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."
The team found that including firn depth fluctuations in calculations of surface height in the Amundsen Sea Embayment in West Antarctica increased the amount of surface lowering, while a large area of the East Antarctic ice sheet slowly grew due to increased accumulation.
"We now know that a highly variable accumulation rate has a large influence on elevation changes," said Helsen. "When ice sheet altimetry is used to quantify the mass balance of the Antarctic ice sheet, this should be taken into account. Since the density of surface snow is only one third of the ice density, a separation of the influence of ice dynamical changes versus a varying accumulation rate on the observed elevation change makes a big difference."
According to the scientists, firn layer thickness varies over timescales of days to millennia and so may have a large influence on observed elevation trends. While enhanced snowfall favours firn layer thickening it also tends to coincide with higher than average temperatures, which densify the upper firn pack. Previous studies have accounted for temperature-driven changes in firn compaction but not those caused by accumulation rate variability.
According to Helsen, one of the key constraints on the team's modelling results is the long-term mean accumulation rate, which balances the vertical ice velocity. "Unfortunately this long-term mean accumulation rate is poorly known for large parts of the Antarctic ice sheet," he said. "A next step in our research will be to further constrain this accumulation rate, and to collect all available accumulation rate records that cover a period of longer than around 100 years."
The researchers reported their work in Sciencexpress.