SUPPLEMENTARY INFORMATION Supplementary Information DOI: 10.1038/NGEO1481

SUPPLEMENTARY INFORMATION Supplementary information DOI: 10.1038/NGEO1481

An aerial view of 80 years of climate-related glacier fluctuations in southeast Greenland

Anders A. Bjørk1*, Kurt H. Kjær1, Niels J. Korsgaard1, Shfaqat A. Khan2, Kristian K. Kjeldsen1, Camilla S. Andresen3, Jason E. Box4,5, Nicolaj K. Larsen6 and Svend Funder1

1Centre for GeoGenetics, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark

2DTU Space - National Space Institute, Technical University of Denmark, Department of Geodesy, Copenhagen, Denmark.

3Geological Survey of Denmark and Greenland, Department of Marine Geology and Glaciology, Copenhagen, Denmark.

4Department of Geography, The Ohio State University, Columbus, Ohio, USA

5Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, USA

6Department of Geocience, Aarhus University, Aarhus, Denmark

*e-mail: [email protected]

1. Data

2. Methods 2.1 Image selection strategy 2.2 1981/1985 Digital Elevation Models and ortho-mosaics 2.3 Ortho-mosaics from 1981 and 1985 aerial photos 2.4 Control of the imagery 2.5 Digitization and error assessment 2.6 Glacier length measurements 2.7 Land-terminating elevation change 2.8 Regional differentiation and glacier subdivision

3. Additional climate data

4. Results 4.1 List of measured glaciers 4.2 Supplementary regional changes 4.3 Mid-Century advance 4.4 Unknown time of exposure for “1943” images

5. Supplementary information references

1. Data

1932/1933 Imagery

Images were recorded primarily for cartography during the 7th Thule Expedition to southeast Greenland, led by the Danish/Greenlandic scientist and explorer Knud Rasmussen. After topographic maps were produced, the images were classified and archived in a citadel on the outskirts of Copenhagen. Some of the aerial images had been published after the expedition25 and in the official report of the expedition26. Besides these images, the data have remained unexplored until recently rediscovered in Copenhagen.

Figure S1. Photo positions and examples of aerial oblique images recorded during the 7th Thule Expedition. Image A is taken from the side of the aircraft with a low shooting angle at an altitude of 3700 meters. Image B is a tail-shot from 3300 meters with a higher shooting angle – the glacier depicted is the Thrym Glacier (1700 m wide) in the inner part of Skjoldungen Fjord (63°33’N; 41°45’W).The markings on the upper image are from geodesists generating the maps from the original images.

The aerial images recorded during the 7th Thule Expedition (Fig. S1) fall into two categories: a series of flight lines recorded at 4,000 meters altitude off the coast towards land, and a series of tail images shot

backwards relative to the direction of flight altitudes between 2,500 – 3,500 m. The set of tail flight lines are shot with a larger inclination, thereby more applicable for mapping purposes.

The images are of varying quality, with the earliest recordings generally being the poorest. Many images were discarded due to poor quality. During the first year of flying in 1932 the technology and environment was new to the operators, and many film rolls are useless due to over exposure25. We have used images from the 1932 series, which are of high contrast, and the remaining images are from the 1933 campaign. Contact prints of the images have been scanned at 600 DPI; this resolution has proven sufficient to recognize all details from the prints.

1933 Terrestrial Imagery

Due to the rough ice - and sailing conditions off the southeast coast of Greenland - very few historical terrestrial oblique images are available. From the historic expeditions few photographs were recorded and even fewer containing glaciers which can be geo located today. Five glaciers in the northern part of the study area have had their 1933 positions established based on terrestrial images recorded during The 7th Thule Expedition by Knud Rasmussen (Fig. S2). The work was an independent project carried out by the geologist Keld Milthers25, who’s results were not published. Five Land-terminating glaciers were recorded with a high precision photogrammetric camera for the purpose of measuring summer ablation. The glacier margins have been reestablished after revisiting the glaciers during the summer of 2010.

Figure S2. Terrestrial images recorded in 1933 (left) and 2010 (right) from a Land-terminating glacier northeast of Tasiilaq (65°58’N; 35°51’W). During the 77 year period the glacier has retreated 1.7 km. Photo: Kjeld Milthers & Niels J. Korsgaard.

From the exact 1933 camera positions new images were recorded (Fig. S2, right). This work was primarily done in order to create new 2010 digital elevation models and measure mass loss. In this study only results from the front positions have been used.

Other examples of terrestrial images exist, however, at these sites aerial images from the same period and with a better resolution have been used instead.

1931 Images

The images from the British Arctic Air Route Expedition (BAARE) from 1931 (Fig. S3) are used to fill in the blanks where no suitable images from the 7th Thule Expedition are available. This is primarily the coastline from Umivik to the beginning of Sermilik Fjord, where no tail-images were acquired during the 7th Thule Expedition. The images from the 1931 survey are all oblique photos recorded with a handheld camera from the airplane. The main objective of the survey was to investigate the area for a potential landing strip, leaving less attention to glacierized areas45. The angle of the recordings and the distance to the ice margin varies greatly from close-ups of glacier fronts to coverage of entire regions.

Figure S3. Examples of images recorded during BAARE in 1931. Recorded with a handheld camera, the angle and distance to the ice vary. Left is the inner part of Johan Petersen Fjord (66°00’N; 38°25’W), right is a local ice cap on the southern part of Jens Munks Ø (64°29’N; 40°17’W). Photos are courtesy of Scott Polar Research Institute.

The BAARE photos were provided in digital form from the Scott Polar Research Institute, with a resolution of c. 300 DPI. A number of the images have been scanned at 1200 DPI resolution from the negatives with little influence in the final precision.

1943 Images

Figure S4. Position of the photos recorded during World War II, example from Thrym Glacier (1700 m wide) in the inner part of Skjoldungen Fjord (63°33’N; 41°45’W)

During the 2nd World War an intensive photo campaign took place by the U.S. military. A large number of aerial photos were recorded throughout the coasts of Greenland (Fig. S4). The vertical photos were recorded in scale 1:40,000. The film rolls were handed over to the Danish National Survey and Cadastre after the war. Records of the photo campaign are elusive on when the flights took place, as the precise dates and camera details are no longer available. The best estimate is that these photos were recorded in the second half of the war. The registration cards at the archive of the Danish National Cadastre and Survey associated with these flight lines have the year “1943” and “1943?” noted on them, hence 1943 has also been the year used in this study. The true age of these images is still being investigated both in Denmark, and in the U.S. The implications of a possible incorrect year for our results and main conclusions are discussed in section 4.4. All 1943 vertical photos were scanned from the contact prints at 600 dpi, resulting in a ground resolution of c. 2 m.

1965 Images

Recently unclassified US intelligence satellite images from the Corona program covering the southeast Greenland coast line have been used (Fig. S5). The images were provided in digital form scanned on a photogrammetric precision-scanner at 7 µm from the original negatives from the U.S. Geological Survey. The camera model used for all images was the J-1 stereo photogrammetric camera with a ground resolution of less than 0.61 m46.

