C32A-01 10:20h
Sea-level and geoid changes around Greenland
Present-day sea-level and geoid changes around Greenland are largely the result of mass fluctuations in the Greenland Ice Sheet (GIS). However, there is still a very strong effect arising from the on-going glacial-isostatic adjustment of the Earth due to the demise of the other late-Pleistocene ice sheets, especially those that covered North America. The GIS's contribution may be divided between that which arises from the ice-sheet retreat following Last Glacial Maximum (21 ka BP), and those changes that are occurring today. Predictions of sea-level change have been made and are compared with values inferred from tide-gauge time series. Similarly, predictions of geoid change that may be detectable by the GRACE gravity space mission are discussed, and compared with currently available data.
C32A-02 10:40h
The neoglacial mass balance of the Greenland Ice Sheet inferred from sea level observations: reconciling observations and models over different timescales
This paper uses relative sea level (RSL) data to infer patterns of Greenland Ice Sheet re-growth during the neoglacial, a period of late Holocene climate cooling that began c. 4.5 ka cal. yr BP and culminated in the Little Ice Age. We argue that there is widespread evidence from Greenland for a switch from early and mid Holocene RSL fall to late Holocene RSL rise. This rise reflects the combined effects of the collapsing forebulge of the Laurentide Ice Sheet and the neoglacial re-growth of the Greenland Ice Sheet. Spatial variations in RSL broadly mirror present-day variations in precipitation and temperature across the ice sheet, that are strongly influenced by Greenland's oceanographic and atmospheric setting. Our analysis suggests that neoglacial ice sheet re-loading was greatest in the relatively warm, high precipitation maritime regime of south Greenland. Less re-loading occurred in the cold polar regime of northeast Greenland. The identification of significant late Holocene RSL rise along much of the east coast of Greenland provides support for recent GPS observations of vertical land motions, but contradicts geophysical model outputs from east Greenland which predict only uplift in east Greenland. Widespread regrowth of the ice sheet suggests that the Greenland Ice Sheet has made a stronger negative contribution to global sea level during the late Holocene than previously thought.
C32A-03 INVITED 10:55h
GPS Measurement of Glacial Isostatic Adjustment in Antarctica: Current Results and Future Prospects
Global Positioning System (GPS) measurements at bedrock sites in Antarctica hold the potential for partially testing chronological models for the mid-to-late Holocene (8-0 Ka) collapse of the West Antarctic Ice Sheet. A network of permanent GPS sites exists in Antarctica mainly at coastal sites, but sites in the interior of Antarctica, near present and paleo ice accumulation centers, are key to shedding light on the history of the Antarctic ice sheet. Considerable effort has been expended by several groups of investigators during the last decade to acquire these data. These efforts have resulted in several new data points with which to test current models of postglacial rebound. We have collected GPS data quasi-continuously between November 1996 and January 2001 at two autonomous GPS stations in the northern Transantarctic Mountains, only one of which produced a high-quality vertical rate time series. The vertical rate at Mt. Coates in the Dry Valleys indicates uplift of 4.5 ± 2.3 mm/yr, most likely due to glacial isostatic motion. Uplift at Mt. Coates deviates significantly from uplift predictions based on deglaciation models ICE-3G and ICE-4G, but is consistent with the D91-1.5 model of variable and continued ice sheet ablation to 2 kyr and a viscosity of 10$^{21}$ Pa s in the upper mantle and 10$^{22}$ Pa s in the lower mantle beneath the region. The lack of spatial coverage at reasonable wavelength results in a predicted uplift rate that is largely insensitive to lithospheric thickness. The range of upper mantle viscosities indicated by the deglaciation model that includes persistent drawdown to 2 kyr suggests a subcratonic upper mantle. However, the large error estimates on this uplift rate allow upper mantle viscosities ranging from about 2 x 10$^{20}$ Pa s to nearly 10$^{22}$ Pa s. High-quality uplift data from the interior of the continent are key to providing stronger constraints on both deglaciation history and Earth rheology. Longer and more continuous time series from a network of stations that sample the spatial gradients in uplift are needed to advance better models of glacial isostatic adjustment and complement GRACE estimates of present-day ice mass change in Antarctica.
