Mass budget of the glaciers and ice caps of the Queen Elizabeth Islands, Canada, from 1991 to 2015

The glaciers and ice caps of the Queen Elizabeth Islands (QEI), Canada cover an area of 105 000 km², which represents 25% of the Arctic land ice outside Greenland (Lenaerts et al 2013, Van Wychen et al 2014). This region is divided into eight major ice caps and ice fields: 1) Northern Ellsmere Icefield (NEI), 2) Agassiz Ice Cap, 3) Prince of Wales Icefield (POW), 4) Devon Ice Cap (DIC), 5) Müller Ice Cap, 6) Steacie Ice Cap, 7) Sydkap Ice cap, and 8) Manson Icefield (figure 1). Time series of time-variable gravity and altimetry data indicate that the mass loss from QEI increased significantly to an average of 39 ± 9 Gt yr⁻¹ for the time period 2004–2009, mostly the result of ice melt by warmer surface air temperature (Gardner et al 2011). A combination of surface mass balance (SMB) output products from the Regional Atmospheric Climate Model Version 2 (RACMO-2) and in-situ observations suggest that the QEI was close to a state of mass balance before year 2000 and started to lose mass from enhanced runoff after 2005 at an average rate of 35 ± 18 Gt yr⁻¹ for the time period 2005–2011 (Lenaerts et al 2013).

Complete mapping of 2012 ice velocity (Van Wychen et al 2014) suggested that the loss from ice discharge is small compared to enhanced runoff and less than 10% of the total mass balance (Gardner et al 2011, Van Wychen et al 2014, Short and Gray 2005, Van Wychen et al 2016). The Randolph Glacier Inventory (RGI 3.2) (Arendt et al 2012), however, indicates that about half of the QEI is drained by marine-terminating glaciers: 254 marine terminating glaciers drain an ice area of 48 360 km² versus 4 284 land-terminating glaciers draining an area of 56 513 km². Changes in ice dynamics of tide-water glaciers may play a strong role in the mass balance of the QEI. In Greenland, for instance, 40%–50% of the mass loss is driven by the flow of marine-terminating glaciers, with the rest of the mass loss being controlled by an increase in runoff (e.g Rignot et al 2008). It is therefore of interest to investigate the partitioning of the mass balance of the QEI glaciers and extend the current time series over several decades to understand how it has been evolving with time and identify the physical processes behind those changes.

Here, we present a comprehensive analysis of the tidewater glaciers in the QEI between 1991 and 2015, including Devon Ice Cap, one of the largest contributors to ice discharge in this region, hence significantly expanding upon recent time series (Van Wychen et al 2016). We combine ice velocity from satellite data with ice thickness data from NASA's Operation IceBridge (OIB) from year 2012 and 2014 and surface mass balance (SMB) output products from RACMO-2.3 to determine the mass balance of the QEI, its temporal evolution over the last 25 years, and the exact partitioning of the mass loss between ice dynamics and surface mass balance processes, which has never been directly measured before on such time period. We conclude on the relative importance of ice dynamics in the overall mass budget of the QEI over the last 25 years.

2.1. Ice discharge

2.2. Surface Mass Balance

The velocity maps clearly separate marine-terminating glaciers where ice speed increases continuously up to the ice front, from land-terminating glaciers where ice speed is maximal near the equilibrium line elevation and then decreases to zero at the ice front (figure 1). The mapping extends from the ice divides to the ice fronts, i.e. covers the entire ice masses. We observe significant fluctuations in glacier speed over time.

