The SPIRIT project's objective was to enhance our knowledge of the polar region topography. During the fourth international polar year, couples of stereoscopic images of many polar glaciers were collected by the Spot 5 HRS instrument. High-resolution images and digital field models were thus distributed to glaciologists of some twenty countries. This new topography of the polar regions is being used to study their response to climate change.
During the last decades, the cryosphere experienced important changes and sometimes spectacular ones. The melting of mountain glaciers and small polar icecaps accelerated and major changes of the glacial dynamics were identified in Greenland and Antarctica.
Ice caps and glaciers contribute to two-third of the 3-mm/year sea-level rise. So the continental cryosphere appear to be one of the main actor and witness of the current climate change.
An homogeneous and precise topography of these remote regions is crucial to characterize their response to the recent climate change, quantify their contribution to the sea-level rise and anticipate their future evolution using run-off patterns. Nevertheless, the topography of the glaciers, ice caps and glacial platforms is still poorly known, especially because in situ observations are rare and difficult to perform.
A reference topography
In this context, CNES, Spot Image, IGN Espace and LEGOS initiated the SPIRIT project (Spot 5 stereoscopic survey of Polar Ice: Reference Images and Topographies) in order to establish a reference topography during the fourth international polar year (IPY).
The objectives of the SPIRIT project were to make up a significant Spot 5 HRS image collection of polar ices and, for some regions and to produce digital field models (DFM) and high-resolution images in a bid to freely distribute them to the science community. Some glaciologists of some twenty countries took advantage of these data thanks to the SPIRIT project.
The Spot 5 HRS sensor was designed to produce DFMs by acquiring couples of stereoscopic images. It features two telescopes pointing forward and backwards and tilted by +/- 20º according to the vertical. The short time interval (90s) between the two stereoscopic scene acquisitions ensures limited changes at the surface of the glacier and an almost identical solar illumination.
The zone selection included the Greenland and Antarctica coastal areas as well as every other glacial masses (Alaska, Iceland, Patagonia, etc.) surrounding the Arctic and Antarctic oceans. The central, flat and homogeneous regions of the Antarctic and Greenlandic ice caps were deliberately excluded because the SPIRIT DFMs could not compete against the decimetric accuracy of space, radar and laser altimeters.
Images of approximately 75 % of the target areas could be taken without any cloud.
SPIRIT and the glacial mass losses
The images and digital land models resulting from this project allowed the Alaska's glaciers contribution to the sea-level rise between 1962 and 2006 to be recalculated.
The volume variations were estimated for every glacial region of Alaska by comparing the digital land models derived from the recent satellite data with the maps from the 1950's and 1960's.
The evolution of the Chugach Mountains' glaciers translates the complexity of the glacial response to climate change. In this region, some rare glaciers are getting thicker and their snouts are moving forwards (in blue) even if the great majority of them tends to get thinner (in red).
On the contrary, the Columbia glacier has significantly retreated: since 1980, when its started retreating, this glacier lost up to 400 m thick, i.e. more than 20 m/year. The altitude variations of the Alaska glaciers are not the same and it is thus impossible to sample such a spatial variability on account of some ground and airborne measurements.
In total, the melting of the Alaska glaciers made the average sea-level rise by 0.12 mm/year between 1962 and 2006.
Thanks to their regional coverage, the SPIRIT data do enhance the observations of the global warming impacts on the glaciers and help to make the measurements of the freshwater flows to the ocean more precise. The SPIRIT data are now used by many glaciologists in a bid to map the changes in most of the large glacial regions such as the Svalbard, Iceland, Antarctica and Greenland.
Coming next, the same data should also help the glaciologists to diagnose the glaciers' conditions in the Himalayan Arc.
Contribution of Alaskan glaciers to sea level rise derived from satellite imagery, Berthier E., Schiefer E., Clarke G.K.C., Menounos B. & Remy, F., Nature Geoscience, 3(2), 92-95