CR
Cryospheric Sciences

Glacial retreat

Image of The Week – The Pulsating Ice Sheet!

Image of The Week – The Pulsating Ice Sheet!

During the last glacial period (~110,000-12,500 years ago) the Laurentide Ice Sheet (North America) experienced rapid, episodic, mass loss events – known as Heinrich events. These events are particularly curious as they occurred during the colder portions of the last glacial period, when we would intuitively expect large-scale mass loss during warmer times. In order to understand mass loss mechanisms from present-day ice sheets we need to understand what happened in the past. So, how can we better explain Heinrich events?


What are Heinrich Events?

During a Heinrich event large swarms of icebergs were discharged from the Laurentide Ice Sheet into the Hudson Strait and eventually into the North Atlantic Ocean. This addition of fresh water to the oceans caused a rise in sea level and a change in ocean currents and therefore climate.

We know about these events by studying glacial debris that was transported from the ice sheet into the oceans by the icebergs and eventually deposited on the ocean floor. From studying ocean-sediment records we know that Heinrich events occurred episodically during the last glacial period but not on at a regular intervals. Interestingly, when compared to temperature records from Greenland ice cores, it can be seen that the timing of Heinrich events coincides with the cold phases of Dansgaard–Oeschger (DO) cycles – rapid temperature fluctuations which occurred during the last glacial period (see our previous post).

the timing of Heinrich events coincides with the cold phases of Dansgaard–Oeschger (DO) cycles

What do we think causes them?

A new study, published last month in Nature, uses numerical modelling to show how pulses of warm ocean water could trigger Heinrich events. Our image of the week (Figure 1) illustrates the proposed mechanism for one event cycle:

  • a) Ice sheet at it’s full extent, grounded on a sill (raised portion of the bed, at the mouth of the Hudson Strait). Notice the sill is around 300m below sea level at this time.
  • b) A pulse of sub-surface water (purple) warms by a few degrees, encouraging iceberg calving at the glacier front and causing the ice begin to retreat from the sill.
  • c) As the ice retreats, it becomes unstable due to an inwards sloping bed (see our previous post on MISI). This leads to sudden rapid retreat of the ice – characteristic of Heinrich events.
  • d) Due to ice loss and thus less mass depressing the bed, the bed will slowly rise (Glacial Isostatic Adjustment), eventually the sill has risen to a level which cuts off the warmer water from the ice front and the ice can slowly advance again.

Once the ice has advanced back to it’s maximum extent (a) it will slowly depress the bed again, allowing deeper, warmer water to reach the ice front and the whole cycle repeats!

The authors of this study used this model to simulate Heinrich events over the last glacial period and were able to accurately predict the timing of Heinrich events, as known from ocean sediment records. Check out this video to see the model in action!!

Why is it important?

This study shows that the proposed mechanism probably controlled the onset of rapid mass-loss Heinrich events in the past and more generally that such mechanisms can cause the rapid retreat of marine terminating glaciers. This is important as it adds to our understanding of the stability (or instability) of present day marine terminating glaciers – such as the West Antarctic Ice Sheet! If such rapid mass loss happened regularly in the past we need to know if and how it might happen in the future!

such mechanisms can cause the rapid retreat of marine terminating glaciers.


Check out the full study and the news article summarising the findings here:

Image of the Week — Historical aerial imagery of Greenland

Image of the Week — Historical aerial imagery of Greenland

A few month ago, we were taking you on a trip back to Antarctic fieldwork 50 years ago, today we go back to Greenland during 1930s!

When geopolitics serves cryospheric sciences

The Permanent Court of International Justice in The Hague awarded Danish sovereignty over Greenland in 1933 and besides geopolitical interests, Denmark had a keen interest in searching for natural resources and new opportunities in this newly acquired colony. In the 1930s the Danish Government initiated three comprehensive expeditions; one of these, the systematic mapping of East Greenland, was set off by The Greenlandic Agency, The Marines’ air services, The Army’s Flight troops and Geodetic Institute. The Danish Marines provided pilots, mechanics, and three Heinkel seaplanes.

Danish expeditioner Lauge Koch, centre, along with his pilots all dressed in suits made from polar bear. (Credit: The Arctic Institute)

Danish expeditioner Lauge Koch, centre, along with his pilots all dressed in suits made from polar bear. (Credit: The Arctic Institute)

Aerial photography in the 1930s – practical constraints

The airplanes had three seats in an open cockpit. The pilot was seated in the front, the radio operator in the center and in the back the photographer – this seat was originally for the machine-gun operator.

