Observations of enhanced thinning in the upper reaches of Svalbard glaciers

Changes in the volume and extent of land ice of the Svalbard archipelago have been the subject of con- siderable research since their sensitivity to changes in cli- mate was first noted. However, the measurement of these changes is often necessarily based on point or profile mea- surements which may not be representative if extrapolated to a whole catchment or region. Combining high-resolution elevation data from contemporary laser-altimetry surveys and archived aerial photography makes it possible to mea- sure historical changes across a glacier's surface without the need for extrapolation. Here we present a high spatial res- olution time-series for six Arctic glaciers in the Svalbard archipelago spanning 1961 to 2005. We find high variabil- ity in thinning rates between sites with prevalent elevation changes at all sites averaging 0.59± 0.04 m a 1 between 1961-2005. Prior to 1990, ice surface elevation was changing at an average rate of 0.52± 0.09 m a 1 which decreased to 0.76± 0.10 m a 1 after 1990. Setting the elevation changes against the glaciers' altitude distribution reveals that signif- icant increases in thinning rates are occurring most notably in the glaciers' upper reaches. We find that these changes are coincident with a decrease in winter precipitation at the Longyearbyen meteorological station and could reflect a de- crease in albedo or dynamic response to lower accumulation. Further work is required to understand fully the causes of this increase in thinning rates in the glaciers' upper reaches. If on-going and occurring elsewhere in the archipelago, these changes will have a significant effect on the region's future mass balance. Our results highlight the importance of under- standing the climatological context of geodetic mass balance measurements and demonstrate the difficulty of using index glaciers to represent regional changes in areas of strong cli- matological gradients.