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The perennial snow, or firn, on the Greenland ice sheet each summer stores part of the meltwater formed at the surface, buffering the ice sheet’s contribution to sea level. We gathered observations of firn air content, indicative of the space available in the firn to retain meltwater, and find that this air content remained stable in cold regions of the firn over the last 65 years but recently decreased significantly in western Greenland.

The firn covering the Greenland ice sheet interior can retain part of the surface melt, buffering the ice sheet’s contribution to sea level, but its characteristics are still little known. Using remote-sensing observations from 2000- 2017, we estimate that firn covers 1,405,500 ±17,250 km2 of the ice sheet. We present 344 firn-core-derived observations of the top 10 m firn air content (FAC10), indicative of the firn’s meltwater retention capacity. FAC10 remained stable in the coldest 74% of the firn area during 1953-2017, while FAC10 decreased in the warmest and driest 12% of the firn area between 1997-2008 and 2011-2017, resulting in a loss of 180 ±78 km3 (-26 ±11%) of air from the near-surface firn.

Translating satellite measurements of ice sheet volume change into sea level contribution requires knowledge of the profile of density as a function of depth within the ice sheet, and how this profile changes over time. This paper describes an interferometric method of inverting ground-penetrating radar returns for the profile of firn density as a function of depth. The method is an interferometric implementation of the common-midpoint approach, performed using a stepped-frequency, phase-sensitive ground-penetrating radar. By recording the phase difference of returns with a range of antenna separations, the different path lengths through the firn allow recovery of a smoothed representation of the density profile. This density model is characterised by three parameters: surface density and two decay lengths for porosity, each operating over a different density range. Our results suggest that the stepped-frequency radar used here can accurately recover differences in two-way travel time and produce useful estimates of the density profile. In a test of the method performed at Summit station in Greenland, the recovered density-depth profile agreed with indepenent density measurements from an ice core and a neutron probe to within 6% root-mean-square error.

After the successful launch of CryoSat-2 in April 2010, the first direct validation campaign of the satellite is planned for spring 2011. DTU Space has been involved in ESA’s CryoSat Validation Experiment (CryoVEx) with airborne activities since 2003. To validate the prelaunch performance of the CryoSat radar altimeter (SIRAL), an airborne version of the SIRAL altimeter (ASIRAS) has been flown together with a laser scanner in 2006 and 2008. Of particular interest is to study the penetration depth of the radar altimeter over both land- and sea ice. This can be done by comparing the radar and laser measurements with in situ observations. Here, an overview of the prelaunch airborne campaigns is given, together with results of the ASIRAS performance over land- and sea ice. The observations, used in this study, are obtained from the Greenland ice sheet and from both multiyear and first year sea ice.

Interferometric synthetic aperture radar data show that Devon Ice Cap (DIC), northern Canada, is drained through a network of 11 glacier systems. More than half of all ice discharge is through broad flows that converge to the southeast of the ice cap, and these are grounded well below sea level at their termini. A calculation of the ice-cap mass budget reveals that the northwestern sector of DIC is gaining mass and that all other sectors are losing mass. We estimate that a 12 489 km(2) section of the main ice cap receives 3.46 +/- 0.65 Gt of snowfall each year, and loses 3.11 +/- 0.21 Gt of water through runoff, and 1.43 +/- 0.03 Gt of ice through glacier discharge. Altogether, the net mass balance of DIC is -1.08 +/- 0.67 Gt a(-1). This loss corresponds to a 0.003 mm a(-1) contribution to global sea levels, and is about half the magnitude of earlier estimates.

We have made side-by-side measurements in several boreholes at Summit, Greenland, using borehole optical stratigraphy (BOS) and neutron-scattering density logging techniques. The BOS logs show strong positive correlation at shallow depths with neutron-scattering logs taken in the same borehole. This supports the hypothesis that BOS detects changes in density. The positive correlation between returned brightness and density decreases with depth and finally becomes negative. We conclude this inversion of correlation is related to changes in densification regime from grain-boundary sliding to pressure sintering.