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The Northern Ellesmere Icefield (NEI) includes large-size marine-terminating glaciers on its western and northern flanks (Otto, Milne, Yelverton, M'Clintock and Disraeli) and land-terminating glaciers along its southern and eastern flanks that represent more than 60% of the ice area (16 871 km², figure 1(a) and table S1). The ice discharge from NEI averaged 0.48 ± 0.08 Gt yr⁻¹ between 1991 and 2000 (table S1). The discharge increased after 2001 to peak at 0.53 ± 0.12 Gt yr⁻¹ in 2006 during the surge of Otto Glacier (table S1 and figure S5). Ice discharge then dropped to 0.31 ± 0.1 Gt yr⁻¹ in 2015 due to a decrease in speed of most glaciers. Otto Glacier experienced a significant slow down between 1991 and 2015 with a frontal speed dropping from > 800 m yr⁻¹ in 1991 to <50 m yr⁻¹ in 2013–2015 (figure S5). After 2010, more than half of the ice discharge of NEI was driven by Yelverton and Milne glaciers with an average total of 0.15 ± 0.06 Gt yr⁻¹ between 1991 and 2015 (table S1).

The Agassiz Ice Cap, between the POW and NEI, comprises a large number of marine-terminating glaciers that represents 8 844 km² (40% of the ice area, figure 1(b) and table S1). The ice discharge was nearly constant between 1991 and 2000 with an average of 0.28 ± 0.05 Gt yr⁻¹. An average mass flux of 0.5 ± 0.04 Gt yr⁻¹ is calculated between 2005 and 2007 due to the combined surge of Parrish Glacier and Glacier 5 (figure S3 and table S1). The mass flux of 60% of the glaciers decreased during 2010–2015 compared to 1991–2010 (figure 3, figure S3, table S1). Cañon, Glacier 4, Dobbin and Tuborg Glaciers are the only ones that maintain a steady state regime during 2000–2015.

The POW is located in the central part of Ellesmere Island between 76 °N and 78 °N (figure 1(d)). The western part only includes land-terminating glaciers whereas the eastern part is drained by large marine-terminating glaciers that drain >60% of the total area (table S1). The ice discharge of POW averaged 1.6 ± 0.1 Gt yr⁻¹ between 1991 and 2008 and increased by a factor of 1.5 since then. During the time period 2009–2015, D increased to 2.46 ± 0.4 Gt yr⁻¹ (figure 3 and table S1). 83% of the glaciers slowed down or maintain a steady regime, however, so the increase in D was only due to the acceleration of two glaciers: Trinity and Wykeham (figure 2, figure 3 and table S1). Draining from a common basin, these glaciers flow several times faster than any other glacier in the QEI (figure 1). Their ice speed (figure 2) was stable in 1991–2009, accelerated by a factor of two to 1 200 m yr⁻¹ for Trinity and 650 m yr⁻¹ for Wykeham in 2015. Examination of the position of the ice fronts reveals a continuous retreat of Trinity Glacier since 1991 by 5 km (figure 2). The retreat amounts to 7.7 km since 1976 (figure 1(d)). Wykeham Glacier retreated 1.8 km between 1991 and 2015 and 4.2 km between 1976 and 2015. At present, the northern part of Wykeham ice front is no longer retreating (figure 2).

South of POW, on Sydkap Ice Cap and Manson Icefield (figure 1(f)), Sydkap Glacier ice discharge dropped from 0.04 ± 0.001 Gt yr⁻¹ in 1991 to 0.02 ± 0.002 Gt yr⁻¹ in 2010 and slightly increased to 0.05 ± 0.001 Gt yr⁻¹ in 2015 (table S1). In contrast, we calculate a mass flux in 2000 of 0.9 ± 0.01 Gt yr⁻¹ for Mittie Glacier, followed by a decrease to an average 0.02 ± 0.002 Gt yr⁻¹ for 2003–2015. Similar abrupt changes in ice discharge are detected for Iceberg and Good Friday Bay glaciers on Axel Heiberg Islands in 1991 (figure 1(c) and S7).

