A new three-dimensional, time-dependent ice-sheet model, including the calculation of the coupled temperature and velocity fields, isostatic adjustment of the bedrock and a mass-balance parameterization, was used to reconstruct the evolution of the Greenland ice sheet in response to a climate history derived from the oxygen-18 measured in the GRIP ice core. Steady-state experiments were done to test the sensitivity of the model, first to variations of poorly known parameters, secondly to different climates. These experiments show that the modelled ice sheet is not very sensitive to variations in the geothermal heat flux, but very sensitive to changes in the accumulation.

Computations over 50 000 years into steady state with Greve’s polythermal ice-sheet model and its numerical code are performed for the Greenland ice sheet with today’s climatological input (surface temperature and accumulation function) and three values of the geothermal heat flux: (42, 54.6, 29.4) mW m−2. It is shown that through the thermomechanical coupling the geometry as well as the thermal regime, in particular that close to the bed, respond surprisingly strongly to the basal thermal heat input. The most sensitive variable is the basal temperature field, but the maximum height of the summit also varies by more than ±100m. Furthermore, some intercomparison of the model outputs with the real ice sheet is carried out, showing that the model provides reasonable results for the ice-sheet geometry as well as for the englacial temperatures.

The energy balance was measured at the ETH/CU research camp, located at the equilibrium-line altitude of the Greenland ice sheet (69°34′N, 49°17′W), in spring 1993. Prior to the onset of melt the latent energy flux is the only energy sink, with an average value of 1.18 MJ m−2d−1. This turbulent transfer of moisture is responsible for an average daily snow sublimation of 5 × 10−4 m. The sensible heat flux (0.91 MJ m−2d−1) is the major energy source, and the net radiation only a minor contributor with a positive balance of 0.12 MJ m−2d−1. The residual of the turbulent fluxes and the net radiation is about 0.14 MJ m−2d−1, which agrees surprisingly well with the in situ ground-heat flux measurements of 0.15 MJ m−2d−1 made at the same location. Assuming a 3°C temperature increase in a global-change scenario, and using the information gained from the energy-balance measurements, a simple calculation showed that 5.4 × 10−2 m w.e. of snow would be lost by sublimation for an area of approximately 400 000 km2. The increased sublimation is due to an increase in the net radiation balance caused by an albedo reduction. This would lead to a total annual mass reduction of 22 km3 w.e. of snow for the entire Greenland ice sheet, which is 4.2% of today’s annual accumulation.

We present the first detailed study of regional and secular changes in accumulation rate from northern Greenland. Four 100–150 m ice cores from this previously little investigated region have been dielectrically profiled and a good chronology for all four ice cores established by modelling the density profiles and identifying volcanic peaks in the records. This made it possible to calculate the accumulation rates of each core. The current accumulation rates show that there is a large region of low accumulation rate to the northeast of central Greenland with drops in accumulation rate of 25% 150 km, and 50% 300 km from Summit.

Relatively large variations in accumulation rate over time are seen in all the cores. We have compared the resulting accumulation-rate record, which should be related to changes in local air temperature over northern Greenland, with Scandinavian tree-ring records and have interpreted the data as showing an early Medieval Warm Epoch, but no pronounced “Little Ice Age” and no unequivocal greenhouse warming effect as yet in northern Greenland.

Net accumulation rates at the Greenland summit have been inferred using layer-thickness data from the GISP2 ice core with corrections for strain using a non-linear, one-dimensional flow model of an ice sheet. The flow model accounts for thickness changes in ice-sheet in response to mass-balance variations. The model is used to investigate how net accumulation-rate changes affect the time evolution of: (1) the ice-sheet thickness. (2) the vertical strain rate, and (3) the corresponding internal annual-layer structure. The model, parameterized to fit the present net accumulation rate and thickness of the Greenland ice-sheet summit, has a characteristic time constant for adjustment to accumulation-rate changes of about 6000a and yields an ice sheet 200-400 m thinner than its present thickness during the last glacial period.

Accumulation-rate histories inferred from GISP2 layer-thickness data using both a constant- and a variable-thickness model are compared. The variable-thickness model predicts accumulation rates about 25% lower than the constant-thickness model. Our results also indicate that high-frequency changes in accumulation rates (i.e. alter the Younger Dryas event) are consistent with earlier analyses. However, sensitivity tests indicate that the accumulation-rate history cannot be precisely determined. Our analysis defines an envelope of likely accumulation histories bounded above by the accumulation history inferred by the constant-thickness model. Predictions become increasingly uncertain for old ice because of (1) intrinsic difficulties associated with this inverse problem, and (2) decreased accuracy of the data.

