Snow covers about 40 million km2 of the land area of the Northern Hemisphere during the winter season. The accumulation and depletion of snow is dynamically coupled with global hydrological and climatological processes. Snow covered area and snow water equivalent are two essential measurements. Snow cover maps are produced routinely by the National Environmental Satellite Data and Information Service of the National Oceanic and Atmospheric Administration (NOAA/NESDIS) and by the US Air Force Global Weather Center (USAFGWC). The snow covered area reported by these two groups sometimes differs by several million km2, Preliminary analysis is performed to evaluate the accuracy of these products.

Microwave radiation penetrating through clouds and snowpacks could provide depth and water equivalent information about snow fields. Based on theoretical calculations, snow covered area and snow water equivalent retrieval algorithms have been developed. Snow cover maps for the Northern Hemisphere have been derived from Nimbus-7 SMMR data for a period of six years (1978–1984). Intercomparisons of SMMR, NOAA/NESDIS and USAFGWC snow maps have been conducted to evaluate and assess the accuracy of SMMR derived snow maps. The total snow covered area derived from SMMR is usually about 10% less than the other two products. This is because passive microwave sensors cannot detect shallow, dry snow which is less than 5 cm in depth. The major geographic regions in which the differences among these three products are the greatest are in central Asia and western China. Future study is required to determine the absolute accuracy of each product.

Preliminary snow water equivalent maps have also been produced. Comparisons are made between retrieved snow water equivalent over large area and available snow depth measurements. The results of the comparisons are good for uniform snow covered areas, such as the Canadian high plains and the Russian steppes. Heavily forested and mountainous areas tend to mask out the microwave snow signatures and thus comparisons with measured water equivalent are poorer in those areas.

The identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice-sheet dynamics and for reconstructing ice sheets and climate. No single landform or diagnostic criterion has yet been found with which to identify surging glaciers. A surging-glacier land-system model is constructed using observations and measurements from contemporary surging-glacier snouts in Iceland, Svalbard, U.S.A. and Canada for differentiating ancient surging margins from other non-surging palaeoglaciers. This integrates the suite of landforms, sediments and stratigraphy produced at surging-glacier margins. Landforms produced during surging include thrust moraines, concertina eskers and subglacial crevasse-squeeze ridges. Sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds, which display severe glaciotectonic contortion and faulting. Hummocky moraine, comprising interbedded stratified sediments and mass-flow diamictons, has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ. An example of the application of the land-system model is presented for east-central Alberta, Canada. A surging palaeo-ice stream is identified within this part of the southwestern Laurentide ice sheet, where thrust-block moraines, crevasse-squeeze ridges, flutings, hummocky moraine and glaciotectonized sediments are juxtaposed.

A parameterization scheme using simple algorithms for unmeasured glaciers is being applied to glacier inventory data to estimate the basic glaciological characteristics of the inventoried ice bodies and simulate potential climate-change effects on mountain glaciers. For past and potential climate scenarios, glacier changes for assumed mass-balance changes are calculated as step functions between steady-state conditions for time intervals that approximately correspond to the characteristic dynamic response time (a few decades) of the glaciers. In order to test the procedure, a pilot study was carried out in the European Alps where detailed glacier inventories had been compiled around the mid-1970s. Total glacier volume in the Alps is estimated at about 130 km3 for the mid-1970s; strongly negative mass balances are likely to have caused a loss of about 10–20% of this total volume during the decade 1980–90. Backward calculation of glacier-length changes using a mean annual mass balance of 0.25m w.e.a−1 since the end of the “Little Ice Age” around 1850 AD gives considerable scatter but satisfactory overall results as compared with long-term observations. The total loss of Alpine surface ice mass since 1850 can be estimated at about half the original value. An acceleration of this development, with annual mass losses of around 1 m a−1 or more as anticipated from IPCC scenario A for the coming century, could eliminate major parts of the presently existing Alpine ice volume within decades.

Retention of meltwater runoff by percolation and/or refreezing in the snowpack cannot be neglected when studying the surface mass balance of the Greenland ice sheet. In this paper, we make a detailed comparison of several treatments proposed in the literature to account for this process in large-scale mass-balance parameterizations. The melt on the Greenland ice sheet is calculated with a revised degree-day model using updated datasets of surface elevation and precipitation rate on a 5 km grid. Crucial model parameters are recalibrated by comparing mass-balance characteristics with available observations on a regional basis. We discuss the role of meltwater retention in the light of the overall mass balance of the Greenland ice sheet and its sensitivity to climatic change, and display patterns of effective-retention fractions for the various methods. As a main conclusion it appears that overall results are quite similar for the various models, but that meltwater retention has a large spatial variation not described by the simple treatments. Using the most comprehensive retention model, the sensitivity of the runoff is found to be +0.35 mm ˚C–1 of sea-level change per year. We also present a new map of the different zones (facies) that characterize the accumulation area of the Greenland ice sheet, which is useful for interpreting field data and calibrating satellite observations.

