The past predicts the future

Old data can take on new life when analysed using newer technologies. This article describes a prime example.

By: Jack Kohler, Emily Geyman and Harald Faste Aas // Norwegian Polar Institute

In 1936 and 1938, Norwegian expeditions used a seaplane equipped with a large-format camera to acquire high-resolution oblique aerial photographs covering nearly all of Svalbard, over 5500 images in total. The photographs were taken to make topographic maps of the region, using the then relatively new technique of aerial photogrammetry.

Converting these photographs into maps started soon after their acquisition, but work was halted by World War II.

Production resumed after the war, but by the 1960s maps were being constructed using newer imagery, and the older photographs were set aside. Some have been used since then, but the entire collection had, until recently, remained largely unexploited.

Sequential oblique photographs covering Eidembreen and Venernbreen, south of St Jonsfjorden.

Photos: Norwegian Polar Institute:

Scientists at the Norwegian Polar Institute have now analysed the full 1936/1938 image archive, using modern photogrammetric techniques, to accurately map Svalbard’s historical glacier cover. Derived products include a digital elevation model, and an orthophoto, which is a virtual image from a birds-eye perspective, correctly placed in a modern reference frame. With these data in hand, it is possible to simulate what glaciers looked like in the 1930s, and compare them with modern images.

In a paper published in Nature, data from this reconstruction are compared to modern elevation data to derive the spatial pattern of ice loss over a 70-year timespan.

Between 1936 and 2010, Svalbard ice thickness has been reduced by an average of over 25 metres, or 0.35 metres per year, and glacier area has shrunk by almost 3000 km², about 10% of its 1936 extent. This corresponds to 1.6 mm of sea level rise, or 1.4% of the global mean sea level signal over that period.

Summer temperature provides the strongest control on glacier thinning. Both ice loss and temperature generally decrease from south to north and from west to east across Svalbard, which allows a relation to be developed between temperature and elevation change. Applying this relation to future temperatures predicted for climate change scenarios yields an estimate of future ice loss.

By 2100, mean rates of glacier ice thickness change are predicted to range from 0.67 to 0.92 metres of ice per year, depending on the scenario, roughly 2-4 times the average rate for 1936-2010. This will not only add to global sea level rise, but the glacier retreat and increased glacier runoff predicted will have significant local impact in the fjords around Svalbard.

These new data provide a baseline for the historical glacier change in Svalbard, and allow an improved assessment of the future development of glacier cover.