Ice-Sheet Mass Balance Estimates Combining a Variety of Techniques

There are many different ways to measure the changes in ice mass on Earth’s polar ice sheets. Each technique has its own strengths and weaknesses, and combining the different measurements can give a more complete and accurate picture of the changes in ice-sheet mass balance. By combining the data from satellite altimetry, interferometry, and gravimetry, Shepherd et al. (2012) were able to further analyze the ice sheets in Greenland (GrIS), East Antarctica (EAIS), West Antarctica (WAIS), and the Antarctic Peninsula (APIS). They found that the changes in mass between 1992 and 2011 were      –142 ± 49 Gt yr –1 in Greenland, +14 ± 43 Gt yr –1 in East Antarctica, –65 ±26 Gt yr –1 in West Antarctica, and –20 ± 14 Gt yr –1 in the Antarctic Peninsula. This amounted to an overall change of –1350 ± 1010 Gt in the Antarctic ice sheet and –2700 ±930 Gt in that of Greenland between 1992 and 2011, equating to a sea level rise of 11.2 ± 3.8 mm during this time period.¾Olivia Jacobs
Shepherd, A., Ivins, E.R., Geruo, A., Barletta, V.R., Bentley, M.J., Bettadpur, S.,
Briggs, K.H., Bromwich, D.H., Forsberg, R., Galin, N., Horwath, M., Jacobs, S., Joughin, I., King, M.A.., Lenaerts, J.T., Li, J., Ligtenberg, S.R., Luckman, A., Luthcke, S.B., McMillan, M., Meister, R., Milne, G., Mouginot, J., Muir, A., Picolas, J.P., Paden, J., Payne, A.J., Pritchard, H., Rignot, E., Rott, H., Sorensen, L.S., Scambos, T.A., Scheuchl, B., Schrama, E.J., Smith, B., Sundal, A.V., van Angelen, J.H., van de Berg, W.J., van den Broeke, M.R., Vaughan, D.G., Velicogna, I., Wahr, J., Whitehouse, P.L., Wingham, D.J., Yi, D., Young, D., Zwally, H.J. 2012. A reconciled estimate of ice-sheet mass balance. Science 338, 1183–1189.[GSS shepherd ivins ice-sheet]

Shepherd et al. (2012) used a variety of data to get the most accurate picture of ice-sheet mass changes over the past 19 years. To accurately reprocess the data, they used common time intervals and definitions of the respective ice-sheets. The ice-sheet surface mass balance (SBM) estimates, collected by RACMO2 between 1979 and 2010, include solid and liquid precipitation, surface sublimation, drifting snow transport, erosion and sublimation, and meltwater formation, refreezing, retention, and runoff.
The scientists accounted for glacial isostatic adjustments (GIA)¾the changes in the ocean basins over time from unweighting of glaciers and ice sheets¾by applying six different types of satellite gravimetry and altimetry models over eight years of data collection. Radar and laser altimetry were also used to give estimates of ice-sheet mass balance through measurements of ice-sheet volume change. Radar altimetry (RA) gives precise measurements of elevation change, but is not able to accurately convert volume changes to mass changes. To do this, Shepherd et al. used laser altimetry (LA) to measure firn-layer thickness.
            The input-output method (IOM) was also applied to quantify the difference between glacier mass gained through snowfall and lost by sublimation and meltwater runoff in both Greenland and Antarctica between 1992 and 2010. This type of analysis is particularly useful because it can isolate different drainage basins and compare the different runoff volumes. Lastly, gravimetry data (including GRACE) estimate changes in ice-sheet mass balance through the ice sheet’s changing gravitational attraction. These data were particularly useful because they give regional averages and allow for monthly samplings. However, these measurements cannot account for the changes in Earth’s crust and mantle, which were then accounted for using GIA models.
            By applying these various techniques, Shepherd et al. were able to get a more accurate image of the changes in ice-sheet mass balance in Greenland and Antarctica. When RA and IOM models of 52 drainage basins in the Antarctic ice-sheet (AIS) were compared, there was an agreement of mass changes in 42 of the 52 basins. The average difference between the models was 1.4 ± 3.8 Gt yr –1, indicating that the RA and IOM models were similar. Knowing this, the scientists used IOM data to account for mass changes in the Antarctic Peninsula (APIS) where RA data were unavailable.
            Next, Shepherd et al. considered the exceptional snowfall in 2009 in East Antarctica (EAIS) to analyze whether or not geodetic techniques can detect fluctuations in SMB. They found that RACMO2, RA, and GRACE estimates, which record firn thickness and mass, volume, and mass fluctuations, respectively, were all able to detect the changes during this year of heavy snowfall.
            The scientists in this study also attempted to find a methodological intercomparison in the sampling techniques used. For this, they used the time period between October 2003 and December 2008, when all four satellite geodetic techniques (IOM, RA, LA, and gravimetry) were operating. By combining these different means of measurement, the errors in estimated ice-sheet imbalances were lessened, and the mass imbalances in the AIS and GIS were  –72 ± 43 Gt yr –1 and –232 ± 23 Gt yr –1, respectively. However, these data also revealed that the mass imbalances vary cyclically and by large amounts over 2 to 4 year periods, and thus short-term fluctuations should be acknowledged.
            The data were also combined to examine trends over both short and long time periods. This integration showed that the GrIS loss in the 1990s was modest, but increased sharply in the new millennium. Currently, there is also an increasing mass loss from WAIS and APIS. The APIS was close to being in balance in the 1990s, but loss from this ice sheet now accounts for about 25% of AIS ice-sheet loss even though it only occupies about 4% of the total area. Comparatively, the EAIS has seen a gain in mass due to an increase in snowfall, but these changes have happened over too short a time period to tell if the change is a short-term fluctuation or a long-term trend.
            In conclusion, the data compiled in this study show that between 1992 and 2011, the AIS has lost 1350 ± 1010 Gt of ice and the GrIS has lost 2700 ±930 Gt. This is equal to a sea level increase of 11.2 ± 3.8 mm. While the combinations of data in this study lessened uncertainty in the changes in ice-sheet mass balance, there is still ample space to improve current sampling techniques. The EAIS could benefit from measurements with greater spatial sampling, while the APIS could benefit from longer temporal sampling. The SMB also shows great fluctuations because of the short time period of sampling, and could thus benefit from longer spans of data. However, Shepherd et al. were able to combine existing techniques to give better estimates despite current shortcomings in the data. 

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