Figure S5. Detail from the declassified Corona satellite recorded in 1965, with the Fenris Glacier in the inner part of Sermilik Fjord (66°21’N; 37°30W).

1981 and 1985 photos

Figure S6. The compilation of aerial imagery from 1981 and 1985 presented as blocks of ortho-rectified mosaics. The 650 km coastline is divided into sections to make file sizes manageable. Cut-out from Ikertivaq Bay (65°32’N; 39°56’W) comprised of six different vertical aerial photos.

Vertical aerial photos from 1981 and 1985 (Fig. S6) covering the entire coast have been used. Flights were conducted in summer from late June to early August, and mages were recorded from an altitude of 13,000 meters to a scale of 1:150,000. The photos are part of a larger collection of images covering the entire ice-free parts of Greenland 1978-1987, processed at the Palaeoclimate-Quaternary Group of the Centre for GeoGenetics, Natural History Museum of Denmark.

The 1981 and 1985 scale 1:150,000 photos have been aero-triangulated to the GR96 reference system, and have been used to produce a DEM which is the basis for a series of orthorectified mosaics (Fig. S6). These ortho-mosaics with a 4 m ground resolution are used as the 1981/85-data and serve as the horizontal reference for rectification of all other imagery in this study.

Landsat Images

The Landsat satellite images (Fig. S7) used in this study have primarily been chosen according to data availability. Years with a complete or near complete cover in the study area have been used. With a desire to cover specific time series, only three years have sufficiently good quality and spatial coverage of the study area: 1972, 2000, and 2010. 1972-imagery were obtained from the Landsat 1 mission’s Multispectral Scanner System (MSS) with four spectral bands and a ground resolution of c. 80 m.

Figure S7. False color composites of Landsat imagery. Left is 1972 Landsat 1 MSS with an 80 meter spatial resolution. Right is a 2010 Landsat 7 ETM+ image pan-sharpened with the panchromatic band (band8) to a spatial resolution of 15 m. The area depicted is the combined outlet of Rimfaxe and Guldfaxe Glaciers (63°12’N; 42°10’W).

2000 and 2010-images were recorded with the Enhanced Thematic Mapper Plus (ETM+) onboard the Landsat 7 satellite. For each scene used in this survey, a false color composite was generated and pan- sharpened with the panchromatic band to a ground resolution of 15 m. Using a false color composite facilitates the identification of glaciological features, thus making the manual digitalization process easier and more accurate.

The latest set of Landsat images (2010) are subject to image gaps resulting from the failure of the scan- line correction (SLC) system. Luckily the orientation of the southeast Greenland coast line is very similar to that of the satellite’s orbital inclination, resulting in a major portion of the coast being unaffected by the SLC failure. For glaciers affected by the SLC-failure, gaps were filled by other Landsat 7 scenes obtained within a short time span of the main scene. In some cases the gap fill could not perform, in which case the glaciers were discarded from the analysis.

2. Methods

2.1 Image selection strategy

The images in the individual years have been chosen as late in the melt season as possible in recognition of the typical seasonal cycle for the front position of marine-terminating glaciers27 and to minimize mapping ambiguities related to snow cover, e.g. during the rectification phase. Nevertheless, imagery acquired in the middle of the melt season has also been used when no other images are available. Most of the seasonal retreat occurs before mid-June. Consequently the historical front position changes will have a minimal influence from seasonal advance/retreat cyclicity. With the higher temporal resolution of satellite imagery this task is easier, but with historical images, we will have to accept the acquisition time or reject the data in cases when seasonal variation is believed to outweigh the annual variation. The scale for seasonal variations in front position varies in the order of kilometers for the largest and fastest outlet glaciers47 to no discernible variation (seen as constant melt/retreat rate) for the land- terminating glaciers.

See also uncertainty regarding the historical aerial images recorded during WWII (SI 4.4).

Recent studies have documented the seasonal variation on selected glaciers on the southeast coast of Greenland17,27, showing varying seasonal advance in the order of less than one kilometer, with the exception of Helheim glacier which have undergone a seasonal variability of more than 2 km in the period 2000-200917.

On a number of representative glaciers we have tested the seasonal variability using Landsat satellite imagery in the period 2008-2010. Satellite imagery is available in spring, summer, and fall, where solar illumination is sufficient. We have mapped the front positions for five glaciers, two local marine- terminating glaciers and three ice sheet originating marine-terminating glaciers (fig. S8).

Figure S8. Mapping seasonal variation on selected marine-terminating glaciers using Landsat imagery. A) Marine-terminating glacier originating from the ice sheet. B) Marine-terminating glaciers from local ice caps. Date (M/D/YY), note the differences in scale.

Besides the clear difference in retreat throughout the period, there is also a difference in seasonal behavior. The large ice sheet outlet glaciers tend to advance more during the winter months, while the smaller local marine-terminating glaciers appear steady in the winter months. Data presented in fig. S8 and longer records17 show that we can with confidence select images late in the melt season, as front positions reflect the least advanced annual front position. Also noteworthy is how the long multi-year trends are larger than the seasonal variation.

2.2 1981/1985 Digital Elevation Models and ortho-mosaics

Between 1978 and 1987, the Danish National Survey and Cadastre (KMS, then Geodætisk Institut) covered all ice-free areas of Greenland, including nunataks, with black-and-white aerial stereo- photography. The photography extended across wide parts of the Greenland ice sheet, as well. Later, the photographs were aero-analytically triangulated using a subset of the geodetic stations of the GR96 reference system48,49. The Greenlandic reference system GR96 is the fundamental 3D-reference system in Greenland and is determined from a global reference system used in conjunction with GPS-surveying. The photos covering the southeast Greenland region of interest were recorded in 1981 and 1985 to a scale of 1:150,000. These were later scanned to a resolution of c. 15 µm corresponding to a ground resolution of c. 2 m. This was done on a photogrammetric scanner with a geometric error of 1-3 µm.

Thus, the source material for the 1981/85 DEM are: the photogrammetrically scanned aerial photos with their respective camera calibration reports; GR96 coordinate lists and image observations.

The work was carried out on the softcopy photogrammetric application SOCET SET 5.5 from BAE SYSTEMS.

Interior orientation of the images, image to sensor registration was accomplished using the corresponding calibration data. A priori camera position and attitude (exterior orientation) data was obtained from the flight line maps.

The photos and supporting data were organized in the Multi-Sensor Triangulation module of SOCET SET, then bundle block adjusted/triangulated using a rigorous simultaneous solution.

For automated measurement and collation of elevation points to a DEM, the NGATE (Next Generation Automated Terrain Extraction) module of SOCET SET was employed50. The adaptive and low contrast strategies were used with a ground sample distance of 25 m. All settings were set for obtaining the highest precision. The DTM filters are set to DSM (Digital Surface Model) as there are virtually no artificial vertical structures nor vegetation in Greenland. Thus, no post-processing is needed as the DSM is the DEM.