The depth–age relation observed in the GISP2 ice core is the result of the integrated effects of ice-sheet changes over time, as well as the accumulation-rate history. Here, we construct a forward model to compute ages at various depths in the core. In the model, these ages are functions of parameters that describe the ice thickness as a function of time. Using the maximum-likelihood inverse method, these parameters are iteratively adjusted until measured and computed ages agree satisfactorily. The results suggest that the thickness along the flowline connecting the GISP2 and GRIP drill sites has not changed significantly since the onset of the Holocene. We also derive bounds on the likely thickness changes. Because these bounds are independent of assumptions concerning the processes driving the ice-sheet evolution, they can provide useful constraints for other ice-sheet modeling efforts.

Summer melt is a primary source of mass loss on the Greenland ice sheet. An understanding of melt processes on the ice sheet, their connection with atmospheric processes, and the redistribution of meltwater is important for ascertaining the mass balance of the ice sheet. High-resolution radar measurements made in the percolation zone of the Greenland ice sheet reveal the evolving radar signature of summer surface melting and subsequent refreezing of meltwater. A traverse over the snow surface has resulted in the first radar map of snow stratigraphy over an extended distance. The dominant sources of back-scatter in the study area are the snow surface and effectively continuous annual ice layers. We suggest applications of our results to help define the extent of the percolation zone and to discriminate between regions where surface melt is lost to the sea and those where melt refreezes nearly in place.

Historical and reconstructed snow-cover data show evidence of a gradual increase in snow cover over the continental interior of North America (NA) during much of the 20th century, primarily in response to increasing snowfall. A rapid decrease in Canadian-prairies snow cover after 1970 is not observed over the Great Plains. Analysis of snow-cover-climate relationships revealed systematic increases in the sensitivity of snow cover to Northern Hemisphere (NH) temperatures over the 1940-65 period. This change is mainly due to an increase in snowfall-temperature sensitivity during this period. Seasonal analysis revealed that the observed increase in snow-cover and snowfall temperature sensitivity is primarily a spring phenomenon. A marked increase in the importance of the spring period is observed around 1960, which coincides with a well-documented change in atmospheric circulation over NA. The post-1960 period is characterized by a significant inverse relationship between snow cover and hemispheric air temperature over the Canadian prairies and northern Great Plains regions.

Horizontal and vertical distributions of melt features (ice layers) were examined using two ice cores (206.6 and 101.5 m deep, 1 m apart) from Site J (66°51.9′ N, 46°15.9′W, 2030 m a.s.l.). The temperature at 10 m was −16.3°C. We observed 2804 melt features, with a total thickness of 30.32 m, in the 206.6 m core, corresponding to 16.4% by volume of the ice-equivalent core length. Horizontal distribution of melt features was examined by correlating melt-feature thicknesses in the two cores. The correlation coefficient was 0.71 (n = 514) for each melt feature in the two cores. It was maximum for data passed through 5 and 40 year low-pass filters. A significant relationship (P = 0.005, n = 36) was obtained for the vertical distribution of melt features and the June temperature on the west coast of Greenland (Jakobshavn). Using this, June temperatures at Jakobshavn since 1550 were estimated. There are three periods (1685-1705, 1835-70 and 1933-45) during which mean June temperatures clearly decreased, when they were estimated to he 0.1°, 0.4° and 0.2°C lower than the average for the whole period (1550-1989). The first two “cold” periods have been identified in melt features of the Dye 3 and Devon Island ice cores and in a tree-ring profile from Yukon Territory, Canada. Melt-feature percentages in the Site J ice core have increased since about 1945, probably reflecting summer-temperature warming on the ice sheet.

During the summer of 1993, a field program was conducted to collect several shallow firn cores from two locations in the southern region of the Greenland ice sheet. Stratigraphic evidence of melt from these cores was used for comparison with satellite-derived indications of melt.

The shallow firn cores were examined for stratigraphic evidence of past melt events and were sampled for oxygen-isotope analysis to delineate the annual accumulation layers in the snowpack. The relative intensity of each year’s summer melt episode was compared to the corresponding melt frequency derived from microwave emissions. This comparison demonstrates that a linkage between the stratigraphic record and microwave data can be established. Both data sets indicate that there was less melt during the late 1970s and early 1980s than during the late 1980s, in general agreement with climate observations.

