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We present a reconstruction of the Greenland Ice Sheet surface mass balance (SMB) from 1870 to 2010, based on merged Twentieth Century Reanalysis (20CR) and European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological reanalyses, and we compare our new SMB series with global and regional climate and atmospheric circulation indices during this period. We demonstrate good agreement between SMB annual series constructed from 20CR and ECMWF reanalyses for the common period of overlap and show statistically significant agreement of long-term modeled snowfall with ice-core-based accumulation data. We analyze variations in SMB for the last 140 years and highlight the periods with significantly increased runoff and decreased SMB since 1870, which have both been enhanced in the period since 1990, as well as interannual variations in SMB linked to Greenland climate fluctuations. We show very good agreement of our SMB series variations with existing, independently derived SMB series (RACMO2) variations for the past few decades of overlap but also a significant disparity of up to ∼200 km3 yr-1 in absolute SMB values due to poorly constrained modeled accumulation reflecting a lack of adequate validation data in southeast Greenland. There is no significant correlation between our SMB time series and a widely referenced time series of North Atlantic icebergs emanating from Greenland for the past century, which may reflect the complex nature of the relationship between SMB and ice dynamical changes. Finally, we discuss how our analysis sheds light on the sensitivity and response of the Greenland Ice Sheet to ongoing and future global climate change, and its contribution to global sea level rise. Copyright 2011 by the American Geophysical Union.

An updated accumulation map for Greenland is presented on the basis of 39 new ice core estimates of accumulation, 256 ice sheet estimates from ice cores and snow pits used in previous maps, and reanalysis of time series data from 20 coastal weather stations. The period 1950-2000 is better represented by the data than are earlier periods. Ice-sheetwide accumulation was estimated based on kriging. The average accumulation (95% confidence interval, or ±2 times standard error) over the Greenland ice sheet is 30.0 ± 2.4 g cm -2 a-1, with the average accumulation above 2000-m elevation being essentially the same, 29.9 ± 2.2 g cm-2 a -1. At higher elevations the new accumulation map maintains the main features shown in previous maps. However, there are five coastal areas with obvious differences: southwest, northwest, and eastern regions, where the accumulation values are 20-50% lower than previously estimated, and southeast and northeast regions, where the accumulation values are 20-50% higher than previously estimated. These differences are almost entirely due to new coastal data. The much lower accumulation in the southwest and the much higher accumulation in the southeast indicated by the current map mean that long-term mass balance in both catchments is closer to steady state than previously estimated. However, uncertainty in these areas remains high owing to strong gradients in precipitation from the coast inland. A significant and sustained precipitation measurement program will be needed to resolve this uncertainty. Copyright 2009 by the American Geophysical Union.

An analysis of 96 snow pit and ice cores distributed over the Greenland ice sheet is used to determine the main drivers of variability in the preserved records of nitrate concentration. The data set provides samples from spatially distributed locations, allowing us to investigate the effect of snow accumulation rate, temperature, and sublimation on nitrate concentration. The mean ice sheet concentration in the dry snow zone (2000 ≥ mean annual sea level (masl)) is 132 ng g-1, ranging between 47 and 265 ng g -1 with a standard deviation of ±37 ng g-1. Nitrate flux varies between 1.1 and 14.7 μg cm-2 a-1 with a mean of 4 ± 2 μg cm-2 a-1. Large-scale spatial variability exists as a result of accumulation gradients, with concentration 5% greater in the northern plateau, yet flux over the northern plateau is 30% lower than the dry snow zone as a whole. While spatially, flux appears to be more dependent on accumulation, preservation of flux shows increasing dependence on concentration with increasing accumulation. The relationship between concentration and accumulation is nonlinear, showing less dependence in the low-accumulation regions versus high-accumulation regions. Accumulation alone is insufficient to account for the observed variability in nitrate flux in the low-accumulation regions, and evidence supports the need for additional components to a transfer function model for nitrate that includes photochemistry, temperature, and sublimation. Spatial variability across the ice sheet is nonuniform, and changes in nitrate concentration have occurred in some regions at a greater rate than others. While the data support that overall the ice sheet acts as an archive of paleoatmospheric concentration despite the effects of postdepositional processing, one needs to consider spatial variables to properly account for trends and variability in the records. This is tested by evaluating past spatial relationships and yields the result that the significant geographic shifts with respect to reactive N concentrations have occurred over the ice sheet in the past century. Copyright 2009 by the American Geophysical Union.