Finally, the DIC is characterized by a dominance of marine-terminating glaciers (10 275 km² vs. 4 722 km² for land-terminating glaciers) (figure 1(e), 2). The discharge of the entire ice cap increased from 0.46 ± 0.1 Gt yr⁻¹ to 0.53 ± 0.1 Gt yr⁻¹ between 1991 and 2015. However the increase in D is within the uncertainty, so we cannot draw solid conclusions on the evolution of this sector. Belcher, South Croker Bay and Fitzroy are the fastest moving glaciers, with speeds of 300 m yr⁻¹ at their calving fronts. Most glaciers have maintained a steady flow regime since 1991 and the increase in ice discharge is mainly due to the 50% acceleration of Belcher during this period (table S1, figure S6). North Croker Bay displays a significant slow down to zero frontal speed in 2015, which suggests that the glacier may have detached from the ocean or have become analogous to a land terminating glacier.

Overall, more than 60% of the ice discharge was measured for 9 years (outside interpolations and extrapolations) compared to the 5 years of recent studies (Van Wychen et al 2016), with crucial new data between 2004 and 2010. Importantly, our discharge results extend these studies further back in time from 2000–2015 to measurements between 1991 and 2016, which is important to draw conclusions about long term changes in ice dynamics. Differences in total surveyed ice discharge for common years results from the addition of Devon Ice Cap in our study, which is the second highest contributor to the total ice discharge and from differences in survey area. We estimate that Devon Ice Cap contributes an average 0.46 ± 0.01 Gt yr⁻¹ discharge over 1991–2015, in agreement with Van Wychen et al (2012) for year 2009. The total ice discharge is on average 1.4 ± 0.5 Gt yr⁻¹ higher than in Van Wychen et al (2016) for complete years (i.e. 2000, 2011, 2013, 2014 and 2015).

The difference between surge and pulse type glaciers is defined by the velocity structure, and particularly in the region of initiation and propagation (Van Wychen et al 2016). As the definition of pulse-type glacier vary from study to study (e.g. Mayo (1978), Raymond (1987), Turrin et al (2014), Van Wychen et al (2016)) and because the difference between these two behaviors is not crucial in the calculation of the mass balance, surge and pulse type glaciers are both referenced as 'surge type' glaciers hereafter.

Among the 43 major glaciers surveyed in this study, 65% did not change speed or even slowed down. We detect 6 major surging glaciers: Mittie, Parrish, Dobbin, Good Friday Bay, Middle and Iceberg glaciers that were also documented in Copland et al (2003) and Van Wychen et al (2016). However, we report here new informations on the important increase in speed of d'Iberville, Glacier 9 and Glacier 11. The 300 m yr⁻¹ speed of d'Iberville glacier in 1991 was three times higher than for the period 2000–2015. In 2007, Glacier 11 quadrupled its ice discharge from 0.01 ± 0.01 Gt yr⁻¹ to 0.04 ± 0.01 Gt yr⁻¹. Similarly, Glacier 9 increased its ice discharge to 0.2 ± 0.06 Gt yr⁻¹ in 2006, which is 8 times higher than in 2000 and for the period 2010–2015. Evidence of surge features such as loop moraines, intensive crevassing or dramatic terminus position are lacking at this time for d'Iberville glacier in 1992. No such features were found on Glacier 11 and Glacier 9. Analysis of the thinning pattern would be necessary to draw conclusions on the changes in dynamics of these glaciers and to determine if it can be attributed to surge episodes. In total, however, the ice discharge from all these glaciers remains low, even after accounting for the surges and short term peak in D. The average D for D'Iberville, Dobbin, Parrish, Otto, Good Friday Bay, Iceberg, Mittie, Glacier 11 and Glacier 9 are, respectively, 0.02, 0.04, 0.04, 0.1, 0.1, 0.04, 0.1, 0.01 and 0.07 Gt yr⁻¹ (average of errors across all surging glaciers is below 0.005 Gt yr⁻¹). As a result of the surges that took place during the period 1991–2015, we estimate that D increased by 0.2 Gt/yr or 6%. The effect of surges and peak in ice discharge on the decadal mass balance of the glaciers is therefore small.

