting of icebergs can dominate the freshwater budget in glacial fjords surrounding the Greenland Ice Sheet, which has important implications for fjord circulation and heat budget, nutrient availability, and primary productivity. Here we use satellite imagery to estimate both iceberg velocity and the seasonal changes in iceberg volume in Sermilik Fjord in southeast Greenland in 2017–2018, from which meltwater fluxes are derived. Iceberg meltwater fluxes are highest in the late summer and fall, when fjord water temperatures are warmer than in the spring and early summer, and when more icebergs have been calved into the fjord. Throughout the year, the volume of freshwater generated from the melting of icebergs is greater than the freshwater entering the fjord at the base of the glacier and sourced from melting at the ice sheet surface. As such, the melting of icebergs provides a significant volume of freshwater to the fjord system, with important implications for fjord‐scale circulation and heat budget, nutrient cycling, and primary productivity. The methodology presented here is effective, simple and inexpensive, and can be applied to a variety of glacial fjord systems, particularly those that are remote and inaccessible.
Key Points
Freshwater fluxes from iceberg melt in Sermilik Fjord have a seasonal signal, peaking across August and September in 2017 and 2018
Fluxes decrease with distance down‐fjord from Helheim Glacier, with ~86–91% of iceberg volume lost before reaching the fjord mouth
We present a simple and effective tool for monitoring iceberg freshwater fluxes across a range of Greenlandic fjords
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Dynamic change at the marine‐terminating margins of the Greenland Ice Sheet may be initiated by the ocean, particularly where subglacial runoff drives vigorous ice‐marginal plumes and rapid submarine ...melting. Here we model submarine melt‐driven undercutting of tidewater glacier termini, simulating a process which is key to understanding ice‐ocean coupling. Where runoff emerges from broad subglacial channels we find that undercutting has only a weak impact on local submarine melt rate but increases total ablation by submarine melting due to the larger submerged ice surface area. Thus, the impact of melting is determined not only by the melt rate magnitude but also by the slope of the ice‐ocean interface. We suggest that the most severe undercutting occurs at the maximum height in the fjord reached by the plume, likely promoting calving of ice above. It remains unclear, however, whether undercutting proceeds sufficiently rapidly to influence calving at Greenland's fastest‐flowing glaciers.
Key Points
We present a first model for submarine melt‐driven undercutting of calving fronts, a key link between the ocean and the Greenland Ice Sheet
For broad plumes, undercutting has a weak impact on local submarine melt rate but increases total melt due to increased ice surface area
Undercut shape is determined by discharge and fjord properties, with steepest undercuts occurring at the maximum height reached by the plume
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Purpose of the Review
This review discusses the role that meltwater plays within the Greenland ice sheet system. The ice sheet’s hydrology is important because it affects mass balance through its ...impact on meltwater runoff processes and ice dynamics. The review considers recent advances in our understanding of the storage and routing of water through the supraglacial, englacial, and subglacial components of the system and their implications for the ice sheet.
Recent Findings
There have been dramatic increases in surface meltwater generation and runoff since the early 1990s, both due to increased air temperatures and decreasing surface albedo. Processes in the subglacial drainage system have similarities to valley glaciers and in a warming climate, the efficiency of meltwater routing to the ice sheet margin is likely to increase. The behaviour of the subglacial drainage system appears to limit the impact of increased surface melt on annual rates of ice motion, in sections of the ice sheet that terminate on land, while the large volumes of meltwater routed subglacially deliver significant volumes of sediment and nutrients to downstream ecosystems.
Summary
Considerable advances have been made recently in our understanding of Greenland ice sheet hydrology and its wider influences. Nevertheless, critical gaps persist both in our understanding of hydrology-dynamics coupling, notably at tidewater glaciers, and in runoff processes which ensure that projecting Greenland’s future mass balance remains challenging.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We apply swath processing to CryoSat‐2 interferometric mode data acquired over the Icelandic ice caps to generate maps of rates of surface elevation change at 0.5 km postings. This high‐resolution ...mapping reveals complex surface elevation changes in the region, related to climate, ice dynamics, and subglacial geothermal and magmatic processes. We estimate rates of volume and mass change independently for the six major Icelandic ice caps, 90% of Iceland's permanent ice cover, for five glaciological years between October 2010 and September 2015. Annual mass balance is highly variable; during the 2014/2015 glaciological year, the Vatnajökull ice cap (~70% of the glaciated area) experienced positive mass balance for the first time since 1992/1993. Our results indicate that between glaciological years 2010/2011and 2014/2015 Icelandic ice caps have lost 5.8 ± 0.7 Gt a−1 on average, ~40% less than the preceding 15 years, contributing 0.016 ± 0.002 mm a−1 to sea level rise.
Key Points
Icelandic ice cap elevation change is the result of complex interaction between climate, ice dynamics, and geothermal and magmatic processes
Estimated rate of Icelandic mass loss between 2010 and 2015 is 5.8 ± 0.7 Gt a‐1 contributing 0.016 ± 0.002 mm a‐1 to sea level rise
Swath processing improves on conventional POCA radar altimetry by providing elevation change with a twofold increase in spatial coverage
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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