The Cordillera Blanca, Peru has 27 peaks over 6,000m, over 600 glaciers and is the highest tropical mountain range in the world. Glaciers are a key water resource from May-September in the region, Mark (2008). The glaciers in this range have been retreating extensively from 1970-2003, GLIMS identified a 22% reduction in glacier volume in the Cordillera Blanca. Vuille (2008) noted that the retreat rate has increased from 7-9 meters per year in the 1970’s to 20 meters per year since 1990. One of the glaciers that is continuing to recede is Llaca Glacier descending the west slopes of Ranralpaca. This glacier has retreated 1700 m from its Little Ice Age moraine, outlined in lime green. Llaca Laguna is impounded by this moraine. The glacier still has a significant consistent accumulation zone and can survive current climate. Stagnant pockets of debris covered ice no long connected to the glacier fill much of the valley between the laguna and the current glacier. The terminus despite ending on a steep slope lacks significant crevassing indicating a lack of vigorous flow which will lead to continued retreat of 20-30 meters per year. This glacier drains into the river which then flows into the Rio Santa in Huarez, Peru. Mark (2008)note the importance of glaciers to the Cordillera Blanca watersheds in the Huarez region receive 35% of their runoff from glaciers, and the upper Rio Santa likely receives 40%.
Zongo Glacier, Bolivia extends 2.9 km down the south side of Huayna Potosi from 6000 m to 4900 m. Zongo Glacier is a small valley glacier located north-east of La Paz, and its runoff is directed to an important hydraulic power station which supplies La Paz. Note Laguna Zongo in foreground of the first image. The dam is visible as is the power station to the right and below the lake. The glacier has considerable snowcover on its upper section and crevassing. This indicates a persistent accumulation zone. In 1991 a glaciological research program (page 46) was established on Zongo Glacier to monitor mass balance, understand its hydrology and energy balance. The long term director of this research Bernard Francou has been called the glacier guardian. The typical Alpine glaciers undergoes a long accumulation period in winter and a short ablation season in summer. The glaciers of the tropical Andes experience snow accumulation during the wet season, austral summer on their upper regions and maximum ablation during the same season low on the glacier. In the dry season winter there is a period of low ablation over the whole glacier. Mean annual air temperature at the long term snowline at 5250 m is -1.5 °C. Mean precipitation is about 0.9 m/year.
Since 1991 the glacier has lost more than 5 m of thickness and has retreated significantly. The mass balance loss has been most pronounced during El Nino periods, thus 2009 should not be a good year for Zongo Glacier. La Nina’s are associated with positive or only slightly negative mass balance. The ongoing mass balance loss has led to retreat of 184 meters of this glacier from 1996-2005. A comparison of satellite images from 2004-2008 indicate a retreat of 70-75 meters, this is consistent with the reported retreat rate of 18 meters per year. The glacier has withdrawn from the new glacier lake formed from the ongoing glacier retreat in the 1990’s. The images below are focussed on the terminus in 2004 and in 2008, note the retreat from the lake shore.
Zongo Glacier continues to have an accumulation zone, a necessary essential for glacier survival, and unlike the nearby Chacaltaya Glacier which has disappeared in 2009, it will exist for sometime. The Chacaltaya Glacier is a small glacier, like 80% of the glaciers in this region of the Cordillera Real, and its disappearance puts more pressure on the water resources provided by the larger remaining glaciers such as Zongo Glacier.