The accuracy of the 1981/85 DEM elevations was assessed using airborne laser altimetry data from the period 1993 to 2010 as reference. Altimetric data were measured with the Airborne Topographic Mapper (ATM) system mounted on a NASA P-3. These data have a vertical uncertainty of 10 cm after 1995, prior to which vertical uncertainty was larger (c. 20 cm)51. This makes the ATM data suitable as

reference data, as they are an order of a magnitude more accurate than what is to be expected from a DEM produced from scale 1:150,000 B/W aerial imagery, scanned to 2 m ground resolution.

The ATM data have been filtered, thereby including only observations on stable bedrock in the analysis. Observations on ice, snow, oceans, and lakes have been manually excluded by superimposing the ATM data on the ortho-mosaics for visual inspection (SI section 2.2).

The ATM elevations are then subtracted from the interpolated 1981/85 DEM elevations. A visual inspection of large residuals for outliers suggests these can be safely deleted as gross error as the maximum random error seems to be c. 45 m. 300 points were inspected for gross error and most were deleted. A total of 81280 ATM observations occur on stable bedrock. The residuals have an uncertainty (std dev) of 6.9 m and a bias of -1.0 m. Although the results are not influenced with the systematic errors and biases, the random error of the DEM affects the horizontal accuracy of the ortho-mosaics.

Another study using the same data sets at other locations calculate an uncertainty of 2.8-5.6m using ground control points measured in situ52.

2.3 Ortho-mosaics from 1981 and 1985 aerial photos

The reference imagery for all rectification in this study are digital orthorectified mosaics made on the basis of the 1981/85 vertical images and using the 1981/85 DEM for the reprojection.

Mosaicing and resampling to Geotiffs was done to a ground resolution of 4 m for a total of 20 blocks of digital ortho-mosaic, covering all glaciers in the region of interest.

Applying the equation (1)53 for calculating the maximum mean error for a point (X,Y) gives a mean error of 8.6 m for the 1981/85 orthomosaics. This error has been included in all retreat rate error calculations.

√ √( ) ( ) (1)

Where; c = calibrated focal length (88 mm) h = flying altitude (13,500 m) = mean error in DEM (6.9 m) = mean error in image orientation (0.02 mm) a ∙ b = effective image area a = (1-q) ∙Ss b = (1-p) ∙Sl Ss = picture format sideways of flight direction (230 mm) Sl = picture format in flight direction (230 mm) q = side lap (20%) p = overlap (60%) Values in parenthesis indicate values used in this study.

2.4 Ground control of the imagery

All imagery used in this study have been co-registered to the digital orthomosaics produced from scale 1:150,000, 1981/85 aerial photos. Individual image rectifications were made for each investigated glacier, before digitization of the particular glacier front.

The strategy used in the rectifying phase is to minimize horizontal error. The image distrotions are minimized by rectifying images on individual glacier basis with tie point registration to the 1981/85 ortho-mosaics, the image distortions are minimized. In addition, bias from the Landsat images, which can be as high as 90 m (Level 1T) to 250 m (Level 1G)54 is removed by this approach. The horizontal accuracy of high altitude vertical imagery (Corona and Landsat) in relation to the 1981/85 reference imagery is narrowed down to the pixel size of the image undergoing the rectification process.

Table S1. Residual RMS values for georeferenced images. Note that the RMS value in itself is no indication of an accurate rectification, but an indicator of how much the image is distorted into its new position.

Residual Original Image source RMS resolution (1st Order) 1931 BAARE Oblique n.a. 1933 7th Thule-exp. Oblique n.a. 1943 USAAF/USN 1,7 m 5.51 1965 Corona 0,61 m 3.93 1972 Landsat 60 m 28.12 2000 Landsat 15 m 9.77 2007 SPOT 10 m 6.99 2008 Landsat 15 m 13.28 2010 Landsat 15 m 12.27

In this study, the strategy to rectify the oblique images is to place a high number of tie points close to the glacier front and, where possible, encircling the area to be digitized. To minimize distortions, only tie points in the same general altitude as the glacier front are used, enabling a rigid rectification using a 2ndorder polynomial transformation55 where 6 or more tie points are available. An adjust transformation where many tie points are available (Fig. S9, left) forces the tie points in the oblique image to the ground truth tie points of the 1981/85 ortho-mosaic and interpolates values between tie points using a polynomial transformation55.

The same process has been used to test the accuracy of the oblique rectifications, using recognizable landforms targets instead of the glacier front (Fig. S9, right). The average difference from the actual to the digitized target is calculated for a number of different distances.

Figure S9. Illustration of rectification and digitization procedure for oblique aerial images. Left are oblique images rectified according to 1981/85 ortho-mosaics using 18 tie points (red crosses) with an adjust-transformation. Right is the digitized 1933- front from the oblique image superimposed on the 1981/85 ortho-mosaic, with test lines for accuracy estimation: trim line (green) and coastline (orange). The glacier displayed is Thrym Glacier (1700 m wide) in Skjoldungen Fjord (63°33’N; 41°45’W).

The varying accuracy on the oblique images after rectification is displayed in fig. S10 along with differences in scale. Since all images were recorded at different elevations and the camera fixed on an airplane not being perfectly leveled uncertainties are varying from image to image. A maximum uncertainty of 60 m has been set as acceptable and as a result glaciers in the upper third of the frame have not been digitized.

Figure S10. Estimation of post-rectification uncertainty level (left) within the frame of a 1933 oblique images. The uncertainty level is found after empirical studies. Right is and an approximate scale.

2.5 Digitization process and error assessment

Once the desired images have been registered to the 1981/85 orthorectified mosaic, the glacier front is digitized manually. The result is a poly-line representing the glacier front at a certain time. When a number of years are available for a certain glacier, the front length change can be calculated.

A number of uncertainties are associated with the digitization process. The uncertainty from the rectification has been found using the method described above. Each dataset is evaluated according to the orthorectified 1981/85 aerial mosaic, which serves the reference dataset (table S4). The second source of uncertainty is the pixel size of the rectified imagery. Larger pixel size results in higher uncertainty. For oblique imagery the spatial resolution can vary within each image, hence affecting the associated uncertainty. Third is the manual digitization, where the user decides where to draw the line representing the glacier front. A maximum digitizing uncertainty is given units for each dataset (table S2).

The uncertainties used for calculating the accuracy estimates of the glacier front change rates are the maximum possible uncertainty within the digitization of that particular dataset.

Table S2. Measured uncertainties found by digitizing objects with known position. The maximum value is presented as mean error. Uncertainties are measured with the 1981/85 orthomosaic as reference. *1981/85 uncertainty is given as one pixel size.

Maximum Image source digitizing uncertainty 1931 BAARE 60.5 m 1933 7th Thule Exp. 60.5 m 1943 USAAF/USN 12.5 m 1965 Corona 12.5 m 1972 Landsat 30.9 m 1981/85 KMS 4.00 m* 2000 Landsat 16.7 m 2010 Landsat 16.7 m

The uncertainty of the front change rate between two observations is calculated using the maximum digitizing uncertainty and the maximum mean error from the orthomosaic (table S3). The retreat rate uncertainty is given as:

√ (2)

Where;

x = uncertainty for a given observation

t = time interval in years between two observations

Table S3. Uncertainty values for observational periods (m yr-1), found using equation (2).