A new snow-soil-vegetation-atmosphere transfer (Snow-SVAT) scheme, which simulates the accumulation and ablation of the snow cover beneath a forest canopy, is presented. The model was formulated by coupling a canopy optical and thermal radiation model to a physically based multi-layer snow model. This canopy radiation model is physically based yet requires few parameters, so can be used when extensive in situ field measurements are not available. Other forest effects such as the reduction of wind speed, interception of snow on the canopy and the deposition of litter were incorporated within this combined model, SNOWCAN, which was tested with data taken as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) international collaborative experiment. Snow depths beneath four different canopy types and at an open site were simulated. Agreement between observed and simulated snow depths was generally good, with correlation coefficients ranging between r(2) = 0.94 and r(2) = 0.98 for all sites where automatic measurements were available. However, the simulated date of total snowpack ablation generally occurred later than the observed date. A comparison between simulated solar radiation and limited measurements of sub-canopy radiation at one site indicates that the model simulates the sub-canopy downwelling solar radiation early in the season to within measurement uncertainty.

Over the period 1972-98 the height of the snow surface at eight Antarctic sites in Palmer Land and on Alexander Island has been measured with respect to fixed points on local nunataks. From these data an empirical relation between height changes over a given period and three key variables has been derived. These variables are (i) the local mean annual surface air temperature, (ii) a regional estimate of energy available for melt over the period (derived from the nearby Rothera air-temperature record) and (iii) a regional estimate of accumulation over the period (derived from the nearby Gomez Nunatak ice-core accumulation record). Using this relation, the contribution of the Antarctic Peninsula to sea-level rise for warming from climatic conditions (averaged over the last 30 years) is estimated to be -0.006 +/- 0.002 mm a(-1) K-1. If recent warm conditions persist, however, and meltwater can run off to the sea, the contribution to sea-level rise from ablation is calculated to be 0.07 +/- 0.02 mm a(-1) K-1.

Mapping surface air temperature in the Antarctic Peninsula region is made unusually difficult by: the scarcity of meteorological stations, strong climatic gradients and recent rapid regional warming. We have compiled a database of 534 mean annual temperatures derived from measurements of snow temperature at around 10-m depth and air temperature measured at meteorological stations and automatic weather stations. These annual temperatures were corrected for interannual variability using a composite record from six stations across the region. The corrected temperatures were then analysed using multiple linear regression to yield altitudinal and temporal lapse rates. A subset of 508 values were then used to produce a map of temperature reduced to sea level and for a specific epoch (2000 A.D.). The map shows the dramatic climate contrast (3-5degreesC) between the east and west coast of the Antarctic Peninsula in greater detail than earlier studies and also indicates that the present limit of ice shelves closely follows the -9degreesC (2000 A.D.) isotherm. Furthermore, the limit of ice shelves known to have retreated during the last 100 years is bounded by the -9degreesC and -5degreesC (2000 A.D.) isotherms, suggesting that the retreat of ice shelves in the Antarctic Peninsula region is consistent with a warming of around similar to 4 degrees C.

Earlier views regarding the relative insignificance of Antarctic science have been rudely shattered by recent developments. The International Geophysical Year (IGY) of 1957–1958 represented a major turning point for research in Antarctica, even if it is a mistake to believe that no useful scientific work was performed during the pre-IGY period. The IGY provided a sound foundation for the development of Antarctic scientific activity in a wide range of disciplines, including glaciology, atmospheric sciences and medicine. The intrinsic value of research in Antarctica is reinforced by its relevance to an improved understanding of global environmental systems; stratospheric ozone depletion was first identified at Halley base station. Minerals, as yet unproven, have highlighted the conservation problems inherent in the utilization of any Antarctic resources. Marine living resources are at present under threat of over-exploitation in some areas. The 1990s hold the promise for a brighter scientific future for Antarctica, but only as long as the Antarctic Treaty system continues in operation.

The perennial snow, or firn, on the Greenland ice sheet each summer stores part of the meltwater formed at the surface, buffering the ice sheet’s contribution to sea level. We gathered observations of firn air content, indicative of the space available in the firn to retain meltwater, and find that this air content remained stable in cold regions of the firn over the last 65 years but recently decreased significantly in western Greenland.