The Yakutat Glacier during the 1894-1895 Alaskan Boundary Survey ended near a terminal moraine on a flat coastal outwash plain. By 1906 the glacier had retreated from the moraine and a new lake was forming. Harlequin Lake. Surveys of the terminus of the glacier indicated a retreat of 1 kilometer in that decade. From 1906-1948 the glacier retreated an additional 5 km. From 1948-1958 the glacier retreated 3.6 km. The retreat is evident in comparing the Yakutat B-3 quadrangle, from 1958 photography, and Landsat imagery from 1984, 2010 and 2013. Points A-D are the same in each image and the yellow dots are the terminus. In 1984 the terminus was just retreating from a peninsula marked A, the valley at D was filled with ice, there was no break in the surface at C and B was well inland of the terminus. By 2010 the glacier had retreated from A, the valley at D was deglaciated, a small strip of bedrock-sediment was exposed at C from what had been beneath the glacier, and B was still well inland of the terminus. By 2013 the northern arm of the glacier had retreated 6.4 km from the peninsula at A toward the peninsula at B. The central arm of the glacier toward C had retreated 7.5 km and the retreat on the southern edge of the glacier was 6.5 km. The glacier had retreated on average more than 6.6 km in 30 years, a rate of 220 m/year. The retreat was most rapid from 2010-2013, when the glacier retreated 3 km.
Yakutat terminus map
Today the glacier is the focus of a study by the University of Alaska, led my Roman Motyka, Martin Truffer and Chris Larsen
They have set up a time lapse camera to record frontal changes. The goal is to understand the controls on calving into Harlequin Lake of this glacier. More amazing than the retreat has been the observed thinning of the glacier. The glacier has thinned by more 200 m on average according to the preliminary thickness change maps from the UAF project (Truessel et al 2013). The Yakutat Glacier does not have a high accumulation zone and the recent increase in the snowline elevation and thinning of the glacier have led to a substantial shrinking of the accumulation zone and thinning of the glacier in the accumulation (Truessel et al 2013). This glacier does not have a persistent significant accumulation zone and cannot survive (Pelto, 2010). For a calving glacier to be in equilibrium it needs to have at least 60 % of its area snowcovered at the end of the summer. The glacier is in the midst of a large ongoing retreat. The retreat rate and calving mechanism is similar to that of Grand Plateau Glacier, Bear Lake Glacier and Gilkey Glacier. However, unlike these Yakutat Glacier lacks an accumulation zone, a better analog is East Novatak Glacier, which also has a lower elevation accumulation zone.
Above is a paired Landsat image from 1984 left and 2013 right indicaing the 3200 m retreat during this period of Gilkey Glacier.The Juneau Icefield Research Program has long monitored the mass balance of the Lemon Creek and Taku Glacier on the Juneau Icefield. This program begun by Maynard Miller in 1946 and continuing through today has also monitored the terminus behavior of the icefields outlet glaciers. Of the 17 significant outlet glaciers 5 have retreated more than 500 m since 1948, 11 more than 1000 m, and one glacier the Taku has advanced. I have a chance to visit the glaciers during a number of summers over the last 25 years as part of this ongoing annual program. The Gilkey Glacier is a 32 km long 245 km2 outlet glacier flowing west from the Juneau Icefield. In 1948 it terminate at the head of a braided outwash plain. At that time it was joined 5 km above the terminus by the Battle and Thiel Glaciers from the south. All three of these glaciers drain from the Juneau Iceifeld accumulation zone between 1500 and 2000 m, which maintain consistent snow cover. From 1948 to 1967 the Gilkey Glacier retreated 600 m and in 1961 a proglacial lake began to form. By 2005 Gilkey Glacier had retreated another 3200 m , generating a proglacial lake that is now 3.9 kilometers long, which is approximately the amount of retreat in the last 60 years as well. The lake is partly filled with large icebergs from disintegration of the, note below in an image from Scott McGee of JIRP,Gilkey terminus. The lake is currently terminating in this still growing lake. Approximately half of this retreat occurred after a 1991 satellite image indicated the lake was close to half its current size. The retreat has been resulted from calving icebergs into the new lake as well as thinning from melting in the lower reach of the glacier. The extensive debris cover and lack of crevassing in the lower 1500 meters of the glacier indicates that this section is stagnant and will break up soon.