Maximum Observation digitizing period uncertainty 1933-1943 6.2 m 1943-1965 1.2 m 1965-1972 4.8 m 1972-1981 3.5 m 1981-2000 1.0 m 2000-2010 2.4 m

2.6 Glacier length measurements

Many different methods have been used to measure glacier effective length changes. A common method used by e.g. Lopez et al.56 measures the length at the centre flow line. A weakness of this method is that glacier fronts are often uneven and changes in front position is not uniform. This diversity is not accounted for using the centre flow line method. Others use a box method where the entire front is used in the calculation20,21,27, thereby including changes outside the centre flow line. Another method is a front area method44,57 where the entire front area is calculated by digitizing the lateral margins as well and width is assumed constant. In this study a similar approach as the bow method is adapted, in the development of an automated glacier length measuring tool designed for ArcGIS. The tool divides the front into points spaced e.g. 15m apart and calculates the mean distance to a reference point placed up glacier (Fig. S11). Using the same reference point for all years enables a direct comparison of changes in front position.

Figure S11. Glacier length tool. Illustration of the method of GlacierLengthTool for ArcGIS, using the 2300 m wide Fenris Glacier (66°20’N; 37°30’W). Each glacier front is digitized as an ESRI shapefile (1933=yellow, 1981=green, 2010=blue). A reference point for is placed up stream (red star) with a minimum distance of 6 times the glacier width from the closest front (this distance is not to scale at this figure for illustrative purposes). The front is divided into sections of desired lengths marks by red circles (in this case 200m) within the red box. Average length from reference point to glacier points is calculated.

To minimize uncertainties from the procedure, the reference point to which the length is measured is placed at least six times the width of the glacier front upstream from the front itself. At this distance the angle between the outermost points on the front and the reference point is sufficiently small not to affect to result. This distance was found by testing the results from several glaciers with varying distances to the reference point (Fig. S12).

Figure S12. Testing distances for placement of reference point. At a distance of six times the width of the glacier, the variation in length caused by changing angles to the reference point is less than 1 m.

Once all preparations are made, the shapefiles containing the front for each glacier can be batched, and an algorithm calculates the distance for all front positions for all glaciers.

The reference point method has been tested against the box- and the centre flow line methods by examining the divergence between methods. We selected 5 representative glaciers with different origin, size, and terminal environment (table S4).

Table S4. Testing the reference point method with two established methods; the box- and the centre flow line method. Values are in m yr-1 and represent the divergence from the point reference method for each observational period.

SEGL157 SEGL070 SEGL031 SEGL057 SEGL053 SEGL091 GrIS GrIS GrIS GrIS Local Local marine marine Lake land marine land ± Box method Average stdv 1933-1943 -4.9 1.4 1.7 0.2 4.9 0.5 0.6 3.2 1943-1965 5.9 -4.0 -2.9 -0.3 -2.5 -0.3 -0.7 3.5 1965-1972 -1.3 2.3 2.4 4.4 -2.7 0.0 0.9 2.7 1972-1981 4.7 1.2 0.8 -1.8 2.9 0.6 1.4 2.2 1981-2000 -4.7 -5.1 3.0 0.3 0.8 0.1 -0.9 3.2 2000-2010 -1.6 -7.3 -1.2 -0.1 -5.2 -1.3 -2.8 2.8 Centre flow line method 1933-1943 -21 .9 -3. 8 1. 9 -1. 5 7. 6 -1 .7 -3.2 10.0 1943-1965 6.8 -3.4 -3.9 -10.8 -1.3 0.5 -2.0 5.8 1965-1972 -11.6 20.9 0.3 43.6 -6.7 1.3 8.0 20.7 1972-1981 7.0 10.0 0.3 -2.8 0.4 -3.8 1.9 5.5 1981-2000 -3.5 -1.8 10.0 0.1 0.0 2.3 1.2 4.7 2000-2010 -5.2 -27.7 -25.1 1.6 -2.8 -2.1 -10.2 12.8

We conclude that the point reference method used in this study performs well in comparison with the box method as all the differences in measured lengths are within the uncertainty described in Table S3. Comparing the results with front change rates found using the centre flow line method, reveals larger differences and much higher standard deviations. This is to be expected since measurements are a result of a single measurement on the glacier front. The differences are not extremely large, but the centre flow line method did in some instances measure an advance, where both the box and the point reference method measured retreat. The box and the point reference matched the advance / retreat in all instances. We have not tested the method with the front area method, since it is primarily used in automated classifications, and as digitizing the lateral margins manually would mean a tripling of the uncertainty.

2.7 Land-terminating elevation change

The changes in land-terminating front elevation is investigated using points extracted from the 1981/85 DEM representing the front position in the early thirties. The historical elevation has a precision of 6.9 m (SI section 2.2). Present elevations are found by extracting data from the SPIRIT SPOT DEM and extracting points from the SPOT orthophotos, and thus elevation change is the difference between extracted values. The SPOT product has an elevation uncertainty of 3.8 m58, resulting in a combined uncertainty of 7.9 m, using equation 2.

2.8 Regional differentiation and glacier subdivision

In order to investigate any possible regional differences in glacial behavior, the study area has been subdivided into five regions (A-E) (Fig. S13). We choose to subdivide the region to display any differences which might get lost when front changes are averaged over the entire region. We have used a global DEM (ASTER GDEM V2) of the ice sheet in combination with topographic maps and ortho- mosaics for the subdivision. The regions represent different orographic settings and different ice sheet surface slope.

Figure S13. Data subdivision. Subdivision of regions and glaciers based on local topography, slope of the ice sheet and glacier catchment and terminal environment. Elevation model presented is a satellite derived ASTER-DEM with ice sheet elevations added from the KMS 1:250.000 map series.

A) The Sermilik Fjord region, characterized by many local glaciations and few large marine-terminating glaciers from the ice sheet. The fjord length exceeds 100 km and is crowded with icebergs mostly from the fastest glacier in south east Greenland, the Helheim Glacier7. In the northeastern part of the region there are several large marine-terminating glaciers originating from local ice caps. The region houses the only lake terminating glaciers in the entire study area, all originating from the ice sheet. B) The Umivik and Køge Bugt region, where the ice sheet is gently sloping directly into the ocean, with the lowest ice

sheet inclination in the region. No local glaciations and few land-terminating glaciers. Here, large dynamic thinning and bedrock uplift have been observed14. C) The Gyldenløve Fjord region, a large broad bay where the southern dome of the ice sheet flows directly into the ocean. Primarily marine- terminating glaciers from the ice sheet, with a few local glaciers from local ice caps on large islands in the Gyldenløve Fjord. D) The Fjord Region with large ice sheet marine-terminating glaciers that flow among complex steep mountains. It is a region with many local glaciations. The ice sheet slope in this region is high. From satellite imagery it is observed that this region has a later winter fjord ice breakup up E) The Puisortoq Region includes areas where the ice sheet flows directly into the ocean and the slope of the ice sheet is less. There are few land-terminating glaciers and local ice caps.

