GLACIER CHANGE IN NORTHERN SWEDEN FROM AD500 - A SIMPLE GEOMETRIC MODEL OF STORGLACIAREN

A simple geometric model of glacier volume is derived. The model is ba sed on the assumption that the net mass balance averaged over the glac ier surface is related to the summer temperature and winter accumulati on at a representative height on the glacier. This height varies with time as climate changes, in ways that are determined by the geometry o f the glacier surface. Expressions are derived for the equilibrium gla cier volume (V-eq) as a function of summer temperature and winter accu mulation and for the glacier-response time (tau) as a function of volu me. The model is used to reconstruct the volume the Storglaciaren over the period AD 500-1992. Measured net mass-balance data for Storglacir en (1946-92) are used to estimate the model parameters. For the summer temperature forcing, the long tree-ring-based temperature reconstruct ion for northern Fennoscandia is used to extend a temperature record n ear the glacier back to AD 500. For the past accumulation forcing, a r ange of assumptions is tested. The results show that the prime cause o f the decrease in volume of Storglaciaren between 1946 and 1992 was re latively low accumulation over 1946-89 as opposed to warm temperatures as previously supposed. Reconstructed volume changes agree well with geomorphological evidence. Where differences occur, deductions can be made about past accumulation. For example, the maximum reconstructed g lacier volume is associated with the cold period from 1580 to 1740 but relatively low accumulation probably limited the volume to values not much larger than that achieved around 1916.