Gilkey Glacier was in 1955 joined by the tributary glaciers Battle and Thiel Glacier. A visit to the Battle Glacier in 1982 indicated that it had separated from the Gilkey Glacier and the Thiel Glacier, but the Thiel Glacier was still connected. By 1991 the Thiel Glacier had separated. Today these glaciers terminate 3200 m and 1700 m up their respective valleys from Gilkey Glacier. Thiel has retreated 1700 m from the Gilkey Glacier. A retreat of 3200 m has created a glacier 70 % its former length. The vast bare valley beyond the terminus is in stark contrast to the map above. Thiel Glacier has extensive lateral moraines extending above the glacier terminus indicating the ongoing retreat. The lower 3 kilometers of this glacier are flat and are downwasting, indicating a substantial retreat is still underway. A view up the valley from the Gilkey toward the terminus of Battle Glacier indicates that most of the area deglaciated was a flat low elevation valley. Now that the glacier is retreating up a steeper slope, the retreat rate of Battle Glacier should slow.
The following pictures give a year by year view of Columbia Glacier within one day of August 1. The best year was 1999, the worst, 2005.The snowy peaks of the Monte Cristo region can be seen from the Everett area. With 30 glaciers many at low altitudes, this region may receive more snow than any other region in the North Cascades. The largest and lowest is Columbia Glacier occupying a deep cirque above Blanca Lake and ranging in altitude from 4600 to 5700 feet. Kyes, Monte Cristo and Columbia Peak surround the glacier with summits over 2000 feet above the glacier. The Monte Cristo range is the first major rise that weather systems coming off the ocean encounter on the way east to the Cascade Crest. As a result precipitation is heavy. During the summer if it is raining anywhere in the North Cascades it will be in the Monte Cristo region. The glacier is the beneficiary of heavy orographic lifting over the surrounding peaks, and heavy avalanching off the same peaks. We measure the mass balance of this glacier each year and report the data to the World Glacier Monitoring Service. The location is gorgeous as seen in this painting by Jill Pelto Despite the advantages of snow accumulation the glaciers mass balance since 1984 has average -0.5 m a year for a cumulative loss of 13 m. For a glacier that averages 60 m in thickness this is over 20% of its volume. Details of the mass balance research and methods are at
Columbia Glacier has retreated 134 m since 1984. Lateral reduction in glacier width of 95 m in the lower section of the glacier and the reduction in glacier thickness are even more substantial as a percentage. The major issue is that the glacier is thinning as appreciably in the accumulation zone in the upper cirque basin as at the terminus. This indicates a glaciers that is in disequilibrium with current climate and will melt away with a continuation of the current warm conditions. The glacier has lost 17 m in thickness since 1984, but still remains a thick glacier, over 75 meters in the upper basin and will not disappear quickly.
A lateral moraine deposited during the Little Ice Age, is visible at the western edge of the glacier, descending below the glacier to 4250 feet. This moraine has little vegetation on the inside, but is vegetated on the outside. Just in front of the terminus are two terminal moraines deposited during retreat in the last 20 years. Facing southeast Columbia Glacier is protected from any afternoon sun except during the summer. During the winters storm winds sweep from the west across Monte Cristo Pass dropping snow in the lee on Columbia Glacier. Avalanches spilling from the mountains above descend onto and spread across Columbia Glacier. The avalanche fans created by the settled avalanche snows are 20 feet deep even late in the summer. Nearly a third of the glacier is covered by avalanche fans, but no summer avalanches have been observed. Avalanches, shading from the sum provided by the high peaks, and wind drift snow deposition permits Columbia Glacier to exist at such a low altitude.