The glaciers have been divided into categories (Fig. S13 & table S5) according to their terminal environment (land, lake or marine) and their catchment (ice sheet or local glaciers and ice caps).

Table S5. Glacier subdivision.

All glaciers 132 Marine-terminating 96 Ice Sheet 55 Local 41 Land-terminating 33 Ice Sheet 5 Local 28 Lake-terminating (all Ice Sheet) 3

A total of 158 glaciers were digitized from the 1930s imagery, but 26 were discarded from the dataset due to high uncertainties from the rectification process and missing observations in the 1943 images. In the 1960s and 1970s some images had to be discarded due to persistent cloud cover, resulting in fewer digitized glaciers (table S6). The number of glaciers is roughly consistent in the 7 observation epochs.

Table S6. Number of glaciers digitized within each year

Year Number of glaciers 1930s 158 1943? 132 1965 101 1972 117 1981 131 2000 132 2010 132

Glaciers in this study are numbered (in the north to south direction). Few southeast Greenland glaciers have been given names throughout the history, and to avoid confusion since these names often are spelled differently, glaciers that do have official names are presented with their identification number, as well as a name relating the glacier’s position in relation to a well established landmark or point (e.g. Glacier 051 Kjaerulfs Nunatak Northeast). All place names and names of glaciers are adopted from the

1:250,000 map series based on the 1932-1933 images recorded during the 7th Thule Expedition as well as a smaller area in the southern region from a 1948 map series. This map series offer the most comprehensive compilation of southeastern place names (table S7)

3. Additional climate data

The southeastern coast of Greenland is characterized by extremely high precipitation (> 2 m water equivalence per year59) compared to the rest of Greenland, resulting in high snowfall accumulation rates59. Observations from the last couple of years have shown record high negative mass balance for a part of region A, due to increasing warm season ablation rates18.

Figure S14. Climatic factors influencing the southeast Greenland coast. Temperature from Tasiilaq60, SE Greenland temperature modified from Box et al.31, measured SST34, accumulation rates modified from Wake et al.28. Periods with front observations used in this study has been marked on the time axis.

The recent temperature history of southeast Greenland is derived from the Tasiilaq meteorological station situated in the northern part of the study area (Fig. S14) and from a compilation of all available meteorological station data from the southeast coast and from stations on the ice sheet, after Box et al.

31. The station at Tasiilaq is the only station with a temperature record dating back to the earliest aerial images used in the study.

Figure S15. Seasonal variations in air temperature anomaly (from 1951-1980 baseline) for the entire southeast Greenland coast. Gauss smoothed (21 pt.). Updated from Box et al.31.

The seasonal changes in temperature are investigated using the temperature reconstruction covering the entire southeast Greenland ice sheet (Fig. S15) (after Box et al.31). The reconstruction reveals a seasonal asymmetry between the early 20th Century warming (ECW) led by c. 4 y and maximized in spring and the recent warming led by c. 6 y and maximized in autumn and winter. The latter warming period in autumn has a larger impact on degree days available for glacier melt. However, all seasons are undergoing warming in the two periods.

In this study, special attention is placed on the 1920’s warming. Studies on the temperature variations in Greenland have been carried out by Box et al.31 and Chylek et al.30 confirming the ECW warming rate to be larger in magnitude (°C /yr) than the recent warming. However, the recent temperatures are highest overall.

Furthermore Box et al.31 tested all Greenlandic meteorological stations with records longer than 100 years and found that the entire southeast coast is best represented by the Tasiilaq station rather than the Qaqartoq station in the south southwest.

The measured SST record34 is a compilation of data gathered by Smed et al.61 from historic oceanographic cruises and data from ICES (International Council for Exploration of the Sea). The modern hydrography by the Southeast Greenland coast is characterized by cold, low-saline surface waters of The East Greenland Current (EGC), originating from the Arctic ocean via Fram Strait and the underlying (below 200-300 m) warm atlantic waters originating in the Irminger Sea9. Fig. S16 illustrates annual SST anomaly (SSTA) between 1912 and 2011 from the Met Office Hadley Centre observations dataset62.

Notable is the recent warming starting in late 1990’s with SSTA of about 2.0 C above the 1961–2000 baseline, and the mid-Century cooling (minima during 1967-1972) with SSTA of -1.5 C along the southeast Greenland coast. The 1967-1972 cooling is most likely linked with a marked fresh water transport from the Arctic Ocean63. With the exception of few years (1934, 1935, 1958) the SSTA during 1927-1961 is about 1 degree above the 1961–2000 baseline. This is consistent with measured air temperature anomaly for the southeast Greenland coast (Fig. S15). The observed re-advance of marine- terminating glaciers during 1965-1972 (Fig. S18), suggests that marine-terminating glaciers are very sensitive to ocean temperatures and react rapidly to both cooling and warming, as was also documented by a study from Helheim Glacier34.

Figure S16: Annual SST anomaly for 1912-2011 from Met Office Hadley Centre observations datasets (http://www.metoffice.gov.uk/hadobs/hadisst/data/download.html). The SST anomaly is computed relative to a 1961–2000 baseline.

4. Results 4.1 List of measured glaciers

Table S7: Complete list of glaciers investigated in the study. All glaciers are given an ID stating with SEGL001 in the north and ending with SEGL158 in the south. Glacier names are applied according to KMS map series 1:250.000, with official glacier names in italics. Where no official glacier names exist, glaciers are named according to the position to know landmarks labeled on the map. Coordinates are given in UTM zone 24. Annual front positions are given in meters and relative to an arbitrary fixed upstream point. Six local marine-terminating glaciers marked* in the “type-field” reached land during the 2000-2010 period. Width ID glacier name type origin UTM N UTM E (m) 1931 1932 1933 1943 1965 1972 1981/85 2000 2010

SEGL001 Point 1001 South marine local 630980 7318624 1300 11785 11277 10189 9911 9323 7960

SEGL003 Apuserajik Glacier land local 621106 7327881 390 4818 4533 4368 4404 4331 4263 4251

SEGL004 Knud Rasmussen Glacier marine local 621283 7332465 2400 14357 14112 14634 14542 14516 14558 14277

SEGL005 Karale North East marine local 610792 7331760 2400 13486 12915 12958 12850 12903 12883 12759

SEGL007 Karale Glacier marine local 605678 7332113 2100 30007 28572 25565 24986 24768 23987 23528

SEGL009 Glacier 15 Tunu land local 597039 7320123 1400 13032 12599 12362 12195 12007 11920

SEGL011 Midgaards Glacier marine GIS 595276 7365702 3400 76963 73766 62442 59674 57074 48201

SEGL012 Point 1240 West land local 580905 7361470 850 11215 10727 11168 10014 9998 9501 9036

SEGL013 Fenris Glacier marine GIS 566006 7360236 2200 20100 17890 17549 16739 16174 15747 14347

