The glacier discharges ice bergs in calving events from West Greenland and it's calving face continues to retreat over a deep depression in the fjord, which adds to it's instability.
“We are now seeing summer speeds more than 4 times what they were in the 1990s on a glacier which at that time was believed to be one of the fastest, if not the fastest, glacier in Greenland,” says Ian Joughin, a researcher at the Polar Science Center, University of Washington and lead-author of the study: Brief Communication: Further summer speedup of Jakobshavn Isbræ.
Reputedly Jakobshavn Isbræ produced the large ice berg that drifted down into the North Atlantic that sank the Titanic in 1912.
Calving events are a natural part of fluctuations in ice sheet mass balance, and counteract precipitation as snow adds to ice sheet mass. However, what we are seeing in Greenland and parts of West Antarctica (Pine Island and Thwaites Glaciers) is that ice streams and glaciers are accelerating increasing the calving rate and ice mass loss and raising sea levels.
The speed of the glacier does change seasonally with slower speeds in winter months.
The abstract for the study says:
We have extended the record of flow speed on Jakobshavn Isbræ through the summer of 2013. These new data reveal large seasonal speedups, 30 to 50% greater than previous summers. At a point a few kilometres inland from the terminus, the mean annual speed for 2012 is nearly three times as great as that in the mid-1990s, while the peak summer speeds are more than a factor of four greater. These speeds were achieved as the glacier terminus appears to have retreated to the bottom of an over-deepened basin with a depth of ~ 1300 m below sea level. The terminus is likely to reach the deepest section of the trough within a few decades, after which it could rapidly retreat to the shallower regions ~ 50 km farther upstream, potentially by the end of this century.
Caption: Plots of (top) terminus position and (bottom) speed through time for Jakobshavn Isbræ determined from TerraSAR-X data collected from 2009 to 2013. Source: Joughin et al (2014)
The study was conducted by researchers from the University of Washington and the German Space Agency (DLR) who measured the dramatic speeds of the fast-flowing glacier in 2012 and 2013, and builds on previous research into the speed of the glacier.
Watch a National Science Foundation youtube video published September 2013 of David Holland investigating what happens when the warm water from the Atlantic meets the ice:
The Glacier is retreating into a depression 1.3km below sea level, before it rises again. As the grounding lines retreat into this depression, it produces more instability and greater speed of retreat. Warm waters from the Atlantic and warming atmospheric temperatures are contributing to destabilisation of the ice sheet.
In recent years the surface of the ice sheet becomes pock marked with moulins - blue glacial lakes - which eventually drain through cracks and crevasses deep into the ice to help lubricate movement to the coast. The blue surface lakes also reduces the surface albedo further increasing warming and melting.
Moulins and unprecedented surface melt
In July 2012 there was unprecedented surface melt over 97 per cent of its surface. Melting is now being detected even in winter months from south east Greenland.
The authors conclude in their study:
Our results show that Jakobshavn Isbræ has accelerated to speeds unprecedented in its observational record as its terminus has retreated to a region where the bed is ∼ 1300 m below sea level. While the current increase in annual discharge flux remains less than a factor of three, the increase plausibly could reach or exceed a factor of 10 within decades. This is a consequence of the fact that retreat into deeper water increases both speed and thickness of the terminus. Conversely, where retreat to shallower depths occurs, losses will be far more moderate. Hence, a tenfold increase in discharge is likely only to be sustained in the few decades before rapid thinning would cause the terminus to retreat out of the deep trough. Thus, the potential for large losses from Greenland is likely to be determined by the depth and inland extent of the troughs through which its outlet glaciers drain.
The NASA Icebridge project has mapped the depth of this fjord canyon which enables better modelling of glacier dynamics as it retreats. NASA's Goddard Space Flight Center researcher Sophie Nowicki highlighted that if the ground slopes downhill as you move inland from the ocean, or if the glacier is channeled through a deep canyon or fjord, then it is highly likely that the glacier will retreat quickly. However, ridges beneath the glacier can act as pinning points and temporarily slow or stop the retreat.
“Before IceBridge, this measurement of Jakobshavn was the best bed we had,” said Nowicki, (see image below left). “With much more data from the radar instrument (on IceBridge and earlier flights), we realized that the bed of Jakobshavn Isbrae is a channel that could be compared to the Grand Canyon. It’s the same width, the same depth.”
