Corrigendum: Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier.

This corrects the article DOI: 10.1038/nature20136.

Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier.

The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate, and several numerical models suggest that unstable and irreversible retreat of the grounding line-which marks the boundary between grounded ice and floating ice shelf-is underway. Understanding this recent retreat requires a detailed knowledge of grounding-line history, but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated. In particular, a history of grounding-line retreat is required to understand the relative roles of contemporaneous ocean-forced change and of ongoing glacier response to an earlier perturbation in driving ice-sheet loss. Here we show that the present thinning and retreat of Pine Island Glacier in West Antarctica is part of a climatically forced trend that was triggered in the 1940s. Our conclusions arise from analysis of sediment cores recovered beneath the floating Pine Island Glacier ice shelf, and constrain the date at which the grounding line retreated from a prominent seafloor ridge. We find that incursion of marine water beyond the crest of this ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years); final ungrounding of the ice shelf from the ridge occurred in 1970 (±4 years). The initial opening of this ocean cavity followed a period of strong warming of West Antarctica, associated with El Niño activity. Thus our results suggest that, even when climate forcing weakened, ice-sheet retreat continued.

Channelized ice melting in the ocean boundary layer beneath Pine Island Glacier, Antarctica.

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.

Tributaries of West Antarctic Ice Streams Revealed by RADARSAT Interferometry.

Interferometric RADARSAT data are used to map ice motion in the source areas of four West Antarctic ice streams. The data reveal that tributaries, coincident with subglacial valleys, provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions. Two tributaries flow into the stagnant ice stream C, creating an extensive region that is thickening at an average rate of 0.49 meters per year. This is one of the largest rates of thickening ever reported in Antarctica.

Changes in the west antarctic ice sheet since 1963 from declassified satellite photography

Comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the Ross Ice Shelf. Ice stream B has widened by 4 kilometers, at a rate much faster than suggested by models, and has decreased in speed by 50 percent. The ice ridge between ice streams B and C has eroded 14 kilometers. These changes, along with changes in the crevassing around Crary Ice Rise, imply that this region's velocity field shifted during this century.

Greenland Ice Sheet Surface Properties and Ice Dynamics from ERS-1 SAR Imagery.

C-band synthetic aperture radar (SAR) imagery from the European Space Agency's ERS-1 satellite reveals the basic zonation of the surface of the Greenland Ice Sheet. The zones have backscatter signatures related to the structure of the snowpack, which varies with the balance of accumulation and melt at various elevations. The boundaries of zones can be accurately located with the use of this high-resolution imagery. The images also reveal a large flow feature in northeast Greenland that is similar to ice streams in Antarctica and may play a major role in the discharge of ice from the ice sheet.

Satellite-image-derived velocity field of an antarctic ice stream.

The surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery. Variations of ice velocity are spatially related to surface undulations, and transverse velocity variations of up to 30 percent occur. Such large variations negate the concept of plug flow and call into question earlier mass-balance calculations for this and other ice streams where sparse velocity data were used. The coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross-correlation of image windows provide significant improvements in the use of satellite imagery for ice-flow determination.

Growth of greenland ice sheet: measurement.

Measurements of ice-sheet elevation change by satellite altimetry show that the Greenland surface elevation south of 72 degrees north latitude is increasing. The vertical velocity of the surface is 0.20 +/- 0.06 meters per year from measured changes in surface elevations at 5906 intersections between Geosat paths in 1985 and Seasat in 1978, and 0.28 +/- 0.02 meters per year from 256,694 intersections of Geosat paths during a 548-day period of 1985 to 1986.