SEGL014 Helheim marine GIS 540881 7359971 5400 55686 56196 57926 59809 59291 59414 54481

SEGL015 Kilikilat Glacier North land local 574646 7346571 350 10581 10367 10198 10192 10189 10086 9971

SEGL016 Kilikilat Glacier South land local 573412 7346130 650 11356 11254 11239 11115 11125 11034 10903

SEGL017 Apuserajik marine GIS 545994 7343133 1600 17680 17025 16311 16178 15274 13310 11578

SEGL018 Apuserajik Glacier lake GIS 544143 7340752 850 11621 11483 11059 10787 10658 10360 9891

SEGL020 Heim Glacier marine GIS 525188 7318712 1600 12335 12250 12185 12230 12223 12032 11925

SEGL021 Bruckner Glacier marine GIS 526158 7311836 2300 12375 11761 11750 11858 11799 11612

SEGL022 Milthers Glacier 5 land local 533123 7301962 800 11333 11077 10894 10790 10626 10507

SEGL024 Mittivakkat Glacier land local 551989 7285564 200 11663 11504 10712 10659 10507 10380

SEGL026 Isertup Imia Glacier lake GIS 527310 7288514 1400 18067 17502 16663 16501 16489 16343 15849

SEGL027 Bussemandgletscher land GIS 512240 7286410 400 13083 12646 12129 11689 11393 10875 10633

SEGL028 Apuserserpia Glacier North marine GIS 513022 7281840 2600 16337 15835 15794 15804 15790 15770 15658

SEGL029 Apuserserpia Glacier South marine GIS 515258 7278283 2000 18774 18561 18374 18280 18294 18269 17854

SEGL030 Isertoq Glacier marine GIS 505330 7276366 1800 15823 15491 14947 14295 14128 13938 13838

SEGL031 Qornorsata Glacier lake GIS 500479 7280427 1200 16012 15636 14952 14659 14533 14086 13537

SEGL032 Nunalarte North marine GIS 496643 7283473 900 18119 17215 16937 16960 16908 16628 16034

SEGL033 Nugatsiakajik North marine GIS 472275 7284150 3000 31185 31431 32081 31548 31436 31672 27704

SEGL034 Nugatsiakajik South marine GIS 470357 7280089 4200 42006 41793 41965 41784 41930 41976 41437

SEGL035 Point 300 Northwest land GIS 467085 7275012 550 16676 16505 16505 16386 16502 16569 16499

SEGL036 Point 300 North land GIS 463701 7273997 800 23228 23039 22771 22855 22583 22464

Width ID glacier name type origin UTM N UTM E (m) 1931 1932 1933 1943 1965 1972 1981/85 2000 2010

SEGL037 Point 300 West marine GIS 456480 7273658 2300 20253 20003 20666 19909 20347 20013 18632

SEGL038 Simitakaja North West marine GIS 457496 7250305 1800 22664 21877 21400 21236 21031 20975 20107

SEGL039 Simitakaja West marine GIS 459899 7247123 900 12138 11868 11870 11863 11859 11853 11772

SEGL040 Igtip Tunua Glacier marine GIS 461100 7241828 1800 17561 17089 17119 17117 16957 16935 16792

SEGL042 Point 300 North marine GIS 458576 7235301 1100 17360 17541 17054 16846 16283 16016 15732

SEGL043 Point 300 South marine GIS 454019 7233761 2200 36239 32971 31136 30936 30339 29091 27887

SEGL045 Point 415 East marine GIS 449702 7218908 1700 16143 15771 15793 15853 15854 15645 15478

SEGL046 Apuseq Glacier marine GIS 444721 7216008 3300 34054 33175 32012 31715 31457 30860 30523

SEGL047 Husryggen East land GIS 439595 7217467 1600 18516 18106 17974 17900 17922 17569 17579

SEGL048 Point 415 East marine GIS 412675 7222656 750 11837 10636 10431 10463 10418 10312 9472

SEGL049 Koege Bugt Northwest marine GIS 400195 7227990 3300 32497 29729 29505 29493 29587 29542 27731

SEGL050 Mandehoved West marine GIS 395937 7209088 5000 47254 42762 41220 41211 41043 40982 41173

SEGL051 Kjaerulfs Nunatak Northeast marine GIS 400117 7199684 900 16775 16688 16787 16333 16792 16774 16670

SEGL052 Jomfruen Northeast marine GIS 401724 7195102 2300 21222 21368 21760 21635 22609 21298

SEGL053 Point 550 East marine local 419810 7198639 1100 11186 11130 10979 11070 10936 10844 10649

SEGL054 Kagtertoq Fjord West marine GIS 420801 7152180 4700 42833 42877 43540 43667 43569 43288 40900

SEGL056 Puisertoq marine GIS 420382 7142210 1400 15615 15332 15134 15143 15209 15109 14710

SEGL057 Kullede Top East land GIS 405694 7134562 800 15946 15772 15579 15582 15521 15434 15321

SEGL058 Upernagtivik North land local 409505 7130527 300 5883 5784 5686 5679 5572 5518

SEGL059 Upernagtivik North marine local 407667 7130034 700 5914 5860 5847 5884 5753 5799

SEGL061 Upernagtivik Northwest marine local 403452 7126133 2200 14849 14565 14384 14375 14320 14181 14158

SEGL062 Qataq East marine GIS 388029 7137850 1800 20202 20110 19995 20230 20300 20180 19998

SEGL063 Graaulv Glacier marine GIS 378105 7136714 2500 25211 25723 25831 26807 26688 26228 25468

SEGL065 Delmenhorsts Nunatak South marine GIS 374817 7128166 5100 42577 42492 42549 45494 43206 42138 42232

SEGL066 Pingasikajit North marine local 415940 7097593 700 5224 5145 5055 5040 5104 5061 4892

SEGL067 Pingasikajit Northwest marine local 412246 7096176 250 4603 4401 4433 4396 4420 4338 4311

SEGL069 Otto Krumpens Fjord Glacier marine local 409159 7091470 1300 12773 12614 12559 12579 12657 12419 12162

Fimbul Glacier / Sleipner SEGL070 Glacier marine GIS 377783 7086908 3000 27670 26632 25711 25930 25658 26014 24590

SEGL071 Bernstorffs Glacier marine GIS 367919 7079766 4100 40800 41552 39152 39546 38447 38575 35471

SEGL072 Store Bjoern Glacier marine GIS 368599 7071773 2400 25197 25973 25026 25201 24894 24758 23187

SEGL073 Lysalfbjerg West marine local 373191 7069392 1100 15960 15604 15594 15064 14703 14331 13985

SEGL074 Ensomheden Northeast marine local 391472 7071348 650 11711 11471 11445 11444 11346 10852

SEGL075 Yduns Glacier marine local 403972 7064801 1700 14047 13854 13870 13862 13891 13852 13803

Width ID glacier name type origin UTM N UTM E (m) 1931 1932 1933 1943 1965 1972 1981/85 2000 2010

SEGL076 Gerds Glacier marine local 407798 7063780 800 8989 8843 8841 8810 8856 8802 8721