The north facing side of the Stubai Glacier, also referred to as the Schaufel Ferner, that comprises the biggest ski area in the area is open all summer down to the Eisgrat lift station. There are two main lifts that traverse up the glacier, some of the towers for the ski lifts are set right on the glacier. The linear features extending down glacier in this satellite view of the glacier are the ski lifts and the ski runs. The Stubai Glacier has been retreating and thinning significantly as have most all glaciers in the Alps. Austria has a long term program monitoring the terminus position of over 100 glaciers. From 2000-2005 of the 115 glaciers observed and reported to the World Glacier Monitoring Service, all 115 experienced net retreat. The mass balance of Austrian glaciers, which represents volume loss, reported to the WGMS has been averaging a loss of more than 0.5 m per year since 1998. The loss of 5 m of ice in a decade on glaciers like the Stubai represents about 10% of their volume lost this decade. Stubai Glacier has experiences a 33% loss in its area since 1969 shrinking from 1.72 to 1.15 square kilometers (Aberman and others, 2009). This ongoing ice loss prompted the ski area in 2003 to begin to explore means to preserve the glacier and maintain there ski season. They turned to the University of Innsbruck’s Andrea Fischer and Marc Olefs, who explored three means to reduce the summer melting. Olefs and Fisher (2007) Innsbruck University.The first was injecting water during the winter into the cold snowpack to make it denser. This did add mass, but did not reduce the melt rate. The second methods was to pack down the snow periodically in the winter, again making it denser. Likewise this did not reduce ablation. This is not surprising given that ablation rates on dense ice and less dense snow are very similar on glaciers. The third method was to cover the glacier with a blanket, they used both felt and plastic. The plastic was more reflective, thinner and easier to deploy and as seen in the next two photographs blends in well with the glacier surface. The top image is from Ineedsnow.com. This technique reduced ablation by 60%. Is snow making now being employed a better answer? The problem is that even one small glacier ski slope is still a large area to cover. Because of this success in 2005, the ski resort continues to employ these white polyethylene sheets to reduce melting in strategic areas on the glacier. They are typically spread out in May. The sheets can be seen emplaced around the lift towers in particular. The bare ice of the main section of the glacier is an area of 400,000 square meters (4,300,000 square feet) tough to cover with material, even if it is a low cost per square meter. This type of geoengineering applied to just part of one small glacier maybe practical, but it is not practical at a significant scale. The severity of the climate change we are experiencing is emphasized by the extent to which the ski area is being forced to adapt to try and maintain its summer ski area. In the pictures below, the problem is illustrated by the extent of bare glacier ice late in summer. The ski lifts are apparent as are the square snow patches around the lift towers in the upper image. In the lower image the view from the gondola shows a glacier with very little snow remaining, this is a sign of a glacier that is quickly losing mass.
This video examines the 2009 North Cascade Glacier Climate Project Field Season. It is a look more at where we work, than what we find. Mass balances were substantial averaging -1.93 m. The winter season was wetter than average, with close to average snowpack in the North Cascades. Summer melt conditions were exceptionally warm, leading to enhanced melting and considerable losses in glacier volume. The mass loss of approximately 2 meters represents about 4% of total glacier volume, gone in one year. In July ablation averaged 8.5 cm per day. For glaciers that average 50-60 m in thickness that is the loss of 2.5 m in one month. Most of the melt was snowpack from the previous winter. However, by mid-August blue ice was exposed on the majority of the glacier surfaces across the North Cascades and any ablation was a loss in long term glacier volume. In the case of Columbia Glacier at the end of August a view of its surface indicates only a few white patches of snow remaining. Glacier retreat was slowed on Rainbow and Ice Worm glaciers where the terminus was buried under avalanche snow. Retreat of Easton Glacier was 20 m, Lower Curtis Glacier 11m, Daniels Glacier 12 m, Lynch Glacier 8 m. All 42 glacier observed retreated in 2009. Easton Glacier developed a new rock outcrop in the midst of the glacier. More details on this project North Cascade Glacier Climate Project
Bugaboo Glacier is the most visible glacier of the Bugaboo Range in southern British Columbia. The Conrad Kain Hut who was the first mountaineer to explore the range is adjacent to the glacier and is the main starting point for climbers lured to the region by the fantastic quartz monszonite spires. Since Conrad Kain’s visit in 1910 with the famous mountain explorer Tom Longstaff the Bugaboo Glacier has retreated 2150 meters. The glacier continues to flow around both sides of Snowpatch Spire, the aptly named peak surrounding by the glacier on the left side of the glacier. Also notice the glacier width declines as it descends past a prominent ridge trending toward the glacier from the right, before expanding in the valley below. The glacier used to flow all the way to the valley bottom. The active front was both steep and crevassed By the early 1970’s the glacier had retreated from the valley bottom and had begun to retreat up the mountain slope adjacent to the Kain Hut. Snowpatch Spire is still surrounded by ice, but ice that is not as crevassed, indicating reduced thickness and velocity. The glacier than ended somewhat below where the ridge approached from the right of the glacier.
