Reduced refreezing accelerates mass loss of northern Greenland glaciers and ice caps

Transcription

1 Reduced refreezing accelerates mass loss of northern Greenland glaciers and ice caps Brice Noël* 1, W. J. van de Berg 1, H. Machguth 2,3,4, I. Howat 5, S. Lhermitte 6, M. Citterio 4, J. T. M. Lenaerts 1, B. Wouters 1 & M. R. van den Broeke 1 1 Institute for Marine and Atmospheric research Utrecht, University of Utrecht, Netherlands. 2 Department of Geography, University of Zurich, Zurich Switzerland. 3 Department of Geosciences, University of Fribourg, Fribourg, Switzerland. 4 Geological Survey of Denmark and Greenland GEUS, Copenhagen Denmark. 5 Byrd Polar Research Center and School of Earth Sciences, Ohio State University, Columbus, US. 6 KU Leuven, Department of Earth & Environmental Sciences, Leuven, Belgium. The Greenland ice sheet (GrIS) and its peripheral glaciers and ice caps (GICs) contribute significantly to current sea level rise. Although patterns of GrIS mass loss are relatively wellstudied, the spatial and temporal evolution of GICs mass loss has remained unclear. Here, we use a novel combination of remote sensing and regional climate modelling to show that mass loss from GICs has accelerated during the last 25 years ( 1.5±0.3 Gt/yr 2 ). Averaged over , Greenland s GICs lost 31±8 Gt/yr of mass, a threefold increase compared to Southern Greenland GICs exhibit steady loss rates, but the northern GICs loss rates have undergone a marked acceleration since We show that pronounced regional warming in north Greenland intensified surface melt with a similar increase in runoff, caused by decreased refreezing capacity of the deteriorating firn layer. Currently, GICs account for 1

2 12% of the total glacial mass loss from Greenland and will play an increasingly important role in future sea level rise. Greenland s peripheral glaciers and ice caps (GICs) cover an area of 90,000 km 2 ( 5% of the GrIS and 12% of the world s GICs glaciated area) with a total estimated volume of 38.9 mm SLE 1. According to recent estimates 2, GICs account for 14 to 20% of the total current Greenland glacial mass loss, although they only store an estimated 0.5% of the total GrIS ice volume. Under continued warming through the 21 st century, GICs might yield up to mm of sea level rise by , with a dominant contribution of meltwater runoff over ice discharge 4 6. Despite multiple in-situ observation campaigns since the early 1950s 7, 8, glacier modelling 9 and satellite-based estimates, little remains known about the spatial and temporal distribution of GICs mass loss. To fill these spatial and temporal gaps, regional climate models (RCMs) are often used to estimate the mass loss of the GrIS and surrounding ice caps However, the horizontal resolution used in RCMs, typically 5 to 20 km, fails to resolve the steep surface mass balance (SMB) gradients in these topographically complex regions 20. To resolve these glaciated features and accurately reproduce their SMB changes, a km resolution is needed. 2, 10 1 Methods and evaluation To quantify the contribution of GICs to total Greenland glacial mass loss, we use a novel, daily SMB product at 1-km resolution ( ), statistically downscaled from the output of the regional atmospheric climate model RACMO2.3 at 11-km resolution. The SMB components were projected onto a down-sampled, 1-km version of the topography and ice mask of the Greenland 2

3 Ice Mapping Project (GIMP) Digital Elevation Model (DEM) 21 (Figs. S1a and b). The downscaled SMB product using elevation dependence alone is hereafter referred to as v0.2. The more recent v1.0 also incorporates a bare ice albedo correction to reduce melt underestimates, especially on low-lying glacier tongues. In this correction procedure, the systematic negative precipitation bias of RACMO2.3 in the accumulation zone ( 37.5 mmwe/yr 20 ) is also eliminated. For further information about the downscaling procedure, the reader is referred to the Supplementary Information and Noël et al. (2016) 20. This method was successfully applied to the contiguous GrIS 20 and has now been extended to the GICs. Here, we used the PROMICE ice mask 22, down-sampled to 1-km, to make the distinction between the contiguous ice sheet and detached ice masses (Fig. S1b). To evaluate the downscaled dataset, ablation measurements (yellow dots in Fig. 1) were retrieved from a newly compiled dataset 8, with 230 sites on the contiguous GrIS and 101 sites on GICs. Downscaled and observed SMB values were compared, using the exact number of days in the observations. Fig. S3 compares a) downscaled SMB v0.2, and b) downscaled SMB v1.0 for all 965 SMB measurements at 101 GICs sites. Note how the downscaled product v1.0 improves on v0.2, especially for strong ablation (SMB < -2 mwe), explaining 77% of variance up from 62% in v0.2. However, a small ( 100 mmwe) negative bias still remains in v1.0 (Fig. S3b) and this information was used to estimate the product s integrated SMB uncertainty. By integrating the SMB bias over the GICs area ( 81,400 km 2 in the GIMP DEM), we obtain a 8 Gt/yr uncertainty. The GICs area used here is smaller than previous estimates by 8% owing to unresolved small (< 1-km 2 ) glaciated bodies in the original GIMP DEM at 90 m. Fig. 1 shows mean 3