SEGL077 Nasigtarfiq Southwest marine local 420808 7063100 600 10582 9698 9490 9489 9498 9511 9218

SEGL078 Rypefjeldet South marine local 406627 7055801 1500 20118 20055 20014 19946 20013 20015 19938

SEGL079 Diabastoppen North marine local 409451 7049188 400 4383 3961 3897 3915 3970 3916 3870

SEGL080 Diabastoppen Northwest land local 406033 7049634 1400 13061 12752 12595 12638 12618 12517 12443

SEGL081 Modes Fjord Gletscher land local 401277 7049188 500 7387 7202 6973 6783 6672 6178 5697

SEGL082 Magnes Fjord Gletscher marine local 398380 7048965 1600 12005 12048 12002 11992 11973 11988 11925

SEGL083 Sigyns Gletscher marine local 386909 7050905 1400 11170 11126 11070 10972 11070 11032 10737

SEGL084 Jaettefjorden Northeast marine* local 379221 7051146 450 5412 5398 5379 5409 5402 5300 5139

SEGL085 Hakkefjeld Northeast 1 land local 377424 7052835 400 12204 11276 11170 11164 11108 11071 10907

SEGL086 Hakkefjeld Northeast 2 land local 376796 7052251 400 12237 12116 11718 11544 11446 11181 10660

SEGL087 Thryms Gletscher marine GIS 362708 7050676 1600 17536 17638 17737 17690 17738 17533 17239

SEGL089 Rumleren Glacier marine GIS 353213 7037486 350 8002 7990 8059 7980 7998 8017 7990

SEGL091 Skjoldmoeen Gletscher land local 363169 7041797 900 12682 12494 12332 12320 12255 12178 11988

SEGL093 Hvide telt North 2 land local 379523 7039977 450 6086 5993 6028 5995 5803 5559

SEGL094 Skruen North land local 366516 7027526 500 6722 6728 6732 6764 6727 6646 6555

SEGL095 Vigges Gletscher marine local 375308 7025763 400 7803 7516 7491 7405 7330 7300

SEGL096 Knopfjeld West 1 land local 378177 7022142 300 5046 4743 4268 4228 4152 3898

SEGL099 Halvdans Fjord Northeast marine local 382295 7018641 400 6258 6234 6229 6227 5903

SEGL100 Bjarkes Gletscher land local 378182 7018644 800 7270 7068 6870 6758 6566

SEGL102 Roars Halvoe South land local 387220 7013997 450 4284 4110 3944 3930 3837

SEGL103 Helges Halvoe Northwest land local 389498 7014543 300 7742 7658 7695 7675 7521

SEGL104 Helges Halvoe Southwest land local 390081 7013377 300 7872 7770 7773 7747 7688

SEGL105 Gedebukken West marine* local 371476 7021616 250 4652 4358 4295 4351 4208 4007

SEGL106 Staerkodder Vig land local 366171 7016834 450 6787 6289 5653 5416 5157

SEGL107 Kaempekeglen Southeast land local 367384 7015536 500 4765 4661 4658 4600 4529 4330

SEGL108 Staerkodders Vig South land local 368682 7014681 800 7325 7178 6863 6742 6478

SEGL110 Kong Skjold Halvoe West marine local 372730 7008375 1300 10902 10385 10338 10205 10145 9928

SEGL112 Skinfaxe marine GIS 357412 7014079 2600 21493 21709 22025 21927 21794 21948

SEGL113 Rimfaxe Guldfaxe marine GIS 340059 7013889 2900 28082 28156 28168 28087 28061 28011

SEGL114 Roeriks Bjerg South marine local 339362 7008939 550 17253 17206 17171 17197 17125 17069

SEGL115 Fensal North land local 340366 7007012 450 17554 17446 17438 17257 16928

SEGL116 Fensal South marine local 336010 7002685 1000 18521 17865 17861 17753 17814 17696

SEGL117 Garm Gletscher marine local 338779 6992581 1500 19783 19659 19621 19661 19597 19360 19035

Width ID glacier name type origin UTM N UTM E (m) 1931 1932 1933 1943 1965 1972 1981/85 2000 2010

SEGL118 Point 1380 West marine local 344025 6989083 1300 11621 11483 10787 10658 10360 9891

SEGL119 Garm Southern Lobe East marine GIS 334612 6982870 1200 9772 9329 8729 8747 8606 8333 7700

SEGL121 Heimdal Glacier marine GIS 315897 6976205 2700 21831 22521 22319 22394 22561 23021 23065

SEGL123 Snehatten Southeast marine* local 305353 6958727 650 7462 7359 7202 7192 7156 7153 7072

SEGL124 Snehatten South marine local 300456 6959007 500 5840 5729 5612 5734 5785 5542 5443

SEGL125 Snehatten West 1 land local 295519 6961785 700 31171 30932 30529 30456 29809 30013

SEGL126 Snehatten West 2 land local 294719 6962025 450 31174 30705 30446 30299 29330 29607

SEGL127 Nunatak 680 East marine GIS 287930 6965063 2200 30310 30901 29990 31235 30644 29987 27894

SEGL128 Point 1060 west marine local 312542 6944642 200 2650 2653 2620 2638 2555 2436

SEGL129 Runde Keglefjeld Northeast 1 marine local 310193 6943595 1400 9838 9658 9431 9312 9300 9027 8552

SEGL130 Runde Keglefjeld Northeast 2 marine local 311594 6941641 2100 13896 13686 12831 12675 12532 12033 11426

SEGL131 Runde Keglefjeld Northeast 3 marine local 313411 6942055 600 6476 6189 5212 4939 4621 4227

SEGL134 Point 850 South marine local 319090 6937507 1200 9992 10032 9976 9954 9983 9891 9846

SEGL135 Grydefjeldet Northwest land local 321051 6934353 700 9811 9720 9619 9714 9592 9460

SEGL136 Rundekeglefjeld South marine local 300673 6936335 1000 8736 8703 8461 8537 8490 8384 7925

Inner Mogens Heinesen Fjord SEGL138 1 marine GIS 297206 6936985 800 8538 8769 8483 8334 8237 8026 8024

Inner Mogens Heinesen Fjord SEGL139 2 marine GIS 292742 6938546 2000 23985 24440 23531 21997 20966 19518 16466

Inner Mogens Heinesen Fjord SEGL140 3 marine GIS 291832 6935209 1800 24516 25129 23842 22571 22188 22013 20450

SEGL141 Point 570 North marine GIS 326688 6907060 750 8030 7307 7224 7136 7128 6905

SEGL142 Point 140 South marine GIS 326804 6905148 650 6269 6028 6209 6123 5927 5842

SEGL143 Point 340 South marine GIS 324313 6893590 2100 28178 28164 28183 28308 28121 28066

SEGL144 Point 470 South marine GIS 320285 6886936 2300 27296 26256 26059 26238 26206 26072 24299

SEGL145 Point 450 South marine GIS 312678 6882366 1700 31077 30946 30474 28684 28854 28286 27048

