Firming up ice sheet mass change

Liz Kalaugher in Environmental Research Web: Using satellites to measure ice sheet elevation is a popular technique for estimating changes in ice sheet mass: combining the sheet's height with a measure of its density indicates mass. Now researchers have corrected these calculations for the Antarctic ice sheet to include fluctuations in the thickness of firn - a layer of old snow above the ice.

Firn becomes more dense over time at a rate that depends on temperature and the weight of new snow added to the surface (accumulation). Michiel Helsen of Utrecht University in the Netherlands and colleagues from the Royal Netherlands Meteorological Institute, University of Missouri, US, and University of Newcastle, Australia, used a firn densification model together with data on snow accumulation and temperature from ice cores to study the impact of variability in these two factors on elevation changes. A simulation of Antarctic firn depth variability for 1980 to 2004 showed that changes in firn layer thickness were comparable in magnitude to measured elevation changes.

"[This work] shows that even insignificant deviations of the accumulation rate will result in significant changes in the thickness of the firn layer, and thereby will change the elevation of the ice sheet," Helsen told environmentalresearchweb. "Researchers were often looking for trends in the accumulation to explain a changing ice sheet surface elevation, while we showed that it is an accumulation rate anomaly with respect to the long-term mean that can cause elevation changes. Since accumulation rate fluctuates on time scales of different orders of magnitude, this complicates the interpretation of satellite altimetry, which covers only a period of around 15 years."…

Firn from the South Cascade glacier in the state of Washington (USA), Agricultural Research Service, Wikimedia Commons