Caption: Old bedmap for Greenland and new bedmap of Jacobshavn Isbrae that was acquired between 1987-2006. Source: NASA Earch Observatory (2011)
The authors of this latest study do point out that the geography of Jakobshavn’s Isbræ is somewhat unique (Bamber et al., 2013), suggesting that the majority of Greenland’s outlet glaciers may not be able to match or sustain similar large increases in ice discharge.
Sea Level Rise
Melting of sea ice and ice shelves doesn't itself add to sea level rise. But as ice shelves melt and grounding lines recede, it allows land based ice from glaciers and ice streams to accelerate into the ocean. It is this ice mass which increases sea level rise.
“We know that from 2000 to 2010 this glacier alone increased sea level by about 1 mm. With the additional speed it likely will contribute a bit more than this over the next decade,” explains Joughin.
Jakobshavn Isbræ drains the ice sheet into Ilulissat Icefjord on the west coast of Greenland. At its calving front, melt water directly contributes to sea level rise while giant icebergs break off also contributing to sea level rise. Both of these processes contribute about the same amount to sea-level rise from Greenland.
A previous study published in June 2013 highlighted the need to understand the dynamic ice loss from calving glaciers. It reported on 17 continuous velocity records from eight major marine-terminating outlet glaciers from the Greenland ice sheet derived from single-frequency standalone Global Positioning System (GPS) receivers placed on the glacier surface, covering varying parts of the period summer 2009 to summer 2012.
Since the dynamic mass loss is believed to constitute roughly half the contribution to sea level rise from the Greenland ice sheet over the last decade (Van den Broeke et al., 2009) and appears to be highly variable with time (Andresen et al., 2011; Bevan et al., 2012; Bjørk et al., 2012), understanding this mechanism is of paramount importance to reduce the uncertainty in predicting the impact of future climate change on the Greenland ice sheet.
Greenland is losing net ice mass at an accelerating rate according to the Polar Portal Season Report 2013:
The Greenland Ice Sheet has therefore been losing mass at a rate of about 200 Gt per year over the last decade. 1 Gt is one billion tons and corresponds to 1 cubic kilometer of water. A mass loss of 100 Gt translates to a sea level rise of 0.28 mm.
The ice sheet gained 166 Gt in 2012/2013 at the surface as the net result of snowfall minus melting. And the total mass loss – which includes both the loss through melting and calving of icebergs along with the gain from snowfall – is estimated to be about 430 Gt.
Caption: the total change in accumulated mass of the Greenland ice sheet contributing to sea level using two different methods Source: Polar Portal- Greenland Ice Sheet Total Mass change
Watch this Youtube video by NASA of ice mass loss on Greenland 2003-2011 based upon GRACE satellite data.
Research from the Potsdam Institute for Climate Impact Research and Universidad Complutense de Madrid published in 2012 has lowered the estimate for irreversible collapse of the Greenland ice sheet from 3.1 °C to a likely mean of 1.6 °C, which the planet is likely to pass in the next fifty years.
Read more of my articles on Greenland.
- Joughin, I., Smith, B. E., Shean, D. E., and Floricioiu, D.: Brief Communication: Further summer speedup of Jakobshavn Isbræ, The Cryosphere, 8, 209-214, doi:10.5194/tc-8-209-2014, 2014.
- Ahlstrøm, A. P., Andersen, S. B., Andersen, M. L., Machguth, H., Nick, F. M., Joughin, I., Reijmer, C. H., van de Wal, R. S. W., Merryman Boncori, J. P., Box, J. E., Citterio, M., van As, D., Fausto, R. S., and Hubbard, A.: Seasonal velocities of eight major marine-terminating outlet glaciers of the Greenland ice sheet from continuous in situ GPS instruments, Earth Syst. Sci. Data, 5, 277-287, doi:10.5194/essd-5-277-2013, 2013.
- NASA Earth Observatory - Looking under Jakobshavn, 10 November 2011.
- European Geosciences Union, media release, 3 February 2014 - Greenland’s fastest glacier reaches record speeds
- Image from Polar Portal of Rate of Greenland ice sheet sea level contribution, ver 20131004
- Lead image is of Jakobshavn Fjord taken from NASA's project icebridge flight April 2013. Credit: NASA / Jim Yungel / Flickr (CC BY 2.0)