SEGL146 Point 1038 South marine GIS 313401 6871524 2300 16503 16468 17499 16546 16469 16063 14881

SEGL147 Puisortoq Fjord West marine GIS 318905 6868355 1400 19502 18773 18274 18388 18311 18311 18318

SEGL148 Puisortoq Fjord South marine GIS 324854 6865241 2500 17953 17840 17835 17940 17933 17816 17806

SEGL149 Point 590 North marine GIS 327412 6863017 900 14688 14089 13825 13959 13933 13822 13655

SEGL150 Point 950 Southwest marine local 305483 6854192 400 8106 8104 8100 8060 8099 8030 7986

SEGL151 Point 650 West marine GIS 301562 6859594 1500 17400 16732 16321 16624 16869 16452 15145

SEGL152 Point 940 Northeast marine GIS 297810 6856802 1800 15871 15884 15784 15729 15825 15714 15681

SEGL153 Pyramiden South marine* local 301632 6830430 1300 11807 11820 11838 11761 11846 11740 11619

SEGL154 Point 1321 South land local 295884 6831678 100 7610 7535 7510 7546 7330 7205

Width ID glacier name type origin UTM N UTM E (m) 1931 1932 1933 1943 1965 1972 1981/85 2000 2010

SEGL155 Point 1080 Southeast marine* local 294706 6831881 650 7673 7668 7692 7610 7673 7552 7374

SEGL156 Point 1080 Southwest Marine local 291749 6832911 700 5965 5946 5951 5922 5971 5883 5826

SEGL157 Kangikitsua Glacier Marine GIS 282923 6836533 2300 14288 14263 14518 15800 15973 14547 14293

SEGL158 Point 1050 North Marine local 299180 6826382 1100 9281 9256 9239 9278 9268 9233 9184

4.2 Supplementary regional changes

4.2.1 Land-terminating glaciers

Figure S17. Front change rates of all land-terminating glaciers. Average rates of selected regions (n>3) are depicted with histograms, red indicates retreat and green indicates advance. Glaciers missing 1965 observations in region D are not used in the calculation of average rates. Note the difference in glacier type, region A and D are local glaciers and ice caps while region B is ice sheet glaciers. For ice sheet glaciers, region C and D are discarded as only a single land-terminating glacier appear in each region.

Collectively, all three regions (A,B, and D) underwent higher retreat rates in the first observation period compared to 2000-2010 (Fig. S17). Most land-terminating glaciers exhibit higher retreat rates in the 1981/85-2000 period compared to the latest and warmest 2000-2010 period. A possible explanation is evident considering the topographic setting of the land-terminating glaciers. Generally as the glacier front retreats, rapid changes are seen in the gradually sloping lower glacier in proximity to the coast where most of the land-terminating glaciers are situated. The ablation area becomes less in proportion

to the accumulation area as the glacier front retreats to higher elevations leaving a larger part of the glacier exposed to a lower temperature and less intense melt regime.

4.2.2 Marine-terminating glaciers from the ice sheet

Figure S18. Front change rates of 55 marine-terminating glaciers originating from the ice sheet. Glaciers missing 1965 observations in region D have not been used in the calculation of average rates.

The overall pattern of retreat in the 1930s is interrupted by an early advance in regions C and D (Fig. S18). This behavior is in sharp contrast to that of other glacier types in the same regions. A large regional variation in front behavior is evident in comparing the histograms. A portion of this erratic behavior comes from several individual glaciers repeatedly switching from retreat to advance. Averaging the 2000s retreat for all regions outweighs the early 20th Century retreat.

Regional surface and basal topographic differences probably contribute to the difference in glacier behavior. Fig. S13 illustrates regions C and D both being fed from a section of the ice sheet with high surface slopes, and deep fjords likely continuing inland. This may lead to a different dynamic behavior than observed from glaciers emerging from the lower sloping ice sheet surface in regions B and E. Further, the ice sheet in region D is also characterized by having a higher bed that is above sea-level64, yielding thinner ice than in other regions.

4.2.3 Marine-terminating glaciers from local glaciers and ice caps

Figure S19. Front change rates of 41 marine-terminating glaciers originating from local glaciers and ice caps. Glaciers missing 1965 observations in region D have not been used in the calculation of average rates. Note that there are no local marine- terminating glaciers in region B.

The large variation in region A retreat magnitude, is due to these glaciers having a much larger area change potential given their larger physical dimensions, with widths >2km reaching into the fjords.

The overall change of the local marine-terminating glaciers, closely resemble that of the land- terminating glaciers (Fig. S17), however, land-terminating glaciers retreated more rapidly in the early period of observation while marine-terminating glaciers retreated more in the later period.

4.3 Mid-Century advance

As our results are based on data with roughly decadal temporal resolution, and the fact that changes in front position is known to occur at time scales of higher granularity, our observations cannot account for changes between observations. Minor retreats and advances could have occurred within periods of opposite trend. It is fair to say that the number of glaciers undergoing an advance during the mid- Century cooling is therefore a minimum, since advances could be missed between the observations. An example of this is the Mittivakkat Glacier (SEGL014) on Ammassalik Island which advanced during a period of overall retreat spanning 1943-1965. The moraines from the advance (Fig. S20, left) were photographed in situ in 195839, but have since been eroded by the melt water from the glacier (Fig. S20, right).

Figure S20. The end moraine left after an advance by the Mittivakkat Glacier (65°40’N; 37°54’W) between the 1943 and 1965 observations photographed in 1958(39). Right, in 2010 the same valley bears no evidence of advance (Photo: Anders Anker Bjørk).

Similar advance episodes have been observed within the general retreat period from 2000-2010, where 17 numerous outlet glaciers advance between 2005 and 2008 .

4.4 Unknown time of exposure for “1943” images

We have tested how our conclusions will be affected in case our assumption of applying the year 1943 to the U.S. military WWII imagery is wrong. Our main conclusion affected by this is the comparison of front change rates between the first observation period and the last. In the case where an extra year is added, and the images are assumed to be recorded in 1944, the number of glaciers experiencing a similar or larger retreat in 1933-1943 compared to 2000-2010 remains the same (73 of 132). However, if the year of exposure is assumed to be 1945, the number is reduced to 71 of 132. Taking this into account, it is fair to say that our conclusions remain insensitive, even in the case where the year of exposure have mistakenly been set to 1945.

The season in which the images were recorded is officially unknown. However, based on the snow conditions, the images must have been recorded from the middle to the end of the melt season (Fig. S21). Hence the images are representative of the annual minimum extent or an extent close to it.

Figure S21. Aerial images recorded in the latter half of World War II. Left is the local land-terminating glacier Skjoldmøen (SEGL091) on Skjoldungen Island. Right is the combined outlet of the ice sheet glaciers Rimfaxe and Guldfaxe (SEGL113). The images are not orientated geographically.

In the southernmost region, Puisortoq, and in the Køge Bugt region there is generally a more snow cover on the images. However, from the other imagery sources with known time stamps of recording we know that these regions tend to be snow covered for longer periods, and hence we cannot conclude that the WWII images were recorded at different times.

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