Aerial photographs from a 1930s expedition by Danish explorer Knud Rasmussen were used by Anders Bjørk at the University of Copenhagen and his colleagues to compare historical changes in glacier fronts in southeast Greenland with those of today. The photos were lost to science after being classified as secret and locked in an archive outside Copenhagen. Pictured is Rasmussen’s Heinkel hydroplane after its return from a surveying mission.
Arctic Institute of Denmark
The rough ice and dangerous sailing conditions off the southeast coast of Greenland mean that few historical images of the area exist. Even Rasmussen’s photos are of varying quality. This photo is an aerial oblique image from Rasmussen's Seventh Thule expedition, shot from the side of the aircraft at a height of 3,700 metres. The pen marks are from geodesists, who used the photos to generate maps.
Danish National Survey and Cadastre
Images from a handheld camera on a British Arctic Air Route Expedition in 1931, which investigated the area for a potential landing strip. These were used to fill in the blanks from Rasmussen’s trips, and were taken at varying angles and distances from the ice. Left: Johan Petersen Fjord; right: ice cap on Jens Munks Ø, an island just off southeast Greenland.
Scott Polar Research Institute
Because the aerial shots were taken at different elevations and were not perfectly level, Bjørk and his team had to geometrically correct them so that they had a uniform scale and the same lack of distortion as a map. By superimposing the photos on satellite images from the 1980s, they could then be used to measure distances accurately. Pictured is the 1,700-metre-wide Thrym Glacier in Skjoldungen Fjord, rectified using tie points (red crosses) from 1980s satellite images.
Natural History Museum of Denmark
The US military conducted an intensive aerial photo campaign of the Greenlandic coast in the Second World War, looking for German weather stations. The film rolls were later given to the Danish National Survey, but there are no precise dates for the flights. The best date estimate for this image is 1943, but its true age is still being investigated because a wrong date would skew the researchers' glacier results. Pictured is the land-terminating glacier Skjoldmøen on Skjoldungen Island during the Second World War.
Natural History Museum of Denmark
The US Landsat Earth-observation programme has been providing researchers with a steady stream of satellite observations since 1972. To fill in the gap since the last aerial photographs, Bjørk and his colleagues gained access to recently declassified images from US intelligence satellites from 1965. Pictured are false-colour composites showing the combined outlet of the Rimfaxe and Guldfaxe glaciers. Left: 1972 image taken by Landsat 1; right, 2010 image taken by Landsat 7, the most recent Landsat satellite.
NASA Goddard Space Flight Center/US Geological Survey
Long-forgotten aerial photographs of Greenland from the 1930s, rediscovered in a castle outside Copenhagen, have allowed researchers to construct a history of glacier retreat and advance in the area. The work, by Anders Bjørk at the University of Copenhagen and his colleagues, aims to provide a deeper understanding of how climate change has affected ice loss and glacier movements over the past 80 years.
Most studies of Greenland's glaciers have been done only since imaging satellites became available in the 1970s, so the data are relatively short-term. But using photographs from 1930s aerial surveys of the southeast coast of Greenland, together with US military aerial shots from the Second World War and recent satellite images, Bjørk and his colleagues have been able to observe changes at high spatial resolution from a period in which few glacier measurements were previously available.
Analysis of the images reveals that over the past decade, glacier retreat was as vigorous as in a similar period of warming in the 1930s. However, whereas glaciers that spill into the ocean retreated rapidly in the 2000s, it was land-terminating glaciers that underwent the fastest regression 80 years ago.