Li, Jun and H. Jay Zwally, 2011, “Modeling of firn compaction for estimating ice-sheet mass change from observed ice-sheet elevation change”, Annals of Glaciology, Vol 52, No. 59, pp 1-7, 2011.
Changes in ice-sheet surface elevation are caused by a combination of ice-dynamic imbalance, ablation, temporal variations in accumulation rate, firn compaction and underlying bedrock motion. Thus, deriving the rate of ice-sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion, which do not involve changes in mass, and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability. We use a 25 year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model. We apply this formulation to ICESat measurements of surface elevation change at three locations on the Greenland ice sheet in order to separate the accumulation-driven changes from the ice-dynamic/ablation-driven changes, and thus to derive the corresponding mass change. Our calculated densities for the accumulation-driven changes range from 410 to 610 kgm-3, which along with 900 kgm-3 for the dynamic/ablation-driven changes gives average densities ranging from 680 to 790 kgm-3. We show that using an average (or ‘effective’) density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign.
Zwally, H. J., M.A. Beckley, A.C. Brenner, and M.B. Giovinetto, 2002, “Motion of major ice-shelf fronts in Antarctica from slant-range analysis of radar altimeter data, 1978-98”, Annals of Glaciology, Volume 34, Number 1, 1 January 2002, pp. 255-262(8)
Slant-range analysis of radar altimeter data from the Seasat, Geosat and European Remote-sensing Satellite (ERS-1and-2) databases is used to determine barrier location at particular times, and estimate barrier motion (km a-1) for major Antarctic ice shelves. The analysis covers various multi-year intervals from 1978 to 1998, supplemented by barrier location maps produced elsewhere for 1977 and 1986. Barrier motion is estimated as the ratio between mean annual ice-shelf area change for a particular interval, and the length of the discharge periphery.This value is positive if the barrier location progresses seaward, or negative if the barrier location regresses (break-back). Either positive or negative values are lower-limit estimates because the method does not detect relatively small area changes due to calving or surge events. The findings are discussed in the context of the three ice shelves that lie in large embayments (the Filchner-Ronne, Amery and Ross Ice Shelves), and marginal ice shelves characterized by relatively short distances between main segments of grounding line and barrier (those in the Dronning Maud Land sector between 010.1° W and 032.5° E, and the West and Shackleton Ice Shelves).The ice shelves included in the study account for approximately three-quarters of the total ice-shelf area of Antarctica, and discharge approximately two-thirds of the total grounded ice area.
Zwally, H.J., B. Schutz, W. Abdalati, J. Abshire, C. Bentley, A. Brenner, J. Bufton, J. Dezio, D. Hancock, D. Harding, T. Herring, B. Minster, K. Quinn, S. Palm, J. Spinhirne, and R. Thomas, 2002, “ICESat's laser measurements of polar ice, atmosphere, ocean, and land”, Journal of Geodynamics, 34(3-4), 405-445, doi:10.1016/S0264-3707(02)00042-X.
The Ice, Cloud and Land Elevation Satellite (ICESat) mission will measure changes in elevation of the Greenland and Antarctic ice sheets as part of NASA’s Earth Observing System (EOS) of satellites. Time- series of elevation changes will enable determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. Other scientific objectives of ICESat include: global measurements of cloud heights and the vertical structure of clouds and aerosols; precise measure- ments of land topography and vegetation canopy heights; and measurements of sea ice roughness, sea ice thickness, ocean surface elevations, and surface reflectivity. The Geoscience Laser Altimeter System (GLAS) on ICESat has a 1064 nm laser channel for surface altimetry and dense cloud heights and a 532 nm lidar channel for the vertical distribution of clouds and aerosols. The predicted accuracy for the surface- elevation measurements is 15 cm, averaged over 60 m diameter laser footprints spaced at 172 m along- track. The orbital altitude will be around 600 km at an inclination of 94° with a 183-day repeat pattern. The on-board GPS receiver will enable radial orbit determinations to better than 5 cm, and star-trackers will enable footprints to be located to 6 m horizontally. The spacecraft attitude will be controlled to point the laser beam to within ± 35 m of reference surface tracks at high latitudes. ICESat is designed to operate for 35 years and should be followed by successive missions to measure ice changes for at least 15 years.
Zwally, H. Jay, Waleed Abdalati, Tom Herring, Kristine Larson, Jack Saba, and Konrad Steffen, 2002, “Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow” , Science, Vol 297, pp. 218-222.
Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming.
