| Determination of glacier mass balance via the comparison of two terrain elevation models is
one of the most popular, and most accurate methods to monitor glacier mass change over
periods of a few years to some decades. However, the differencing of elevation models
provides a change in glacier volume instead of a mass change which is the relevant quantity
for climate impact studies. Observable glacier volume change is usually converted to a mass
change using a straight-forward density assumption that has received little attention so
far, and can represent a significant uncertainty in geodetically determined mass
balance.
This study investigates the factor to convert geodetic volume change to mass change
based on a firn compaction model applied to simplified glacier geometries with idealized
climate forcing, and two glaciers with long-term mass balance series. It is shown that the
’density’ of geodetic volume change is not a constant factor and is systematically
smaller than ice density in most cases. This is explained by the accretion/removal of
low-density firn layers, and changes in the firn density profile with positive/negative mass
balance. Assuming a value of 850±60 kg m-3 to convert volume change to mass
change is appropriate for a wide range of conditions. For short time intervals (-¤3
years), periods with limited volume change, and/or changing mass balance gradients,
the conversion factor can however vary from 0-2000 kg m-3 and beyond, which
requires caution when interpreting glacier mass changes based on geodetic surveys. |