4 downscaled SMB (v1.0) for entire Greenland and observations sites (yellow). The downscaled product generally shows good agreement with observations although significant biases remain. A comparison of GIC-integrated mass loss ( ) between ICESat/Cryosat-2 products and the downscaled SMB (v1.0) also show good agreements (Fig. 2). Bert, could you comment on this last sentence, add a reference to your upcoming paper and update Fig. 2? Could you include SMB v0.2 in Fig. 2 to highlight the improvements in v1.0 and provide a similar plot for the Hans Tausen region? Could you also provide a short description of IceSat and Cryosat-2 datasets in the Supplementary Information? Could you provide a number for the XX±XX Gt/yr in the manuscript and in Tab. S2? 2 SMB spatial distribution: case study The GICs downscaled fields, for the first time, enable us to study in detail the spatial and temporal variability of SMB. We select 5 GICs regions (black boxes in Fig. 1) with SMB transects spanning more than 500 m of surface elevation difference, to highlight the downscaled SMB dataset. Fig. 3 shows Landsat satellite imagery (Google Earth) for four of these selected regions, with superimposed downscaled annual mean total precipitation (solid and liquid), runoff and SMB (v1.0). Subsequently, region 5 (north Greenland) is discussed in more detail in Section 4. 4

5 Mittivakkat (region 1; o N, o W; Figs. 1 and 3a) is a small ice cap in southeast Greenland. This region experiences relatively large precipitation ( 1600 mmwe/yr) combined with pronounced summer runoff ( 2000 mmwe/yr). The downscaled SMB shows realistic gradients (Fig. 1). Figs. 1 and 3b show two detached ice caps in southwest Greenland, Sukkertoppen and Amitsuloq (region 2; o N, o W). These ice caps have well-defined accumulation zones owing to pronounced precipitation maxima in their interior and runoff being restricted to the margins. Interestingly, the area with large ablation in the southeastern corner of the downscaled product mirrors the region of dark bare ice exposed at the surface in the satellite image. Figs. 3c and d show the A.P. Olsen (74.64 o N, o W) and Qaanaq regions (77.50 o N, o W) in northeast and northwest Greenland, respectively (regions 3 and 4 in Fig. 1). Compared to southern Greenland, these regions are characterized by a relatively cold and dry climate resulting in smaller marginal runoff, narrowing the ablation zone. The modelled SMB profile for south Qaanaq shows a systematic positive bias of 0.5 mwe (Fig. 1), potentially as a result of inaccurate representation of the narrow ablation zone. 3 Recent mass loss acceleration Fig. 4 shows time series of GIC-integrated annual mean SMB components (Gt per year) for the period Prior to 1990, GICs average SMB was -11±8 Gt/yr, i.e. close to zero. Note the importance of refreezing for these Arctic GICs, buffering on average 38% of surface melt. This keeps the GICs stable, i.e. SMB 0, as precipitation (57.4 Gt/yr) almost balance runoff (67.1 Gt/yr), in spite of melt (94.4 Gt/yr) significantly exceeding precipitation (Tab. S1). After 1990, 5

6 SMB decreases at a rate of -1.5±0.3 Gt/yr 2, signifying mass loss acceleration. As a result, the SMB averaged -31±8 Gt/yr for This trend in SMB is almost exclusively driven by intensified runoff (1.5 Gt/yr 2 ), as precipitation show no trend for this period. Enhanced surface melt accounts for 80% (1.2 Gt/yr 2 ) of the total runoff increase, the remaining 20% (-0.3 Gt/yr 2 ) can be ascribed to reduced refreezing in the firn layer (Tab. S1). A comparison with previous estimates of Greenland GICs mass loss 2, 7, 9, 10 shows good agreement (Tab. S2). In line with our results, these previous studies also suggest a recent increase of GICs mass loss, from 21±10 Gt/yr ( ) to XX±XX Gt/yr ( ), compared to 17±8 Gt/yr and 41±8 Gt/yr in the downscaled product v1.0. For the first time, our results resolve GICs SMB on an annual time scale and at full spatial coverage. This proves important to understand the mass loss, as discussed in the next Section. 4 Accelerated mass loss in north Greenland To assess regional contributions to total Greenland GICs mass loss, we calculated changes ( minus ) in annual mean runoff and 2 m air temperature in 12 regions (boxes in Fig. 5). Fig. 5 shows that all regions have been affected by a recent increase in 2 m air temperature and runoff. However, marked contrast in runoff change since the 1990s is found between north (+34 52%, orange boxes) and south Greenland (+14 28%, black boxes). This latitudinal difference is primarily caused by a more pronounced warming in northern ( o C) relative to southern areas ( o C) 23. This results in northern GICs showing a significantly larger mass loss 6

