The conventional wisdom is that glaciers are retreating as a result of global warming. That is that glacier fronts are drawing back and consequently the glacier area is reducing. This is somewhat simplistic. Blåmannsisen is the fifth largest glacier on mainland Norway. It is 87 km^2, much larger than any Swedish glacier. The icecap also supplies meltwater for hydropower plants run by two companies.
Satellite imagery from Landsat-7 shows us that the area of Blåmannsisen has not changed significantly since 2001, despite a warming climate. That is not to say that the glacier is unchanged. The firn line, which is essentially the long-term boundary between accumulation and ablation (melt), has retreated 1 km in 10 years (see figure).
The firn line was mapped in winter synthetic aperture radar (SAR) data from the European Space Agency. Images from 1992-2010 were analysed. The advantage of using the SAR data, as opposed to optical imagery from Landsat, is that the SAR can penetrate clouds, and does not require solar illumination. Furthermore, the SAR penetrates the snow cover and is backscattered (reflected) by the firn and ice beneath. This allows us to identify a range of features or zones on the glacier. In doing so we can make more sophisticated analyses of climate impacts on the glacier. Samples of backscatter from the sites shown in grey (FLA96, Sh06, FLA06, Centre) show that backscatter drops as the firn line retreats past the sample sites. Site Deep was used as a control to ensure the images used were comparable.
What is more, comparing digital elevation data from the Norwegian Mapping Authority and differential GPS data from measurements made on the glacier in winter 2011, we find that the glacier has down-wasted: i.e. it has been shrinking vertically, though not horizontally. Near the firn line the surface of Blåmannsisen has dropped 10 m.
Blåmannsisen shows us that the paradigm of retreating glaciers in response to warming, is both simplistic and flawed. It also calls into question the validity of mapping glacier areas without any analysis of volume or surface change. That is not to say glacier front mapping is redundant: by sampling a large population we increase the utility of the data. Glacier area data are also useful to other communties, such as modellers and paleo-glaciologists. Nevertheless, the case of Blåmannsisen reminds us that climate impacts and glacier responses to climate are complex and require more thorough observational approaches.