At the outset, the idea was to take vertical images, but that was impossible at the time due to the height of the mountains and the limited capability of the aircraft to reach adequate heights. The airplanes couldn’t reach more than 4000 m – similar to the height of mountains in Greenland. Oblique images were therefore recorded. The reduced view of the terrain when photographing in oblique angles required many more flights than originally planned. The photographic films were processed immediately after each flight. 45,000 km were covered during the first season, which lasted about two and a half months. In the following years, each summer a flight covered parts of the Greenlandic coast. During the Second World War, the mapping was temporarily stopped due to safety reasons.

The aircraft had an open hole in the floor for the photographer, originally where the machine gunner would sit. (Credit: The Arctic Institute)

The aircraft had an open hole in the floor for the photographer, originally where the machine gunner would sit.(Credit: The Arctic Institute)

An unexplored treasure trove of climate data

The tremendous volume of aerial images obtained from several expeditions and hundreds of flights not only constitutes the cornerstone of mapping in Greenland, but is invaluable data for studying climate change in these remote regions. The 1930s survey, compared to modern imagery, provides crucial insight into coastal changes, ice sheet mass balances, and glacier movement. Glacier fluctuations in southeast Greenland have been identified, showing that many land-terminating glaciers underwent a more rapid retreat in the 1930s than in the 2000s, whereas marine-terminating glaciers retreat more rapidly during the recent warming (Bjørk et al, 2012).

An ongoing project between the University of Copenhagen, INSTAAR (Institute of Arctic and Alpine Research) in Boulder, Colorado, and Natural History Museum of Denmark is currently focusing on analysing deltaic changes in Central and Southern Greenland; linking shoreline development to climate changes – these historic aerial images are essential for detecting such coastal evolution. However, there are still many other links between the past and present climate to be discovered from these images. Interested in hearing more about the project or the aerial images? Please contact Mette Bendixen (mette.bendixen@ign.ku.dk)

Bibliography

Bjørk, A. A., Kjær, K. H., Korsgaard, N. J., Khan, S. A., Kjeldsen, K. K., Andresen, C. S., … & Funder, S. (2012). An aerial view of 80 years of climate-related glacier fluctuations in southeast Greenland. Nature Geoscience, 5(6), 427-432. http://dx.doi.org/DOI:10.1038/ngeo1481

Edited by Alistair McConnell, Sophie Berger and Emma Smith


Mette BendixenMette Bendixen is s a PhD student at the Center for Permafrost in Copenhagen. She investigates the changing geomorphology of Greenlandic coasts, where climate changes can have huge impact on the local environment.

Image of the Week — Last Glacial Maximum in Europe

Image of the Week — Last Glacial Maximum in Europe

During the last ice age*, ~70,000 to 20,000 years ago, the climate was much colder in Europe.

As a result, the northern part of Europe was fully covered by the Fennoscandian (a.k.a the Scandinavian ) ice sheet, which extended up to the British Isles and some parts of Poland and Germany. In central Europe, the Alps were also almost fully glaciated.

The storage of all this ice on the continent lowered the sea level (seedark green), which substantially reduced the extent of the North Sea.

*This period is referred to as the Weichselian glaciation and the Würm glaciation in Northern Europe and the Alps, respectively.

 

More information

A more complete and accurate dataset (including GIS maps) of Europe during the last glacial maximum is freely available :

Becker, D., Verheul, J., Zickel, M., Willmes, C. (2015): LGM paleoenvironment of Europe – Map. CRC806-Database, DOI: http://dx.doi.org/10.5880/SFB806.15

LGM_Europe_Map_v1

 

Image of the Week : 63 years of the Muir Glacier’s retreat

Image of the Week : 63 years of the Muir Glacier’s retreat

The Muir is a valley glacier (Alaska) that has significantly retreated over the last 2 centuries. The 3 pictures have the same field of view and record the changes that occurred during the 63 years separating 1941 and 2004.

In the 1941, the terminus of the glacier is on the lower right corner of the photo. The Muir is then a tidewater glacier up to 700m thick and is well connected to its tributary, the Riggs Glacier (upper right part of the photo).

9 years later, in 1950, the Muir Glacier has retreated by more than 3 km, is more than 100m thinner but is still connected to Riggs Glacier.

By 2004, the Muir glacier has retreated further inland and its terminus is no longer visible on the picture. The Riggs glacier is now disconnected to the Muir and has retreated by 0.25km. Vegetation has invaded the place.

Credits:

The photo comes from and the text is inspired from the section “Repeat photography of the Alaskan Glaciers” on U.S. Geological Survey website.
Photo 1: W. O. Field, # 41-64, courtesy of the National Snow and Ice Data Center and Glacier Bay National Park and Preserve Archive.
Photo 2 : W. O. Field, # F50-R29, courtesy of the Glacier Bay National Park and Preserve Archive.
Photo 3: B. F. Molnia, USGS Photograph