To obtain fundamental quantitative knowledge of the relationship between microwave signatures and the microwave properties of the snow and ice cover experiments are being conducted on naturally grown saline ice and naturally occurring snow. We need to understand the characteristics of microwave emission, surface and volume scatter and reflection, incidence angle, azimuth and polarization as they are affected by saline ice with different growth and deformation histories and metamorphism of the snow cover. Experiments are being conducted on the CRREL pond with microwave radiometers under a variety of conditions that can be reasonably well controlled. Recent results have shown statistically significant differences in microwave signatures at 10 GHz, vertical polarization, for the same ice sheet measured on different days. This suggests that short-term aging processes significantly affect the microwave signatures. Measurements at different incidence angles also show significantly different results, suggesting a scale of homogeneity related to the area of ice lying within the instrument footprint.

The Arctic, where calving glaciers and melting sea ice are part of a larger warming trend.Sea-level rise affects coastal environments, economies, and societies. Effectively planning for and respond...

We present subannual observations (2009-2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered a few decades ago by increased ocean-induced thinning, which may have initiated marine ice sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by<4%, with flow recovering as the ocean warmed to prior temperatures. During this cold-ocean period, the evolving glacier-bed/ice shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin-wide thinning signal did not change. Thus, as predicted by theory, once marine ice sheet instability is underway, a single transient high-amplitude ocean cooling has only a relatively minor effect on ice flow. The long-term effects of ocean temperature variability on ice flow, however, are not yet known.

Nature 541, 77–80 (2017); doi:10.1038/nature20136 In this Letter, on page 78, the sentence beginning “At this ‘null zone’20” should have cited ref. 18, instead of ref. 20. The original Letter has been corrected online.

Landsat 8, a NASA and USGS collaboration, acquires global moderate-resolution measurements of the Earth's terrestrial and polar regions in the visible, near-infrared, short wave, and thermal infrared. Landsat 8 extends the remarkable 40 year Landsat record and has enhanced capabilities including new spectral bands in the blue and cirrus cloud-detection portion of the spectrum, two thermal bands, improved sensor signal-to-noise performance and associated improvements in radiometric resolution, and an improved duty cycle that allows collection of a significantly greater number of images per day. This paper introduces the current (2012–2017) Landsat Science Team's efforts to establish an initial understanding of Landsat 8 capabilities and the steps ahead in support of priorities identified by the team. Preliminary evaluation of Landsat 8 capabilities and identification of new science and applications opportunities are described with respect to calibration and radiometric characterization; surface reflectance; surface albedo; surface temperature, evapotranspiration and drought; agriculture; land cover, condition, disturbance and change; fresh and coastal water; and snow and ice. Insights into the development of derived ‘higher-level’ Landsat products are provided in recognition of the growing need for consistently processed, moderate spatial resolution, large area, long-term terrestrial data records for resource management and for climate and global change studies. The paper concludes with future prospects, emphasizing the opportunities for land imaging constellations by combining Landsat data with data collected from other international sensing systems, and consideration of successor Landsat mission requirements.

The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02–0.14 m; 90% range: 0.0–0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04–0.21 m; 90% range: 0.01–0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these values increase to 0.09 m (66% range: 0.04–0.17 m; 90% range: 0.02–0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07–0.28 m; 90% range: 0.04–0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets.

Ice shelves play a key role in the mass balance of the Antarctic ice sheets by buttressing their seaward-flowing outlet glaciers; however, they are exposed to the underlying ocean and may weaken if ocean thermal forcing increases. An expedition to the ice shelf of the remote Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet that has rapidly thinned and accelerated in recent decades, has been completed. Observations from geophysical surveys and long-term oceanographic instruments deployed down bore holes into the ocean cavity reveal a buoyancy-driven boundary layer within a basal channel that melts the channel apex by 0.06 meter per day, with near-zero melt rates along the flanks of the channel. A complex pattern of such channels is visible throughout the Pine Island Glacier shelf.

Thwaites Glacier, West Antarctica, has the potential to directly contribute approximate to 1m to sea level and currently is losing mass and thinning rapidly. Here, we report on regional results for the Sea-level Response to Ice Sheet Evolution (SeaRISE) experiments and investigate the impact of i) spatial resolution within existing data sets, ii) grounding-zone processes, and iii) till rheology on the dynamics of this outlet glacier. In addition to the SeaRISE data sets, we use detailed aerogeophysical and satellite data from Thwaites Glacier as input to a coupled ice stream/ice-shelf/ocean-plume model that includes oceanic influences across a several kilometers wide grounding zone suggested by new, high-resolution data. Our results indicate that the ice tongue provides limited stability, and that while future atmospheric warming will likely add mass to the surface of the glacier, strong ice stream stabilization on bedrock highs narrower than the length of the grounding zone may be ephemeral if circulating waters substantially reduce basal resistance and enhance melting beneath grounded ice within this zone. However, we find that stability is significantly enhanced by effectively plastic till beds. Accurate projections of future sea level change relies on correct understanding of the till rheology as well as local basal processes near the grounding line.