Zwally, H. J., M. B. Giovinetto, J. Li, H. G. Cornejo, M. A. Beckley, A. C. Brenner, J. L. Saba, and D. Yi., 2005, “Mass Changes of the Greenland and Antarctic Ice Sheets and shelves and Contributions to Sea-Level Rise: 1992-2002”, Journal of Glaciology, Vol. 51, No. 175, pp 509-527, 2005.
Changes in ice mass are estimated from elevation changes derived from 10.5years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (-42 ± 2 Gt a -1 below the equilibrium-line altitude (ELA)) and growing inland (+53 ± 2 Gt a -1 above the ELA) with a small overall mass gain (+11 ± 3 Gt a -1; -0.03 mm a -1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (-47 ± 4 Gt a-1) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 ± 11 Gt a-1) for a combined net change of -31 ± 12 Gt a-1 (+0.08mm a-1SLE). The contribution of the three ice sheets to sea level is +0.05 ± 0.03 mm a-1. The Antarctic ice shelves show corresponding mass changes of -95 ± 11 Gt a-1 in WA and +142 ± 10 Gt a -1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice-dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.
Zwally, H. Jay, Jun LI, Anita C. Brenner, Matthew Beckley, Helen G. Cornejo, John DiMarzio, Mario B. Giovinetto, Thomas A. Neumann, John Robbins, Jack L. Saba, Donghui Yi, Weili Wang, 2011, “Greenland ice sheet mass balance: distribution of increased mass loss with climate warming; 2003-07 versus 1992-2002”, Journal of Glaciology, Vol. 57, No. 201, pp 88-102, 2011.
We derive mass changes of the Greenland ice sheet (GIS) for 2003-07 from ICESat laser altimetry and compare them with results for 1992-2002 from ERS radar and airborne laser altimetry. The GIS continued to grow inland and thin at the margins during 2003-07, but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s. The net balance changed from a small loss of 7 ± 3 Gt a -1 in the 1990s to 171 ± 4 Gt a -1 for 2003-07, contributing 0.5 mm a-1 to recent global sea-level rise. We divide the derived mass changes into two components: (1) from changes in melting and ice dynamics and (2) from changes in precipitation and accumulation rate. We use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation-driven changes to mass changes. Increased losses from melting and ice dynamics (17-206 Gt a-1) are over seven times larger than increased gains from precipitation (10-35 Gt a-1) during a warming period of ∼2 K (10 a)-1 over the GIS. Above 2000 m elevation, the rate of gain decreased from 44 to 28 Gt a-1, while below 2000 m the rate of loss increased from 51 to 198 Gt a-1. Enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers, as well as accelerating ice flow. Increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales.
Zwally, H. Jay and Mario B. Giovinetto, 2011, “Overview and Assessment of Antarctic Ice-Sheet Mass Balance Estimates: 1992-2009”, Surveys in Geophysics, Volume 32, Numbers 4-5, pp 351-376, DOI 10.1007/s10712-011-9123-5.
Mass balance estimates for the Antarctic Ice Sheet (AIS) in the 2007 report by the Intergovernmental Panel on Climate Change and in more recent reports lie between approximately +50 to -250 Gt/year for 1992 to 2009. The 300 Gt/year range is approximately 15% of the annual mass input and 0.8 mm/year Sea Level Equivalent (SLE). Two estimates from radar altimeter measurements of elevation change by European Remote-sensing Satellites (ERS) (+28 and -31 Gt/year) lie in the upper part, whereas estimates from the Input-minus-Output Method (IOM) and the Gravity Recovery and Climate Experiment (GRACE) lie in the lower part (-40 to -246 Gt/year). We compare the various estimates, discuss the methodology used, and critically assess the results. We also modify the IOM estimate using (1) an alternate extrapolation to estimate the discharge from the non-observed 15% of the periphery, and (2) substitution of input from a field data compilation for input from an atmospheric model in 6% of area. The modified IOM estimate reduces the loss from 136 Gt/year to 13 Gt/year. Two ERS-based estimates, the modified IOM, and a GRACE-based estimate for observations within 1992-2005 lie in a narrowed range of +27 to -40 Gt/year, which is about 3% of the annual mass input and only 0.2 mm/year SLE. Our preferred estimate for 1992-2001 is -47 Gt/year for West Antarctica, +16 Gt/year for East Antarctica, and -31 Gt/year overall (+0.1 mm/year SLE), not including part of the Antarctic Peninsula (1.07% of the AIS area). Although recent reports of large and increasing rates of mass loss with time from GRACE-based studies cite agreement with IOM results, our evaluation does not support that conclusion.
