A suite of spatially distributed cores collected under the Program for Arctic Regional Climate Assessment (PARCA) provides an unprecedented opportunity to assess local to regional variability of annual accumulation rates over the Greenland ice sheet. PARCA cores are unique in their broad spatial distribution and accurate dating of annual layers using multiple seasonally varying indicators. The core data provide (1) a more rigorous evaluation of spatial and temporal variations in accumulation rates, (2) critical input to ice sheet mass balance estimates, (3) ground truth measurements for satellite observations and climate model-based precipitation estimates, and (4) important constraints on paleoclimatic interpretations from ice cores. Multiple closely spaced cores demonstrate that signals of high-frequency (annual to possibly decadal scale) climate variability preserved in the ice sheet are partially masked by glaciological noise. Two 350-year accumulation histories, one from northwest Greenland and one from the summit area, reveal significant multidecadal variability. The regional trends show long periods (60-90 years) of strong positive correlation and an equally long period of strong negative correlation. Since 1940 the trends have been decoupled. This spatial variability reflects the strong modulation of Greenland precipitation (and the climate information it contains) by changes in North Atlantic atmospheric circulation patterns. Proxy records from the PARCA cores document that climate reconstructions from a single core must be interpreted cautiously, with application of appropriate filters to reduce local noise and careful extrapolations from local to regional scales. Richer, more robust ice core-derived data sets should result from combining multiple, more widely spaced cores to produce regional stacked records.
Interferometric synthetic‐aperture radar (InSAR) observations of southeast Greenland glaciers acquired by the Earth Remote Sensing Satellites (ERS‐1/2) in 1996 were combined with ice sounding radar data collected in the late 1990s to estimate a total discharge of 46 ± 3 km3 ice per year between 62°N and 66°N, which is significantly lower than a mass input of 29 ± 3 km3 ice per year calculated from a recent compilation of snow accumulation data. Further north, Helheim Glacier discharges 23 ± 1 km3/yr vs 30 ± 3 km3/yr accumulation; Kangerdlugssuaq Glacier discharges 29 ± 2 km3/yr vs 23 ± 2 km3/yr; and Daugaard‐Jensen Glacier discharges 10.5 ± 0.6 km3/yr vs 10.5 ± 1 km3/yr. The mass balance of east Greenland glaciers is therefore dominated by the negative mass balance of southeast Greenland glaciers (−17 ± 4 km3/yr), equivalent to a sea level rise of 0.04 ± 0.01 mm/yr. Warmer and drier conditions cannot explain the imbalance which we attribute to long‐term changes in ice dynamics.
Abstract. Satellite radar and aircraft laser altimeter data and a volume budget comparison of total snow accumulation with total ice discharge give three independent estimates of the recent mass balance of parts of the Greenland ice sheet above ---2000 m elevation. Results show the entire region, on average, to be in balance to within 10 mm yr -1, with very low rates of regional thickness change (dH/dt) in the northeast but high rates with large spatial variability in the south. Only the volume budget estimates show significant thinning in the northwest, but thinning in this area is also inferred from local measurements of ice vertical velocities down boreholes. South of latitude 68øN, there has been rapid thickening west of the ice divide, with equally rapid thinning in the southeast, but with large differences between the three estimates of dH/dt throughout this region. The radar data apply to the period 1978-1988, the laser data are for 1993/1994-1998/1999, and the volume budget calculations represent conditions over at least the last few decades. Consequently, many of the differences between results could be caused by temporal changes, particularly in snow accumulation rates, that occurred since the 1970s. However, taken with other information, our results suggest long-term thickening in the southwest and possibly quite recent onset of rapid thinning in the southeast. " IntroductionThe major goal of NASA's Program for Arctic Regional Climate Assessment (PARCA) is to measure and understand the mass balance of the Greenland ice sheet. When the program began, uncertainties in snow accumulation (_+20%) and ablation rates (_+35%) and in iceberg calving (_+30%) were very large [Reeh, 1989]. Consequently, we did not know to better than -100 mm of water equivalent per year whether, on average, the ice sheet was thickening or thinning, and we had
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