The rare melt features in the GISP2, central Greenland deep ice core have decreased in frequency over the most recent 7000 years. Calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central Greenland, and against modern temporal variability of temperatures in central Greenland, indicates that mean mid-summer temperatures have cooled over the most recent 7000 years, probably by slightly more than l°C if variability of summer temperatures has not changed. Comparison to GRIP isotopic records from central Greenland and to the melt record from the Agassiz Ice Cap, Arctic Canada, suggests some seasonal and regional coherence for this cooling signal, as well as for a cold event about 8000-8500 BP.

An analysis of snow-cover variability over Northern Hemisphere land masses reveals a continuation of the subnormal coverage that began in the late 1980s (relative to the 1972–present interval). While the 1994 snow year (September 1993–August 1994) exhibited a return to near-normal hemispheric extent, only three months during this period had above-normal coverage. Only 11 of the past 88 months (through October 1994) have been above the norm. Deficits have been most common in spring, over both the Eurasian and North American continents. This is a hemisphere-wide situation; positive correlations are identified between hemispheric and regional snow extents in spring, as well as in fall and winter. A number of significant associations are also recognized between regions during these three seasons; however, it is uncommon to see more than 50% of the variance in one region explained by another. These correlations are most common between adjacent regions, but some are found between regions on different continents. Only in spring are significant positive relationships between non-adjacent regions on the same continent observed.

Data from a rawinsonde network are used to provide aerological estimates of monthly precipitation minus evaporation (P – E) averaged over the region north of 70° N. Using published climatological estimates of E, area-averaged P is obtained for each month and year as a residual. Using surface temperatures from the rawinsonde network, the fraction of precipitation falling as snow is then estimated. Over the 1974–91 study period, precipitation and snowfall (water equivalent) have annual means of 26.6 and 19.0 cm. respectively. Assuming a representative aged snowpack density of 330 kg m−3 yields a total snow depth of 57.5 cm. The mean annual cycles of both variables display an autumn maximum, but because of the temperature dependency, nearly all precipitation falls as rain during July and August. Composite analyses reveal that increased precipitation for all seasons and increased snowfall for winter and autumn are favored by a “winter-type” circulation pattern, characterized by stronger troughs over the Atlantic and Eurasian sectors of the Arctic, associated with increased cyclonic activity over the Arctic peripheral seas.

The simulation of the northern and southern polar climates for 1979–88 by 14 global climate models (GCMs), using the observed monthly averaged sea-surface temperatures and sea-ice extents as boundary conditions, is part of an international effort to determine the systematic errors of atmospheric models under realistic conditions, the so-called Atmospheric Model Intercomparison Project (AMIP), In this study, intercomparison of the models’ simulation of polar climate is discussed in terms of selected surface and vertically integrated monthly averaged quantities, such as sea-level pressure, cloudiness, precipitable water, precipitation and evaporation/sublimation. The results suggest that the accuracy of model-simulated climate features in high latitudes primarily depends on the horizontal resolution and the treatment of physical processes in the GCMs. AMIP offers an unprecedented opportunity for the comprehensive evaluation and validation of current atmospheric models and provides valuable information for model improvement.

A global coupled atmosphere–ocean model is used to examine the hydrologic cycle of the Arctic Ocean. The model has a horizontal resolution of 4° × 5°, nine vertical layers in the atmosphere and 13 in the ocean. River discharge into the Arctic Ocean is included by allowing runoff from each continental grid box to flow downstream according to a specified direction file and a speed that depends on topography. A 74 year control simulation of the present climate is used to examine variability of the hydrologic cycle, including precipitation, sea ice, glacial ice and river discharge. A 74 year transient simulation in which atmospheric CO2 increases each year at a compound rate оf 1% is then used to examine potential changes in the hydrologic cycle. Among these changes are a 4°C increase in mean annual surface air temperature in the Arctic Ocean, a decrease in ice cover which begins after 35 years, and increases in river discharge and cloud cover. There is little change in the net difference between precipitation and evaporation. Also in the transient simulation, glacial ice on Greenland decreases relative to the control.

Past modelling studies have shown that the energy balance of the ice-sheet surface is of primary importance in representing the 100 000 year glacial cycle. In particular, modelling of the net mass-balance function is an important part of coupled ice-sheet/climate models. We conduct a series of palaeoclimatic simulations with a vertically integrated ice-flow model coupled to the two-dimensional statistical-dynamical LLN (Louvain-la-Neuve) climate model. The models are coupled through a land-surface model which computes seasonal cycles of surface temperature and precipitation at the real altitude of the surface and evaluates the annual snow and/or ice-mass budget. The present-day climate of the Northern Hemisphere, the Greenland mass balance and the snowfield characteristics are quite well represented despite the relative simplicity of the model. Total ice-volume and sea-level variations during the last glacial cycle are well simulated. This suggests that the physical mechanisms included in the models are sufficient to explain the most striking features of the ice-age cycle. Introducing an improved and more detailed topography improves the simulation of the total ice volume but fails to correct inadequacies in the simulated ice distribution on the surface of the Earth.