Ice cores collected in 2000 (ITASE 00-1) and 2005 (WDC05A, WDC05Q) from the West Antarctic Ice Sheet Divide (WAIS Divide) project site were used to investigate the spatial and temporal variability in accumulation. The ice cores were dated based on annual layer counting of multiple glaciochemical measurements resulting in bottom depth ages for WDC05A, VVDC05Q, and ITASE 00-1 of 1775, 1521, and 1653 A.D., with mean annual accumulation rates of 0.200, 0.204, and 0.221 mweq a-1, respectively. Small-scale spatial variability (SSV) was determined using an analysis of variance of accumulation in the ice core array, thereby quantifying the uncertainty in individual accumulation records. Results indicate that the spatial variability was 0.030 mweq a-1, or approximately 15% of the average annual accumulation. An accumulation record representative of the WAIS Divide local area over recent centuries was developed using a principal component analysis to identify the coherent accumulation signal. The WAIS Divide local record exhibited 14% interannual variability (1 standard deviation of the mean) with the SSV reduced to 0.017 mweq a-1. Correlations of the WAIS Divide local accumulation record with atmospheric indices (e.g., Antarctic Oscillation) exhibited periods when the records oscillate in and out of phase. Thus, reconstructing local and global atmospheric indices from WAIS Divide accumulation records over recent centuries may prove problematic. Copyright 2008 by the American Geophysical Union.

Nitrate records from six Greenland ice cores covering the period 1789 to 1995 show a significant correlation in concentration for averaging periods greater than 10 years, as well as an approximately 60% increase in average concentration during the last 75 years. Annual nitrate fluxes contain low-frequency trends driven primarily by changes in concentration, while higher-frequency variability is driven by changes in snow accumulation. Increases in concentration yield nearly 30% higher nitrate flux (2.5 to 3.2 μg m-2 yr-1) and an 11% increase in variability during the 1895 to 1994 period versus the prior 100 years. Nitrate trends in the cores during the last 100 years are also correlated with global nitrate emissions, with a highly significant average r value of 0.93 for the six cores. During the period of anthropogenic influence, nitrate is positively correlated with the North Atlantic Oscillation, while prior to that the correlation is negative, and less significant, suggesting a link between transport of anthropogenic emissions and the North Atlantic Oscillation. Significant preanthropogenic periodicities identified through singular spectrum analysis show decadal variability in the nitrate record leading to shifts as great as 30% from the mean state but none as great as the anthropogenic-driven deviation. Copyright 2006 by the American Geophysical Union.

We report new century-scale ice core records of hydrogen peroxide (H2O2), a major atmospheric oxidant, from 23 locations across the West Antarctic Ice Sheet (WAIS) and use the spatial variability of (multi-) annual mean H2O2 concentrations in snow and firn to investigate the sensitivity of ice core H2O2 preservation to mean annual temperature and accumulation rate. In agreement with the ice-air equilibrium partitioning, H2O2 uptake in near-surface firn was found to be greatest at low temperatures, while postdepositional losses from degassing increase as accumulation rates decrease. This resulted in almost complete loss of H2O2 at warm (>-25°C), low-accumulation sites (94% deviations from the ice-air equilibrium at high-accumulation sites (>30 cm yr-1), but close-to-equilibrium values on the East Antarctic Plateau, where it is dry ( 0.6, p < 0.05) during extended El Niño-La Niña events and explain the occurrence of significant spectral peaks at ENSO-like periodicities (2-7 years) in the H2O2 record. Core records of H2O2 at high-accumulation sites (>30 cm yr-1) are most suitable for detection of temporal changes in atmospheric concentration, although a long-term H2O2 record will be well preserved under the current environment at the WAIS Divide core site. Copyright 2006 by the American Geophysical Union.

The troposphere above the West Antarctic Ice Sheet (WAIS) was sampled for hydroperoxides at 21 locations during 2-month-long summer traverses from 2000 to 2002, as part of the U.S. International Trans-Antarctic Scientific Expedition (US ITASE). First-time quantitative measurements using a high-performance liquid chromatography method showed that methylhydroperoxide (MHP) is the only important organic hydroperoxide occurring in the Antarctic troposphere and that it is found at levels 10 times those previously predicted by photochemical models. During three field seasons, means and standard deviations for hydrogen peroxide (H2O2) were 321 ± 158 pptv, 650 ± 176 pptv, and 330 ± 147 pptv. While MHP was detected but not quantified in December 2000, levels in summer 2001 and 2002 were 317 ± 128 pptv and 304 ± 172 pptv. Results from firn air experiments and diurnal variability of the two species showed that atmospheric H2O2 is significantly impacted by a physical snow pack source between 76° and 90°S, whereas MHP is not. We show strong evidence of a negative correlation between stratospheric ozone and H2O2 at the surface. Between 27 November and 12 December in 2001, when ozone column densities dropped below 220 Dobson units (DU) (means in 2000 and 2001 were 318 DU and 334 DU, respectively), H2O2 was 1.7 times that observed in the same period in 2000 and 2002, while MHP was only 80% of the levels encountered in 2002. Photochemical box model runs match MHP observations only when the production rate from CH3O2 + HO2 was increased to the upper limit of its estimated range of uncertainty. Model results suggest that NO and OH levels on WAIS are closer to coastal values, while Antarctic Plateau levels are higher, confirming that region to be a highly oxidizing environment. The modeled sensitivity of H2O2 and particularly MHP to NO offers the potential to use atmospheric hydroperoxides to constrain the NO background and thus estimate the past oxidation capacity of the remote atmosphere using ice cores. Copyright 2005 by the American Geophysical Union.