Environment Counts | Reassessment of satellite data finds that polar ice sheet melting has contributed 11 mm to sea-level rise since 1992
Author: Geoff Zeiss – Published At: 2012-12-01 16:23 – (1089 Reads)
Since 1993 global sea-level has risen by about 60 mm. Of this a significant, but until now uncertain, proportion can be ascribed to the melting of the world’s largest ice sheets, Antarctica and Greenland. Increases in ocean temperature that have been predicted for the next century could trigger substantial ice-sheet mass loss through faster melting of ice shelves and outlet glaciers. These processes were not included in the ice sheet models that were used for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC 2007). Satellite geodesy, including altimetric, interferometric, and gravimetric techniques, has revolutionized how ice-sheet mass balance is estimated. The authors find that combining satellite data sets leads to greater certainty in estimating mass balance. Between 1992 and 2011, the authors estimate that the major ice sheets changed their mass by
Greenland â€“142 Â± 49 gigatonnes/year
Since 1992, the polar ice sheets have contributed 0.59 Â± 0.20 millimeter/year to the rate of global sea-level rise. This has resulted in a cumulative estimate between 1992 and 2011 that the Antarctic and Greenland ice sheets lost 1350 Â± 1010 and 2700 Â± 930 Gt of ice, respectively, equivalent to an increase in global mean sea level of 11.2 Â± 3.8 mm. Science 30 November 2012: Vol. 338 no. 6111 pp. 1183-1189
Since 1998, there have been at least 29 ice-sheet mass balance estimates, based on the various satellite techniques. But individually these results have not permitted reliable estimates of ice loss for the polar ice sheets. This paper compares and combines estimates of ice-sheet mass balance from all three satellite geodetic techniques to produce a reconciled estimate of ice-sheet mass balance. The authors find that combining satellite data sets leads to greater certainty in estimating mass balance.
This assessment uses 19 years of satellite radar altimeter (RA) data, 5 years of satellite laser altimeter (LA) data, 19 years of satellite radar interferometer data, 8 years of satellite gravimetry data, 32 years of surface mass balance (SMB) model simulations, and estimates from several glacial isostatic adjustment models, to produce a reconciled estimate of ice-sheet mass balance. The satellite data sets spans the period 1992 to 2011. Complete data from all geodetic techniques are available between January 2003 and December 2008.
Rate of mass change of the four main ice-sheet regions, as derived from the four techniques of satellite RA (cyan), IOM (red), LA (green), and gravimetry (blue), with uncertainty ranges (light shading).
Radar and laser altimetry
Altimetry measurements provide ice-sheet mass balance estimates through measurements of ice-sheet volume change. The technique has been applied to both Greenland and Antarctica and is unique in spatially resolving the detailed pattern of mass imbalance, with monthly temporal sampling. Radar altimetry (RA) provides the longest continuous record of all geodetic techniques. Altimeter measurements of elevation change are precise, because they require only modest adjustments to account for sensor drift, changes in the satellite attitude, atmospheric attenuation, and movements of Earthâ€™s surface. The greatest uncertainty lies in the conversion from volume to mass change. In the case of laser altimetry (LA), the conversion has been performed using an external model of fluctuations in the firn-layer thickness. In the case of RA, the conversion to mass has been performed by using a prescribed density model and by allowing for temporal fluctuations in snowfall in the uncertainty.
European Remote-Sensing (ERS-1 and ERS-2) satellite and Envisat 35-day repeat satellite RA observations were used to determine changes in the mass of the EAIS and WAIS between May 1992 and September 2010.
ICESat (Ice, Cloud, and Land Elevation Satellite) LA observations acquired between September 2003 and November 2008 (the period of optimal instrument calibration) were used to estimate changes in the mass of the AIS and GrIS (16).
The Gravity Recovery and Climate Experiment (GRACE) satellite mission observes changes in gravitational attraction to estimate fluctuations in ice-sheet mass. The GRACE method provides regional averages without the need for interpolation, measures the effect of mass fluctuations directly, and permits monthly temporal sampling. However, the technique requires differentiating fluctuations in ice-sheet mass from changes in the underlying crust and mantle. To do this requires using glacial isostatic adjustment (GIA) models.
The modeling techniques that were part of the analysis include ice-sheet surface mass balance (SMB), glacial isostatic adjustment (GIA), and the input-output method (IOM).
Ice-sheet surface mass balance
SMB includes solid and liquid precipitation, surface sublimation, drifting snow transport, erosion and sublimation, and meltwater formation, refreezing, retention, and runoff. In this study the estimates of the Antarctic Ice Sheet (AIS) and the Greenland Ice-sheet (GrIS) SMB are derived from a regional atmospheric climate model over the period 1979 to 2010.
Input-output method (IOM)
The IOM quantifies the difference between glacier mass gained through snowfall and lost by sublimation and meltwater runoff and the discharge of ice into the ocean. The approach has the advantage of allowing changes in SMB and ice dynamics to be examined separately for individual glacier drainage basins.
Glacial isostatic adjustment
GIA of the solid Earth is an important contributor to the signals observed by satellite gravimetry and satellite altimetry. The GIA must therefore be considered when estimating ice-sheet mass balance with either technique.
In the last two Intergovernmental Panel on Climate Change (IPCC) assessments, the sum of observed contributions to seaâ€level rise has consistently been less than the observed rise. In a recent article reviewed here GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L18601, 8 PP., 2011, energy and sea level budgets are considered together for the past five decades from 1972 to the present. It is concluded that the sum of the observed contributions agrees with the observed rise within the estimate errors. The contribution to sea-level rise from Greenland and Antarctica ice-sheet melting between 1993 to 2008 was estimated to be
Greenland 0.31 +/- 0.17 mm/year
Antarctica 0.43 +/- 0.20
Total 0.74 +/-0.37
The current result 0.59 Â± 0.20 agrees with the previous estimate to within the estimated error, but the error bars are considerably tighter.