A numerical model is used to simulate ice-sheet behaviour in Europe through the last glacial cycle. It is used in two modes: a forward mode, in which the model is driven by a proxy palaeoclimate record and the output compared with a geological reconstruction of ice-sheet fluctuation; and an inverse mode, in which we determine the climate function that would be required to simulate geologically reconstructed ice-sheet fluctuations.

From these simulations it is concluded that extra-glacial climates may be poor predictors of ice-sheet surface climates, and that climatic transitions during the glacial period may have been much more rapid and the intensity of warming during the early Holocene much greater than hitherto supposed. Stronger climate forcing is required to drive ice-sheet expansion when sliding occurs at the bed compared with a non-sliding bed. Sliding ice sheets grow more slowly and decay more rapidly than non-sliding ice sheets with the same climate forcing.

A three-dimensional time-dependent thermomechanical ice-sheet model was used together with a two-level (snow-accumulation/runoff) mass-balance model to investigate the Quaternary ice sheets of the Northern Hemisphere. The model freely generates the ice-sheet geometry in response to specified changes in surface temperature and mass balance, and includes bedrock adjustment, basal sliding and a full temperature calculation within the ice. The mass-balance parameterization makes a distinction between snowfall and melting. Yearly snowfall rates depend on the present precipitation distribution, and are varied proportionally to changes in surface temperature and the moisture content of the air. The ablation model is based on the positive-degree-day method, and distinguishes between ice and snow melting. This paper discusses steady-slate characteristics, conditions for growth and retreat, and response time-scales of ice sheets as a function of a prescribed lowering of summer temperature. Most notably, the modelled extents of the Eurasian ice sheet for a summer temperature lowering of 6–7 K and of the Laurentide ice sheet for a cooling of 9–10 K are in reasonable agreement with most reconstructions based on geological evidence, except for the presence of a large ice sheet stretching from Alaska across the Bering Strait to most of eastern Siberia. In addition, wet basal conditions turned out to be always confined to the margin, whereas central areas in these reconstructions remained always cold-based. This is of relevance for processes involving reduced basal traction.

Precipitation predictions from globai-climate models (GCMs) for the ice-covered Arctic Ocean and the ice sheets of Antarctica are among the most important aspects of the inferred response of the polar areas to climate change. It is generally recognized that the atmospheric hydrologic cycle, which includes precipitation as a key part, is one of the components of the climate system that GCMs do not handle particularly well.

The present-day atmospheric-moisture budget poleward of 70° latitude in both hemispheres, as represented by two versions of the NCAR (U.S. National Center for Atmospheric Research) community climate model (CCM1 and CCM2), is compared with observational analyses. The quantities examined on the seasonal and annual timescales are precipitation, evaporation/sublimation and atmospheric poleward moisture transport. The results are discussed in terms of the physiographic and climatic characteristics of both polar regions and how the particular models handle moisture transport: CCM1 uses the positive-moisture fixer and CCM2 the semi- Lagrangian transport. A particularly important test both for models and for observations is the degree to which the independently determined moisture-budget quantities actually balance. Deficiencies of both observations and models are discussed.

The annual rate of net mass accumulation at the surface in the Antarctic and Greenland ice sheets is determined from firn emissivity based on Nimbus-5 ESMR and Nimbus-7 THIR data. In this study the determinations are limited to the areas of dry-snow facies and are based on a hyperbolic function of emissivity. Two coefficients of the function are selected for particular regions of each ice sheet after a comparison with field data selected for their reliability (82 stations in East Antarctica, 69 stations in West Antarctica and 89 stations in Greenland). Derived accumulation values are produced for grid-point locations 100 km apart which cover 56–94% of the dry-snow areas and 32–58% of the accumulation areas of each ice sheet. These values are compared with interpolated values obtained from the latest contoured compilations of field data. The means of derived values for East and West Antarctica are 12% and 39% larger, respectively, than the mean obtained from interpolated values, suggesting that the isopleth patterns as drawn in the compilation of field data lead to underestimates. The mean of derived values for Greenland is 5% smaller than the mean obtained from interpolated values, suggesting that the compilation of field data may lead to small overestimates that are within the error of determination and the variability of accumulation. Improving facies zonation and the determination of coefficients for the areas of upper percolation facies should improve these preliminary assessments.