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In total for QEI, we find that D decreased from 4.5 ± 0.5 Gt yr⁻¹ in 1991 to 3.5 ± 0.5 Gt yr⁻¹ in 2000 due to a decrease in flow speed of several glaciers (figure 3 and table S1). Between 2000 and 2015, D remained constant because the slow down of most glaciers was compensated by the speed up of Trinity and Wykeham glaciers. At the local scale, however, D is a significant component of the mass balance for Trinity and Wykeham glaciers. Both glaciers experienced the largest increase in ice speed of the entire archipelago. Analysis of the SMB record (figure 2) reveals that these glaciers were in balance between 1991 and 2008 with an ice discharge of 0.6 ± 0.02 Gt yr⁻¹ (figure 2) versus a SMB of 0.83 ± 0.2 Gt yr⁻¹. After 2008, the mass balance decreased from +0.25 ± 0.1 Gt yr⁻¹ (i.e. a mass gain) to −1.0 ± 0.2 Gt yr⁻¹. This increase in mass loss is due to a doubling in runoff production from 0.43 ± 0.02 Gt yr⁻¹ to 0.83 ± 0.1 Gt yr⁻¹ and a doubling in ice discharge from 0.6 ± 0.02 Gt/yr to 1.5 ± 0.1 Gt yr⁻¹ after 2009. The increase in glacier speed tripled the mass loss caused by enhanced runoff alone. In total, these two glaciers contributed a 37% increase in total discharge from QEI since 2010 (figure 3). Yet, this is the only sector in QEI where changes in speed have had a significant impact on total mass balance.

During the time period 1991–2005, the QEI was losing mass at a rate of 6.3 ± 1.1 Gt yr⁻¹ (figure 3). Ice discharge contributed 52% of the losses (3.4 ± 0.1 Gt yr⁻¹ discharge vs 3.0 ± 1.1 Gt yr⁻¹ from SMB). After 2005, the mass budget became strongly negative due to a significant increase in runoff that reduced SMB to −55.6 ± 16.6 Gt yr⁻¹ in 2012 (figure 3(b)). During that time period, there was no trend in precipitation, which averaged 28 ± 2.9 Gt yr⁻¹ (figure 3(b)). The loss due to ice discharge of 3.5 ± 0.2 Gt yr⁻¹ therefore contributed only 10% of the total loss, i.e. the loss was dominated by enhanced runoff. This is consistent with previous studies over shorter time periods (Gardner et al 2011).

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The transition in mass balance coincides with a marked increase in summer air temperature around year 2004 that has continued until present (Gardner et al 2011). In the period 2013–2014, runoff production decreased and SMB increased from −55.6 ± 16.6 Gt yr⁻¹ to +8.2 ± 2.5 Gt yr⁻¹. This pause in mass loss resulted from colder-than-average consecutive summers following the large melt event of 2012 (Tedesco et al 2015, Harig and Simons 2016). Over the entire time period 1991–2014, the mass loss averaged 16.0 ± 1.7 Gt yr⁻¹, with an acceleration of 1.0 ± 0.1 Gt yr⁻².

Interestingly, we can notice that mass balance estimates vary significantly with respect to the methods. Indeed, calculation from straight altimetry or gravimetry shows values of 27 ± 7 Gt yr⁻¹ and 24 ± 6 Gt yr⁻¹ for 2003–2010 (Nilsson et al 2015, Colgan et al 2014). This number are much lower than the 34 ± 13 Gt yr⁻¹ from Gardner et al (2011) for 2004–2009 that are closer from the 35 ± 18 Gt yr⁻¹ during 2005–2011 (Lenaerts et al 2013) and the 33.0 ± 3.0 Gt yr⁻¹ during 2005–2014 from this study. Thus, it seems that the uses of an input-output method, with a surface mass balance model gives larger estimates than a mass balance derived only from altimetry or gravity.

