A year/long ice camp centered around a Canadian icebreaker frozen in the arctic ice pack successfully collected a wealth of atmospheric, oceanographic, and cryospheric data.

Research involving a yearlong drift with the ice pack in the Arctic Ocean witnessed surprisingly thin ice at the start and even thinner ice at the end. Also, the extent of open water during the summer of 1998 in the Beaufort and Chukchi Seas was the greatest of the past 2 decades. As the ice is melting from under your feet there is an understandable tendency to blame global warming. But the project, known as the Surface Heat Budget of the Arctic Ocean (SHEBA), though motivated by climate change, was not designed to detect global warming. Definitive climate change pronouncements can not be made based on a single experiment.

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.

Active Ice How, exactly, does warm ocean water erode an ice shelf? In a field study of an ice shelf at Pine Island, Antarctica, Stanton et al. (p. 1236 ) collected data from radar, seismic surveys, and oceanographic sensors inserted in holes bored through the ice shelf. The results show that localized, intensive melting occurs in a complex network of discreet channels that are formed on the underside of the shelf. This pattern of melting may limit the absolute rate of ice-shelf mass loss.

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