In the ensuing 34 years the glacier has continued to retreat up the slope and is notably thinner coming down the final steep slope. The glacier ends at the termination of the ridge on the right of the glacier. The glacier has retreated 420 m since 1972. . The first view is nearly identical to the 1972 view and the second more similar to the 1910 view A Google Earth image from 2005 of the terminus region indicates that the lower 700 meters of the current glacier is thin and lacks any crevasses indicating it nearly stagnant. By 2012 the terminus had retreated 80-90 m from 2005, yellow arrow indicates 2012 terminus, pink arrow 2005. This is in contrast to an active front, which indicates a healthy glacier, that would be thick and crevassed. A healthy glacier can still be retreating, the front was active at the time of my first visit to the glacier in 1984. The glacier is still 3.5 kilometers long and even after the retreat the glacier will still have a substantial length and area.
Honeycomb Glacier is one of the longest and largest glaciers in the North Cascades. It is currently 3.7 km long and has an area of 3.1 km2. It has retreated 2.05 kilometers since its Little Ice Age Maximum. The glacier was an imposing site to C.E. Rusk who recounted his early 20th century exploration (1924). Like all 47 glaciers observed by the North Cascade Glacier Climate Project it has retreated significantly since 1979.
A 1960 photograph taken by Austin Post shows the glacier ending with no lake at its terminus. The terminus is gentle and has no crevasses, indicating it is relatively stagnant and poised to melt away. The glacier has retreated 1.3 km from its Little Ice Age moraines at this point. In 1967 another Austin Post image indicates a new small lake forming at the terminus.
In 1995 we visited the margin of this lake, where the glacier ended in 1967 and took a photograph back to the glacier. As seen below retreat to this point was 400 m. A pair of images from Bill Arundell in 1973 and Lowell Skoog in 2006 indicate the scale of the retreat, these images do not show the actual terminus but do show the main nunatak-rock island and how much it has become exposed in the 33 years. This nunatak was hardly evident in 1960, and in a 1940 image of the glacier literally did not yet exist. The terminus had retreated 400 m from the 1967 position to 1995. In 1987 a new lake began to form at the terminus of the glacier at 1680 m. The glacier is shown ending in this lake in 2002 from both the far end of the lake and the nunatak above the lake, the glacier had retreated 210 m since 1995. In 2006 the glacier retreated from the end of this lake. This is a shallow lake that may eventually be filled in by glacier sediments. The terminus is flat and stagnant ending at 1680 m in the lake. Thus, the rapid retreat will continue, the glacier is still not close to acheiving a post LIA equilibrium. Glacier retreat from 1940-1967 averaged 9 m/year. Retreat was minor between 1967 and 1979. The retreat rate since 1979 has been greater than 38 m/year, with a total retreat of 700 m. The nunatak in the middle of the glacier, which was beneath the ice in 1940 is now 90 m above the ice. The section of the glacier below the nunatak in 2002 is stagnant with no crevasses. Indicating this glacier will retreat at least to the base of this rock knob, which will then no longer be a nunatak. A comparison of Google Earth Imagery from 1998 and 2009 illustrate the appearance of numerous new bedrock knobs in the area where there was an icefall in 1995.
survive The upper portion of the glacier has retained its snowcover in recent years indicating the glacier can survive current climate at a much smaller size.
Zemu Glacier is a 26 km long glacier draining the east side of Kanchenjunga the world’s third highest mountain. The importance of the glacier is that it is a key water source for the Teetsa River. The glacier acts as a natural reservoir releasing water due to melting. The Teetsa River is the focus of a hydropower development project being undertaken by the Government of Sikkim. To date 510 mw of the proposed 3500 mw potential are operating. This is a run of the river project, with the water extracted from the river without a dam, run along the valley wall and dropped back to the river through a series of turbines. Run of river is much less expensive than a dam in this remote, earthquake prone, mountainous valley. Zemu Glacier has received little attention, and hence we will have to rely on Digital Globe imagery to observe its changes. The glacier has been observed to retreat at 27 m per year from 1967-1984. Given the length of the glacier the retreat was fairly slow. The glacier has a heavy debris cover on most of its length, insulating it from ablation, and leading to know detectable retreat of the main terminus from 2000 to 2013 Basnett et al (2013). A view of the lower glacier indicates this heavy debris cover, with some scattered small glacial lakes on its surface.