Ten ice-sheet models are used to study sensitivity of the Greenland and Antarctic ice sheets to prescribed changes of surface mass balance, sub-ice-shelf melting and basal sliding. Results exhibit a large range in projected contributions to sea-level change. In most cases, the ice volume above flotation lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments, suggesting that nonlinear feedbacks are modest. Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is more sensitive to increased ice-shelf basal melting. An experiment approximating the Intergovernmental Panel on Climate Change's RCP8.5 scenario produces additional first-century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm. Linear interpolation of the sensitivity results closely approximates these projections, revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice-sheet response to complicated forcings over 200 years can be linearized.

Sophie Nowicki, Robert A. Bindschadler, Ayako Abe-Ouchi, Andy Aschwanden, Ed Bueler, Hyeungu Choi, Jim Fastook, Glen Granzow, Ralf Greve, Gail Gutowski, Ute Herzfeld, Charles Jackson, Jesse Johnson, Constantine Khroulev, Eric Larour, Anders Levermann, William H. Lipscomb, Maria A. Martin, Mathieu Morlighem, Byron R. Parizek, David Pollard, Stephen F. Price, Diandong Ren, Eric Rignot, Fuyuki Saito, Tatsuru Sato, Hakime Seddik, Helene Seroussi, Kunio Takahashi, Ryan Walker, and Wei Li Wang

This chapter contains sections titled: Introduction Equilibrium Line and Glacier Facies Glacier Velocity Glacier and Ice Sheet Topography Mapping Glaciers, Glacier Change, and Characteristics Surface Features and Snow Pack Characteristics Conclusions and Outlook

The Sea‐level Response to Ice Sheet Evolution (SeaRISE) effort explores the sensitivity of the current generation of ice sheet models to external forcing to gain insight into the potential future contribution to sea level from the Greenland and Antarctic ice sheets. All participating models simulated the ice sheet response to three types of external forcings: a change in oceanic condition, a warmer atmospheric environment, and enhanced basal lubrication. Here an analysis of the spatial response of the Greenland ice sheet is presented, and the impact of model physics and spin‐up on the projections is explored. Although the modeled responses are not always homogeneous, consistent spatial trends emerge from the ensemble analysis, indicating distinct vulnerabilities of the Greenland ice sheet. There are clear response patterns associated with each forcing, and a similar mass loss at the full ice sheet scale will result in different mass losses at the regional scale, as well as distinct thickness changes over the ice sheet. All forcings lead to an increased mass loss for the coming centuries, with increased basal lubrication and warmer ocean conditions affecting mainly outlet glaciers, while the impacts of atmospheric forcings affect the whole ice sheet.

Gradual reduction of a small ice shelf in the Pine Island Bay area is measured using eleven Landsat images spanning 1972 to 2003. Measurements of Ice shelf area indicate that it expanded slightly during the first two decades of observations from approximately 6.19 km 2 measured on December 7, 1972 to a maximum of about 6.82 km 2 observed in 1986. This maximum was followed by a nearly continuous decrease in area and ultimate disappearance of the ice shelf by January 17, 2003. No ice shelf has reappeared since 2003 as observed in subsequent Landsat images. Ten of the eleven Landsat images were co-registered and warped to one of a pair of 2003 geographic reference images before area measurement. Individual study team members made independent measurements of the ice shelf area apparent in each image. The average of these measurements had a standard deviation of 0.14 km 2 . The specific cause of this ice shelf disappearance is unknown, but is probably related to increased basal melting by warmer ocean waters reaching Pine Island Bay. Intrusions of warm “Circumpolar Deep Water” are related to ice shelf and outlet glacier thinning and retreat as reported throughout the Amundsen Sea region. This is the first report of complete ice shelf loss so far south or in the Amundsen Bay region. This small, previously unnamed ice shelf formerly occupied what is now known as the Elizabeth City State University Bay.

This chapter contains sections titled: Introduction Seeing More Clearly Some Arguments on the Possibility of West Antarctic Ice-Sheet Collapse Collapse Mechanisms Summary