http://geodynamics.jpl.nasa.gov/antarctica
C32A-04 INVITED 11:15h
Ice Sheet Mass Balance From Satellite Altimetry
A major uncertainty in understanding the current sea level rise of about 2mm/yr has been the uncertainty in the present mass balance of the Greenland and Antarctica ice sheets. For predicting changes in ice balance over the next century, in response to predictions of climate change, a principal uncertainty has been in the prediction of polar precipitation and the consequent rates of ice accumulation. Predictions of surface temperature change and consequent surface melting and runoff are more reliable. However, the non-linear dynamical effects of increased melting that cause accelerated flow of grounded ice are only beginning to be taken into account. Now, decadal-length measurements of changes in the surface elevations of the ice sheets by satellite and airborne altimeters are providing good estimates of the recent ice-sheet mass balance and the current contributions of the ice sheets to the rise of sea level. Time-series of ice-sheet surface elevations constructed from about 10 years of ERS-1 and 2 radar altimeter data show patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. Generally, the Greenland ice sheet has been growing inland and shrinking at the margins, which is the expected response of the ice sheet to climate warming. Much of the West Antarctic ice sheet has been losing mass while the East Antarctica ice sheet and grounded ice of the Antarctic Peninsula are gaining mass. The net contribution to sea-level rise is only a few tenths of a mm/yr. The floating ice shelves show corresponding regional mass losses and gains and a significant net loss, which may lead to an increased loss of grounded ice. Overall, the measurements are beginning to show patterns of growth and shrinkage characteristic of ongoing climatic change. For the future, the race between increasing precipitation and increasing melting is underway. A program of continuous monitoring of ice sheet elevation, similar to the continuous monitoring of sea-level rise, is clearly needed.
C32A-05 11:35h
Snow Accumulation, Ice Discharge and Mass Balance in Greenland and Antarctica from GRACE
The GRACE satellite mission, launched in March of 2002, provides monthly measurements of time variable gravity at scales of a few hundred kilometers and larger. These data can be used to estimate temporal variations in the distribution of surface mass. We examine here the nineteen monthly GRACE gravity field solutions that are currently available for analyses. We find that these fields can be used to recover mass imbalance and net snow accumulation for the Greenland and Antarctic ice sheets. GRACE estimates of Greenland mass change agree with ice mass imbalance estimates from altimeter heights within measurement errors. We also estimate net snow accumulation from the nonsecular component of the GRACE signal. If precipitation minus evapo-transpiration (P-E) and runoff (R) are known it is possible to obtain monthly estimates of ice discharge through the grounding line from GRACE. We estimate ice discharge for the Greenland ice sheet by combining GRACE estimates of nonsecular net snow accumulation with ERA40-derived P-E-R.
C32A-06 11:50h
Constraints on recent mass changes in the Antarctic and Greenland ice sheets from an analysis of the global water cycle
Secular mass changes in the large ice sheets introduce a particular fingerprint in relative sea level, which can be identified in local trends in relative sea level determined from tide gauge observations [Plag and J\"uttner, 2001]. However, on decadal to century time scales, coastal relative sea level is affected by other factors, including steric changes in the volume of the sea water, mass exchange with the continental cryosphere and hydrosphere, post-glacial rebound, local vertical movement of the crust, and variations in the atmospheric circulation. In order to derive firm constraints on mass changes of the large ice sheets from sea level observations, all these factors need to be taken into account in a global analysis focusing on the ocean as the major reservoir in the water cycle. The different forcing factors for sea level are associate with distinct spatial "finger-prints". In a first step, these "finger-prints" are obtained from model predictions (general circulation models for steric effects, geophysical model for post-glacial rebound), theoretical finger-print functions (for mass exchange with the cryosphere, where the static sea-level equation is used to compute the finger-print functions for the sea-level signal due to linear trends in the Antarctic and Greenland ice sheets), and observational data (steric effect, atmospheric circulation, local vertical land motion). Then, in a second step based on the large and global databaseof monthly mean sea level values collected at the Permanent Service for Mean Sea Level, the "finger-prints" are scaled to fit the observed monthly sea levels, thus allowing maximum use of the available observations. For that, a complex regression model is used which approximates each tide gauge record through a function of the forcing factors. For each forcing factor, a parameterised contribution is included in the regression, with the parameters being both local (e.g. for the air pressure and wind effects) and global (e.g. for the ice sheet mass changes). These parameters are determined for a number of models and/or assumptions and the dependency of the parameters on models and assumptions is discussed. The results indicate a global sea level change of the order of +2.5 mm/year over the last 50 years with significant contributions from both the Antarctic and Greenland ice sheets.
C32A-07 12:05h
Greenland ice sheet surface mass balance sensitivity
Results from a 16-year data assimilation (1988-2003) using the Polar MM5 regional climate model, reveal coherent spatial and temporal variability patterns in temperature, precipitation, cloud radiative forcing, albedo, and surface mass balance. Constraint of systematic model biases was provided by automatic weather station, glacier survey, and remotely-sensed data. Spatial variability in temperature is concentrated along the western slope of the ice sheet, while precipitation variability exhibits a more complex pattern related to the influence of topography on prevailing storm tracks. Albedo variability during this period contributes significantly to ablation trends. Downward longwave radiation increased during this period over much of the ice sheet. Temperature and precipitation trends were also positive over much of the ice sheet. An increase in equilibrium line altitude around much of the ice sheet and a consequent expansion of the ablation zone is evident in the model results. While increasing temperatures led to an increase in the duration of the melt period and an increase in meltwater runoff, increases in precipitation offset a negative perturbation of the surface mass balance. We may expect an increasing dynamic response of the ice sheet to this period of apparent warming and steepening of the ice sheet balance profile.