Present and future variations in Antarctic firn air content

A firn densification model (FDM) is used to assess spatial and temporal (1979-2200) variations in the depth, density and temperature of the firn layer covering the Antarctic ice sheet (AIS). A time-dependent version of the FDM is compared to more commonly used steady-state FDM results. Although the average AIS firn air content (FAC) of both models is similar (22.5 m), large spatial differences are found: in the ice-sheet interior, the steady-state model underestimates the FAC by up to 2 m, while the FAC is overestimated by 5-15m along the ice-sheet margins, due to significant surface melt. Applying the steady-state FAC values to convert surface elevation to ice thickness (i.e., assuming flotation at the grounding line) potentially results in an underestimation of ice discharge at the grounding line, and hence an underestimation of current AIS mass loss by 23.5% (or 16.7 Gt yr-1) with regard to the reconciled estimate over the period 1992-2011. The timing of the measurement is also important, as temporal FAC variations of 1-2m are simulated within the 33 yr period (1979-2012). Until 2200, the Antarctic FAC is projected to change due to a combination of increasing accumulation, temperature, and surface melt. The latter two result in a decrease of FAC, due to (i) more refrozen meltwater, (ii) a higher densification rate, and (iii) a faster firn-to-ice transition at the bottom of the firn layer. These effects are, however, more than compensated for by increasing snowfall, leading to a 4-14% increase in FAC. Only in melt-affected regions, future FAC is simulated to decrease, with the largest changes (-50 to -80 %) on the ice shelves in the Antarctic Peninsula and Dronning Maud Land. Integrated over the AIS, the increase in precipitation results in a similar volume increase due to ice and air (both ∼150 km3 yr-1 until 2100). Combined, this volume increase is equivalent to a surface elevation change of +2.1 cm yr-1, which shows that variations in firn depth remain important to consider in future mass balance studies using satellite altimetry