Surges are common in all the major ice caps in Iceland, and historical reports of surge occurrence go back several centuries. Data collection and regular observation over the last several decades have permitted a detailed description of several surges, from which it is possible to generalize on the nature of surging in Icelandic glaciers. Combining the historical records of glacier-front variations and recent field research, we summarize the geographic distribution of surging glaciers, their subglacial topography and geology, the frequency and duration of surges, changes in glacier surface geometry during the surge cycle, and measured velocity changes compared to calculated balance velocities. We note the indicators of surge onset and describe changes in ice, water and sediment fluxes during a surge. Surges accomplish a significant fraction of the total mass transport through the main outlet glaciers of ice caps in Iceland and have important implications for their hydrology. Our analysis of the data suggests that surge-type glaciers in Iceland are characterized by gently sloping surfaces and that they move too slowly to remain in balance given their accumulation rate. Surge frequency is neither regular nor clearly related to glacier size or mass balance. Steeply sloping glaciers, whether hard- or soft-bedded, seem to move sufficiently rapidly to keep in balance with the annual accumulation.

Variations in global ice volume and temperature over the Cenozoic era have been investigated with a set of one-dimensional (1-D) ice-sheet models. Simulations include three ice sheets representing glaciation in the Northern Hemisphere, i.e. in Eurasia, North America and Greenland, and two separate ice sheets for Antarctic glaciation. The continental mean Northern Hemisphere surface-air temperature has been derived through an inverse procedure from observed benthic δ18O records. These data have yielded a mutually consistent and continuous record of temperature, global ice volume and benthic δ18O over the past 35 Ma. The simple 1-D model shows good agreement with a comprehensive 3-D ice-sheet model for the past 3 Ma. On average, differences are only 1.0˚C for temperature and 6.2 m for sea level. Most notably, over the 35 Ma period, the reconstructed ice volume–temperature sensitivity shows a transition from a climate controlled by Southern Hemisphere ice sheets to one controlled by Northern Hemisphere ice sheets. Although the transient behaviour is important, equilibrium experiments show that the relationship between temperature and sea level is linear and symmetric, providing limited evidence for hysteresis. Furthermore, the results show a good comparison with other simulations of Antarctic ice volume and observed sea level.

The Arctic sea ice has been pointed to as one of the first and clearest indicators of climate change. Satellite passive microwave observations from 1979 through 2005 now indicate a significant –8.4±1.5% decade–1 trend (99% confidence level) in September sea-ice extent, a larger trend than earlier estimates due to acceleration of the decline over the past 41 years. There are differences in regional trends, with some regions more stable than others; not all regional trends are significant. The largest trends tend to occur in months where melt is at or near its peak for a given region. A longer time series of September extents since 1953 was adjusted to correct biases and extended through 2005. The trend from the longer time series is –7.7±0.6% decade–1 (99%), slightly less than from the satellite-derived data that begin in 1979, which is expected given the recent acceleration in the decline.

During the recent Italian expedition ‘K2 2004 – 50 years later’ (June–July 2004) on Baltoro glacier, Karakoram, Pakistan, glaciological field experiments were carried out on the debris-covered area of this high-elevation glacier. The aim was to investigate the ice ablation and its relations with debris thermal properties and meteorological conditions. Ablation measurements along the glacier up to about 5000 m and within a dedicated test field were combined with meteorological data from two automatic weather stations located at Urdukas (4022 ma.s.l.) and at K2 Base Camp (5033 m a.s.l.). In addition, temperature measurements of the debris cover at different depth levels along the glacier allowed the calculation of debris surface temperature and of the debris thermal resistance (R). Using the air temperature, the local mean lapse rate (0.0075˚C m−1) and the measured ablation, the degree-day factors (K) at different locations on the glacier were calculated. The ice ablation rates were related to debris thickness and elevation. They are typically on the order of 4 cm d−1 during the observation period. However, it was found that the surface topography (slope, aspect) has an influence on the total ablation similar to that of the debris thickness. Thermal resistance of the debris cover and its distribution over the glacier were estimated. Finally, a best-guess estimate of the total meltwater production was calculated from available climate data.

Changes of Larsen Ice Shelf, Antarctica, and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery, with emphasis on changes in the glaciers which previously nourished the ice shelf north of Seal Nunataks and now calve directly into the sea. The large glaciers retreated several kilometres inland of the previous grounding line. The velocity field of Drygalski Glacier, the largest glacier in this area, was mapped by means of interferograms derived from pairs of European Remote-sensing Satellite synthetic aperture radar images from 1995 and 1999. The main part of the glacier showed a significant acceleration of flow over these 4 years, with an increase of velocity up to three-fold at the terminus. Similar accelerations were observed by means of interferometry on several other grounded glaciers, suggesting that the removal of ice shelves could lead to an effect on eustatic sea level. For Larsen B, the northernmost surviving part of Larsen Ice Shelf, the retreat of the ice front to October 2000 is documented.