The newly devegetated zone beyond retreating glaciers is small, indicating the slow retreat. Thinning has been significant. The lateral moraine ridges on either side of the main glacier average 150 feet above the main glacier surface. These were built during the Little Ice Age advance. Lateral moraines do not reach above the glacier surface that built them. Thus, the lower glacier has thinned by approximately 150 feet in the last century or so.
A view of a portion of the upper glacier indicates one issue for the glacier. Several of the tributaries no longer join the Zemu, depriving it of a portion of a portion of its former accumulation sources. Near the head of the glacier the walls of Kanchenjunga delivers the debris and large amounts of snow in the form of avalanches to the glacier basin at 5900 to 5200 m. The lower 18 km of the glacier is in the ablation zone where melt dominates. A comparison of 2000 and 2013 Landsat images indicates the lack of change in location of main terminus, red arrows, but recession of surrounding glaciers in the Zemu Basin, yellow arrows.
This area from 5200 m to the 4200 m terminus would quickly melt away without the natural debris cover. The glacier receives considerable snow input from up to 8000 m via avalanches, which are deposited in this region between 5200 m and 5900 m. This glacier will continue to be a large water source for the Teetsa River for the foreseeable future. The glacier has not been retreating as fast or developing a proglacial lake as has happened to Southh Lhonak Glacier, Middle Lhonak Glacier and Changsang Glacier to the north, this should be anticipated in the near future.
Above is a pair of Landsat images from 1984 and 2013 indicating the 2600 m retreat of Antler Glacier in that period. Below is a detailed analysis of the glacier.
The Antler Glacier is an outlet glacier of the Juneau Icefield. It is actually a distributary glacier of the Bucher Glacier. It splits from the Bucher Glacier 8.5 km above where the Bucher Glacier joins the Gilkey Glacer as a tributary. In 1948 it spilled over the lip of the Antler River valley from the Bucher Glacier and flowed 6 kilometers downvalley to end in a proglacial lake. The glacier was 6200 m long in 1948. Note the comparison of the USGS map based on 1948 photographs and the 2005 satellite image below. My only chance to see this glacier in person was in August, 1981 scouting the geology along the Bucher Glacier. Antler Glacier disappeared downvalley into the fog and light snow flurries. The terminus not in site, and icefall to daunting to wish to descend. By 2005 the glacier has retreated almost to the lip of the valley, a 5400 m retreat which is 85-90% of it total length. The Lake -Antler Lake- has expanded from a length of 1.6 km to 4.2 km. The lake is a gorgeous sight, and the valley once filled by the glacier is now nearly devoid of glacier input. The retreat is largely a result of reduced flow from the thinning Bucher Glacier which no longer spills over the valley lip significantly. As the Bucher Glacier continues to thin, the Antler Glacier will cease to exist. This thinning is due to increased melting (ablation) of the glacier. The neighboring glaciers Field and Gilkey Glacier have also thinned and retreated considerably.
