Jakobshavn Isbræ, West Greenland, underwent a large, rapid and not well understood change in flow dynamics in 1998, leading to a doubling of its ice discharge rates. We calculate the width-averaged forces controlling flow of Jakobshavn Isbræ in 1995, 2000 and 2005 to elucidate processes responsible for this change in flow speed. In contrast to earlier suggestions, we conclude that the observed acceleration was not caused by the loss of back-stress due to weakening and subsequent break-up of the floating ice tongue alone. Gradients in longitudinal stress are small at all times considered (∼3% of the driving stress) and basal and lateral drag provide resistance to flow. Over the 10 year period considered, the average driving stress increased by 20 kPa, which was balanced by a comparable increase in lateral drag. We surmise that the velocity changes resulted from weakening of the ice in the lateral shear margins and perhaps a change in properties at the bed. Possible mechanisms for weakening of ice in the lateral shear margins include cryo-hydrologic warming of subsurface ice in the ablation zone and hydraulic weakening due to higher water content of ice in the shear margins.

Imaging spectroscopy is emerging as a critical tool in our ability to move beyond simple detection of changes in cryosphere–climate interactions to attribution of these changes to forcings by changes in greenhouse gases and absorbing impurities (Dozier and others, 2009; Singh and others, 2010). Fresh in the growing literature related to imaging spectroscopy of snow and ice, Singh and others (2010) have explored the impact of dust on the hyperspectral reflectance of snow with the intent of understanding the capacity of imaging spectroscopy to quantify snow properties. The paper reaches relatively robust conclusions about the impact of soil on snow reflectance. As such, the efforts of the authors and the importance of their paper in providing foundational work on imaging spectroscopy of the cryosphere should be appreciated.

Sliding experiments were performed at −10°C on smooth surfaces of freshwater columnar-grained S2 ice sliding against itself at a velocity of 8 × 10−4 m s−1, with the purpose of examining whether normal stress parallel to the sliding plane affects frictional resistance. This component of the stress tensor was varied (0.20–1.83 MPa) using a loading system operated under biaxial compression, by orienting the sliding plane at two different angles, 26° and 64°, with respect to the principal loading direction. Under these conditions, no evidence was found to indicate that the normal stress in the direction of sliding affects the friction coefficient.

We explore the robustness and transferability of parameterizations of cloud radiative forcing used in glacier melt models at two sites in the Swiss Alps. We also look at the rationale behind some of the most commonly used approaches, and explore the relationship between cloud transmittance and several standard meteorological variables. The 2 m air-temperature diurnal range is the best predictor of variations in cloud transmittance. However, linear and exponential parameterizations can only explain 30–50% of the observed variance in computed cloud transmittance factors. We examine the impact of modelled cloud transmittance factors on both solar radiation and ablation rates computed with an enhanced temperature-index model. The melt model performance decreases when modelled radiation is used, the reduction being due to an underestimation of incoming solar radiation on clear-sky days. The model works well under overcast conditions. We also seek alternatives to the use of in situ ground data. However, outputs from an atmospheric model (2.2 km horizontal resolution) do not seem to provide an alternative to the parameterizations of cloud radiative forcing based on observations of air temperature at glacier automatic weather stations. Conversely, the correct definition of overcast conditions is important.

Isvand is an ice-dammed lake situated on the western flank of Kangiata Nunaata Sermia, a large ice stream from the Inland Ice, producing calf ice into Kangersuneq fjord at the head of Godtha˚bsfjord (Nuup Kangerlua), West Greenland (Fig. 1). This outlet of the Inland Ice is the only significant calving outlet of the ice sheet met with between 628 and 698 N.

Glaciers respond to long-term climate changes and also to the year-to-year fluctuations inherent in a constant climate. Differentiating between these factors is critical for the correct interpretation of past glacier fluctuations and for the correct attribution of current changes. Previous work has established that century-scale, kilometre-scale fluctuations can occur in a constant climate. This study asks two further questions of practical significance: how likely is an excursion of a given magnitude in a given amount of time, and how large a trend in length is statistically significant? A linear model permits analytical answers wherein the dependencies on glacier geometry and climate setting can be clearly understood. The expressions are validated with a flowline glacier model. The likelihood of glacier excursions is well characterized by extreme-value statistics, although probabilities are acutely sensitive to some poorly known glacier properties. Conventional statistical tests can be used for establishing the significance of an observed glacier trend. However, it is important to determine the independent information in the observations which can be effectively estimated from the glacier geometry. Finally, the retreat of glaciers around Mount Baker, Washington State, USA, is consistent with, but not independent proof of, the regional climate warming that is established from the instrumental record.

We provide an assessment of the supraglacial water budget of a moulin basin on the western margin of the Greenland ice sheet for 15 days in August 2009. Meltwater production, the dominant input term to the 1.14 ± 0.06 km2 basin, was determined from in situ ablation measurements. The dominant water-output terms from the basin, accounting for 52% and 48% of output, respectively, were moulin discharge and drainage into crevasses. Moulin discharge exhibits large diurnal variability (0.017–0.54 m3 s−1) with a distinct late-afternoon peak at 16:45 local time. This lags peak meltwater production by ∼2.8 ± 4.2 hours. An Extreme Ice Survey time-lapse photography sequence complements the observations of moulin discharge. We infer, from in situ observations of moulin geometry, previously published borehole water heights and estimates of the temporal lag between meltwater production and observed local ice surface uplift (‘jacking’), that the transfer of surface meltwater to the englacial water table via moulins is nearly instantaneous (<30 min). We employ a simple crevasse mass-balance model to demonstrate that crevasse drainage could significantly dampen the surface meltwater fluctuations reaching the englacial system in comparison to moulin discharge. Thus, unlike crevasses, moulins propagate meltwater pulses to the englacial system that are capable of overwhelming subglacial transmission capacity, resulting in enhanced basal sliding.

We use repeat-track laser altimeter data from the Ice, Cloud and land Elevation Satellite (ICESat) to map the grounding zone (GZ) of the Filchner–Ronne Ice Shelf, Antarctica. Ice flexure in the GZ occurs as the ice shelf responds to ocean-height changes due primarily to tides. We have identified three ‘ice plains’, regions of low surface slope near the GZ where the ice is close to hydrostatic equilibrium: one on Institute Ice Stream; another to its east; and another west of Foundation Ice Stream. The vertical information from repeated ICESat tracks enables us to study the topography, state of flotation and flexure characteristics across these features. In regions of ephemeral grounding, tidal migration of the grounding line allows us to estimate bed slope (∼1–2 × 10−3). From these studies we develop a classification scheme for ice plains, expressed in terms of the evolution, or ‘life cycle’, of these features. A lightly grounded ice plain progresses to a state of ephemeral grounding as the ice sheet thins near the GZ. Once sufficient thinning has occurred, the ice plain becomes a fully floating, relict ice plain with an undulated surface topography similar to that of lightly grounded ice; we expect viscous relaxation to a smooth ice-shelf surface to occur over a timescale of decades. Our improved insight into ice-plain evolution suggests added complexity in modeling ice in the vicinity of the GZ, and a role for ice-plain observations as a guide to relatively rapid changes in ice-sheet mass balance.