To understand the evolution of marine-terminating glaciers in POW, it is essential to investigate their interaction with the surrounding ocean because the intrusion of warm ocean waters have the potential to melt ice in contact with it, undercutting the ice fronts and increasing iceberg calving (Motyka et al 2003, O'Leary and Christoffersen 2013, Bartholomaus et al 2013). Ocean currents along POW are driven by the inflow from Nares Strait (see figure 1 for location). Munchow et al (2011) report an increase in ocean temperature in Nares Strait after year 2007 which would suggests that ice melting by the ocean must have increased significantly around that time. Enhanced melting of the calving margins may have dislodged the glaciers from their stabilizing positions after 2010, when Trinity and Wykeham start to accelerate and increase their ice discharge significantly (table S1, figure 2). From these observations, we suggest that ice melting by the ocean must have started to increase prior to 2010. Details on the bathymetry, glacier bed topography, and ocean temperature in front of the glaciers are, however, lacking at this time to make any firm conclusions about the changes in this sector.

We find that 9 glaciers reduced their frontal speed to zero during the time period (figure S3 to S7), suggesting that they lost contact with the ocean, hence stopped calving, as they retreated to higher ground. Alternatively, these glaciers are surge type glaciers that transitioned to a quiescent phase. An analysis of Landsat-8 2015 images indicate that the glaciers were still all in contact with the ocean because we find no moraine deposit separating the ice front from seawater (figure S8). Hence, these glaciers remain marine terminating. We note, however, that most of them are rather thin at the ice front, and less than 100 m thick (figure S8). At this level of ice thickness, the driving stress is quite low, i.e. ice may not deform internally (Hutter 1982). Hence, these glaciers, while reaching the ocean, are more similar to land-terminating glaciers than to tidewater glaciers.

The mass loss of the QEI averaged over its 105 000 km² area is 0.17 ± 0.05 m yr⁻¹ water-equivalent (w.e) for the time period 1991–2014 (0.06 ± 0.02 m yr⁻¹ for 1991–2005 and 0.34 ± 0.03 m yr⁻¹ for 2005–2014). For comparison, Alaskan glaciers experienced a mass loss of 75 ± 11 Gt yr⁻¹ for the time period 1994–2013 (Larsen et al 2015), i.e. an average loss of 0.7 ± 0.1 m yr⁻¹ w.e over their 85 000 km² area, or 4 times larger. Conversely, the mass loss of the Patagonia Icefields averaged 24.4 ± 1.4 Gt yr⁻¹ between 2000 and 2012 (Willis et al 2012b), which corresponds to an average mass loss of 1.3 ± 0.1 m yr⁻¹ w.e over their area of 21 000 km², or 7.6 times larger than for the QEI. Hence, the average mass loss per unit area decreases markedly from low latitudes (Patagonia) to high latitudes (QEI). The QEI currently contributes more mass loss than Patagonia because of its larger area, but we posit that a change in global mean surface air temperature of 0.3 °C–0.5 °C for the period 2016–2035 as projected by the IPCC report (IPCC 2013) could potentially increase the mass loss of the QEI to reach the values observed in Patagonia, which is one order magnitude larger per unit area.

We present the largest time series of glacier velocity and mass balance for the Queen Elizabeth Islands, in Canada spanning from 1991 to 2015 with a complete velocity mapping at 100 m spacing. The acceleration of Trinity and Wykeham between 2010–2015 increased the ice discharge by 37% and accounted for 50% of the total ice discharge from the QEI to the ocean in 2015. Yet, even if ice discharge can play an important role at the basin scale, its contribution to the total mass loss from these icefields remains small. Similarly, glacier surges do not have a significant impact on the long-term mass balance of the QEI. The vast majority of the mass loss is controlled by SMB processes, mainly runoff. Prior to 2005, the rate of mass loss was low and controlled by ice discharge. After 2005, the mass loss increased markedly to transform the QEI into a major contributor to sea level change. With ongoing, sustained, rapid warming of the high Arctic, the mass loss of QEI should continue to increase significantly in the coming decades to century.

This work was performed at the University of California Irvine and at Caltech Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration Cryosphere Science Program (NNX13AI84A, NNX14AN03G, NNX14AB93G). Velocity data from QEI are available upon request from the authors and will be posted at the National Snow and Ice Data Center, Boulder, Colorado.