The Jakobshavn Isbrae (glacier) has captured our attention over the last 30 years because it has the highest long term average velocity of any glacier in the world. At the ice front the velocity has remained above 16 meters per day for all measurements completed over the last 50 years. The ability of this glacier which is 10 km wide at its front and 800 m thick at the calving front to drain 6.5 % of the Greenland Ice Sheet is its importance. The annual volume of discharge is 40 cubic kilometers. This prompted the University of Maine’s Terry Hughes to take a close look at the glacier in 1985. I participated in that project and one key conclusion we reached was that the Jakobshavn Isbrae was in approximate equilibrium (Pelto and others, 1989). The terminus had not shifted significantly in the past 30 years and no thinning was evident either. The image below of terminus change The top image above is from Jason Box, Byrd Polar Research Center, Ohio State University, and is a mosaic of Landsat and ASTER images indicates a substantial retreat from 1850-1964 of about 30 km. The first image is from the fall of 2009 and the second a Modis image from June 2010. The third from July 2010,from NASA. The last two in the sequence are Landat 8 imagery from May 9 and June 1, 2014 indicate an early summer retreat that was first noticed by the ongoing sharp observations of Espen Olsen. The retreat follows the typical winter advance, and is not back to the 2013 furthest retreat also identified by Espen Olsen. Notes on these latest images below the sequence.
From 1964 to 2001 the glacier terminus did not recede significantly and observations of terminus velocity remained relatively constant at 16 to 20 m year at the glacier front. Then in 1997 an acceleration began. The velocity reached 34 m per day by 2003, twice its normal speed, the glacier thinned by up to 15 m year and retreated 10 km, from 2001 to 2003. From 2004-2007 an additional retreat of 5 km occurred.
The pink arrow notes a prominent lateral rift, the yellow arrow a reference point. Red dot is a transverse rift near the ice front. No large rifts are apparent in the area of the main calving on May 9.[/caption] A bedrock high beneath the glacier is reflected by the sudden increase in slope below point A. What is fascinating is the speed at which the glacier surface below A at Point C was transformed from an ordinary set of transverse crevasses to the chaotic scene typically indicative of an area of rapid acceleration and failure of seracs, those walls betweens crevasses. The glacier has had a profound response to the rifting-calving retreat of the previous day. The area of crevasse transformation is an indication of the connection of this area of the glacier to action at the terminus, the crevassed areas response was so swift that it was effectively involved in the calving retreat incident. The area around C is a zone of weakness to watch for further appearance of rifting. The area in front of the bedrock high is clearly not a place for the terminus to stabilize. The bedrock high itself could well be a point of greater stability for the terminus. Upglacier 2010 is not a good year for the glacier either the snowline is high for June exposing larger areas of bare glacier ice with higher albedo for melting, see image at bottom.
On Jakobshavn the acceleration began at the calving front and spread up-glacier 20 km in 1997 and up to 55 km inland by 2003 (Joughin et al., 2004). Luckman et. al., (2006) observed…“The most plausible sequence of events is that the thinning eventually reached a threshold, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. It is reasonable to believe that the 1998 Jakobshavn speed-up, also following a long period of stability, was triggered by the same processes of thinning but occurred earlier and after a shorter period of thinning because the tongue was already afloat.”
On Jakobshavn the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent. This indicates that it is the change in conditions at the calving front where the backforce on the glacier was reduced that allowed acceleration and retreat. This is typical for Greenland marine terminating outlet glaciers, they have accelerated most at the calving front and the acceleration is not seasonal. The acceleration is not significantly due to meltwater enhanced lubrication. Below is the acceleration of the last decade compared to before, illustrating that the greatest acceleration is at the calving front (Thomas et al., 2009).
The Jakobshavn is of particular importance as it has a bed below sea level for at least 80 km inland from the terminus. In this reach there are no significant pinning points, or abrupt changes in slope or width (Clarke and Echelmeyer, 1996) that would help stabilize the glacier during retreat. In particular the bed becomes deeper from 24-40 km behind the calving front, which should reinforce calving acceleration (Thomas et al., 2009). Images of Jakobshavn Isbrae in 2001 indicate substantial rifts on the north side of the glacier near the 2005 terminus position, suggesting the glacier had been preconditioned for retreat. In the image below from June 17, 2010 the snowline is evident on the north side of Jakobshavn as the transition to the much lighter blue tone, in this Landsat image. The red line is the June 2009 snowline and the green line the 2008 June snowline.
A comparison of April 2010 (top image below) and April 2011 Landsat image (middle image) indicates a somewhat lower snowline on the Jakboshavn in 2011. The zoomed in version indicates the amount o the ice that is actually icebergs.