7 acceleration (-0.16±0.03 Gt/yr 2 ) compared to south Greenland (-0.09±0.02 Gt/yr 2 ). To understand the mechanisms responsible for enhanced mass loss in north Greenland, we examine the region of Hans Tausen (region 5; 82.5 o N, 37.5 o W; northernmost orange box in Figs. 1 and 5). Fig. 6 shows a) the annual mean runoff difference between and ; b) time series of area-integrated SMB components for ; c) the difference in the mean ratio between refreezing and surface melt and d) annual runoff (red), snowfall fraction of total precipitation (dark blue) and refreezing fraction of melt (light blue) as a function of 2 m temperature anomalies. For consistency, RACMO2.3 2 m temperature was downscaled to 1-km using the same elevation dependent technique (Fig. S2). The time series of SMB in the Hans Tausen region is qualitatively similar to that of all GICs (Fig. 4 and 6b), with a small steady mass loss (-2.8±1.4 Gt/yr) before 1990 and a doubling in mass loss (-6.1±1.4 Gt/yr) between 1991 and The largest contribution comes from runoff increase (-0.35 Gt/yr 2 ), partitioned in enhanced surface melt (+0.32 Gt/yr 2, 91%) and reduced refreezing (-0.03 Gt/yr 2, 9%). The fact that melt increases but refreezing remains constant or even decreases is remarkable when compared to the contiguous ice sheet 24. Although, the largest absolute runoff increase occurs in the narrow ablation zone along the GICs margins ( mmwe, Fig. 6a), a significant contribution derives from the interior. Since 1990, the inland firn area of northern Greenland GICs has experienced a 20-30% decrease in the refrozen meltwater fraction (Fig. 6c). This refreezing reduction can clearly be related to the recent warming of the Hans Tausen region (+0.9 o C), as refrozen fraction decreases from 50% to 20% in the warmest compared to the coldest 7

8 years (Fig. 6d). In warm years, enhanced surface melt quickly saturates the pore space in the firn. In addition, warmer conditions favour liquid precipitation at the expense of snowfall, decreasing the solid fraction from 95% to 85% with increasing air temperature. As a consequence, less firn is formed and the remaining air content depletes even faster in summer. South Greenland GICs suffer less from a reduction in refreezing fraction, -4.4% since 1990, than northern regions (-8.4%), owing to more active firn formation in winter (Fig. S4). This can be explained by the wetter climate characterizing south Greenland, with more abundant fresh snow deposition, maintaining enough firn air to buffer additional meltwater. 5 Conclusions Since 1990, Greenland peripheral glaciers and ice caps (GICs) show a significant mass loss acceleration (-1.5±0.3 Gt/yr 2 ), primarily governed by an increase in runoff from low-lying ablation zones. Enhanced meltwater production and rainfall account for 80% of the total runoff acceleration, whereas 20% is attributed to reduced refreezing in the GICs interior. North Greenland contributes most to the intensified runoff (+46%), owing to a more pronounced warming (+0.9 o C) relative to the southern regions (+22%, +0.5 o C). The mass loss of GICs in north Greenland is amplified by inefficient refreezing owing to its dry climate, enhanced rainfall and melt. This means that further warming in the future is likely to rapidly erode these northern ice masses. 1. Vaugha, D. G. et al. Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Inter- 8

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12 Acknowledgements B. Noël, W. J. van de Berg, and M. R. van den Broeke acknowledge support from the Polar Programme of the Netherlands Organization for Scientific Research (NWO/ALW) and the Netherlands Earth System Science Centre (NESSC). I. Howat and the GIMP project are supported by the U.S. National Aeronautics and Space Administration (NASA). H. Machguth acknowledges support from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE), funded by the Danish Energy Agency s (DANCEA) program. Competing Interests The authors declare that they have no competing financial interests. Correspondence Correspondence and requests for materials should be addressed to Brice Noël. ( b.p.y.noel@uu.nl). 12