Outlet glaciers draining the Antarctic ice sheet into the Amundsen Sea Embayment (ASE) have accelerated in recent decades, most likely as a result of increased melting of their ice-shelf termini by warm Circumpolar Deep Water (CDW). An ocean model forced with climate reanalysis data shows that, beginning in the early 1990s, an increase in westerly wind stress near the continental shelf edge drove an increase in CDW inflow onto the shelf. The change in local wind stress occurred predominantly in fall and early winter, associated with anomalous high sea-level pressure (SLP) to the north of the ASE and an increase in sea surface temperature (SST) in the central tropical Pacific. The SLP change is associated with geopotential height anomalies in the middle and upper troposphere, characteristic of a stationary Rossby wave response to tropical SST forcing, rather than with changes in the zonally symmetric circulation. Tropical Pacific warming similar to that of the 1990s occurred in the 1940s, and thus is a candidate for initiating the current period of ASE glacier retreat.

Recent satellite investigations revealed that in the 1990s the grounding line of Pine Island and Thwaites Glaciers, West Antarctica, retreated several km, the ice surface on the interior of the basins lowered 10 cm a–1, and Pine Island Glacier thinned 1.6 ma–1. These observations, however, were not sufficient to determine the cause of the changes. Here, we present satellite radar interferometry data that show the thinning and retreat of Pine Island Glacier are caused by an acceleration of ice flow of about 18 ± 2% in 8 years. Thwaites Glacier maintained a nearly constant flow regime at its center, but widened along the sides, and increased its 30 ± 15% mass deficit by another 4% in 4 years. The combined mass loss from both glaciers, if correct, contributes an estimated 0.08 ± 0.03 mma–1 global sea-level rise in 2000.

A model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice-rock interface on glacier sliding. An increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed. Abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content. A similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness. Sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed.

A complete glacier inventory of the Himalaya and Karakoram (H-K) has been created by merging records from the Chinese Glacier Inventory, several regional inventories produced by the International Centre for Integrated Mountain Development, Kathmandu, Nepal, and partial inventories from the Geological Survey of India. The only remaining gap, the Indian part of Kashmir, has been filled by a reconnaissance inventory based on Soviet military maps at 1 : 200 000 scale representing the late 1970s. It contains records for 3526 glaciers covering 9584 km2. The new H-K inventory contains records and outlines for 20 812 glaciers covering 43 178 km2. The extent of ice in the Karakoram is slightly less than in the Himalaya, but the Karakoram glaciers are on average twice as thick (~160m as against ~80 m). A glacier-by-glacier analysis, relying on estimates of mass balance for the entire mountain range and on an extension of the often-used volume–area scaling relation, suggests that up to about one-fifth of the glaciers present in 1985 may have disappeared already. If mass loss were to remain constant at the average rate for 1975–2008, from 3000 to 13 000 more glaciers might disappear by 2035. If mass loss were to continue to accelerate as inferred for 1985–2008, only a few thousand to a few hundred glaciers might remain in 2035. Total area and total mass would each decrease by about one-half (constant-rate assumption) or three-quarters (constant-trend assumption). These projections, which are uncertain and neglect some possibly important mitigating controls, such as variable extents of debris cover and the feedback due to retreat to higher elevations, demonstrate the need for more complete analyses to inform public perceptions of, and policy decisions relating to, the health of H-K glaciers.

Complete Swiss glacier inventories are available for 1850 (reconstructed) and 1973 (from aerial photography). Connected to the Global Land Ice Measurements from Space project, a new Swiss glacier inventory for approximately the year 2000 (SGI 2000) is compiled mainly based on satellite imagery. The developed and applied remotesensing and geographical information system (GIS) methods are described in part I of the contribution. In part II, the inventory design, first-result analyses and comparisons with former glacial conditions are presented. As basic entries SGI 2000 contains the individual glacier identification, planimetric glacier boundaries as derived from image analysis, digitized central flowlines and polygonal glacier basin maps. All other parameters are automatically deduced from the above entries and a digital elevation model within a GIS. Here, we analyze a set of small Bernese and Valais glaciers of area <10 km2. These glaciers lost about 21% of area from 1973 to 1998, in addition to about 80% during 1850–1973, both with respect to the 1973 area. In order to track the latest trend in more detail, anintermediate glacier condition has been compiled from satellite imagery of 1985. This analysis gave an increasing speed of area loss (19%) for 1985–98.

A new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three ‘sliding laws’, in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. Ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal–lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.

Glaciers in China are primarily located in the Tibetan Plateau (TP) and surrounding high mountains. The Chinese Glacier Inventory indicates that there are 46 377 glaciers in western China. Meteorological records indicate that air temperature in western China has risen by 0.2˚C per decade since 1951, and 1998 was the warmest year; precipitation in the region increased by 5–10% per decade from 1953 to 1997. Using remote-sensing and Geographic Information System methods, we have monitored the changes in >5000 glaciers over the past 50 years. We conclude that >80% of glaciers in western China have retreated, losing 4.5% of their combined areal coverage, although some glaciers have advanced. In addition, regional differences characterize glacier changes over the past few decades. For example, glaciers in the central and northwestern TP were relatively stable, while glaciers in the mountains surrounding the TP experienced extensive wastage. Mass-balance variations for some glaciers show accelerated ice shrinkage in the last two decades.

The retreat of the Larsen B ice shelf, Antarctic Peninsula, and the collapse of its northern section are analyzed using satellite images acquired between January 1995 and May 2003. Over 1 week during March 2002, after a period of steady retreat since 1995, 2300 km2 of the ice shelf broke up into many small icebergs. This rapid collapse occurred at the end of an exceptionally warm summer, and after a multi-year period of decreasing surface net mass balance, ice thinning, flow acceleration and widening of rifts. The ice-shelf area decreased from 11 512 km2 in January 1995 to 3463 km2 in March 2002, and 2667 km2 in April 2003. ERS synthetic aperture radar (SAR) images were used to identify ice-shelf zones with different surface morphology, which generated icebergs of different sizes and shapes. The pattern of retreat and break-up, similar to that of Larsen A in 1995, suggests that fracturing enhanced by abundant surface melt played a key role. In addition, the recent changes of grounded and residual floating ice north of Larsen B are analyzed by means of Envisat advanced synthetic aperture radar (ASAR) images up to summer 2003, showing significant loss of grounded ice upstream of those ice-shelf sections which disintegrated in 1995 and 2002.

Some of the ideas about glacier surging are considered, mainly but not entirely in the light of observations of temperate glaciers in Alaska, U.S.A., made within the last 15 years. Climate has an influence on surge recurrence interval. Climate and weather also affect surge initiation, termination and magnitude. Regional studies lead to the speculation that subglacial “till” plays a key role in surging, and it has been found under all surge-type glaciers studied so far, including Black Rapids and Variegated Glaciers, Alaska. In most of the glaciers studied, till deformation processes dominate the motion in quiescence. The linked-cavity model of surge triggering and rapid motion is not consistent with these observations, but the limited coverage of the observations does not rule it out under parts of the glaciers studied. The till observations in Alaska raise old questions about the interaction between till and the hydraulic systems of temperate glaciers. The role of stored water, which observations show to be active even in winter on Black Rapids Glacier, is noted.

A glacier’s most fundamental altitude is the equilibrium-line altitude (ELA) because it divides the glacier into ablation and accumulation areas. The best parameterization of the ELA for glacier inventory is the balanced-budget ELA. We discuss direct estimation of balanced-budget ELA from mass-balance data for individual glaciers, and indirect estimation of balanced-budget ELA from simple topographic parameters available from the World Glacier Inventory (WGI), i.e. the area-median and maximum and minimum altitudes. Mass balance and ELA for individual glaciers are usually strongly correlated and we calculate balanced-budget ELA from the regression equation linking the two. We then compare balanced-budget ELA with area-median and mid-range altitudes for the 94 glaciers for which we have all the necessary data. The different ELA estimates agree well enough (±82 to ±125 m) to describe geographical variations in ELA and for application of glacier–climate models to glacier inventory data. Mid-range and area-median altitudes are already available for tens of thousands of glaciers in the current WGI and should be evaluated in future inventories.

Satellite passive-microwave data have been used to calculate and map the length of the sea-ice season throughout the Southern Ocean for each year 1979–99. Mapping the slopes of the lines of linear least-squares fit through the 21 years of resulting season-length data reveals a detailed pattern of trends in the length of the sea-ice season around the Antarctic continent. Specifically, most of the Ross Sea ice cover has, on average over the 21 years, undergone a lengthening of the sea-ice season, whereas most of the Amundsen Sea ice cover and almost the entire Bellingshausen Sea ice cover have undergone a shortening of the sea-ice season. Results for the Weddell Sea are mixed, with the northwestern portion of the sea having experienced a shortening of the sea-ice season but a substantial area in the south–central portion of the sea having experienced a lengthening of the ice season. Overall, the area of the Southern Ocean experiencing a lengthening of the sea-ice season by at least 1day per year over the period 1979–99 is 5.6 × 106 km2, whereas the area experiencing a shortening of the sea-ice season by at least 1 day per year is 46% less than that, at 3.0 × 166 km2.