Ice shelves are thick, floating platforms of ice that extend from a grounded glacier into the sea. They help buttress the flow of grounded tributary glaciers into the ocean and therefore help slow global sea level rise. Eighty percent of grounded ice in Antarctica drains through ice shelves, and thus the thinning of ice shelves could lead to runaway ice sheets and an acceleration of melt patterns. On the Antarctic Peninsula, ice-shelf collapse has already led to a retreat and acceleration of several local glaciers. By mapping the glacier and ice-shelf elevation change, sea temperatures, and ocean trough depths, Pritchard et al. (2012) concluded that in 20 of 54 Antarctic ice shelves, ice-shelf thinning and glacial acceleration were driven by rapid basal melt. In all cases, tributary glaciers were also thinning, and accounted for about 40% of Antarctic discharge and the majority of ice-sheet mass loss. The scientists found that these areas were concentrated in West Antarctica, where changes in sea currents have brought warmer waters to the coast and into deep bathymetric troughs and melted the ice from below.¾Olivia Jacobs
Pritchard, H.D., Ligtenberg, S.R.M., Fricker, H.A., Vaughan, D.G., van den Broeke,
M.R., Padman, L. 2012. Antartic ice-sheet loss driven by basal melting of ice
shelves. Nature 484, 502–505.
Pritchard et al. used satellite and laser altimetry to understand patterns of ice-shelf thinning and basal melt. They used laser altimetry to measure the surface height change in major Antarctic ice shelves between 2003 and 2008, and compared these changes to measured values of sea temperature. Data showed that thinning was strongly regional and was most rapid along the Amundsen and Bellingshausen Sea coasts.
During this time period, thick ice shelves thinned while thinner ones showed no significant pattern of further thinning in the areas near the Amundsen and Bellingshausen Sea coasts in West Antarctica. Firn modeling indicated that firn layers thickened throughout this region because of increased accumulation. Also, patterns of ice-shelf retreat and glacier influx were similar throughout different ice shelves, and so Pritchard et al. concluded that the changes in ice-shelf elevation were driven by something other than local climate. When these changes were mapped with regional sea temperature data, the data showed a strong correlation between thinning ice shelves and warmer water temperatures, indicating that basal melt was driving elevation changes.
The highest melt rate, 40 m yr –1, was near the grounding line of Pine Island Glacier in West Antarctica. In this glacier, the flow rate of grounded ice increased by 43%, and contributed to a global sea level rise of 1.2 mm per decade. As a whole, Pritchard et al. found that the most rapidly thinning ice shelves occurred in areas with high sea-floor temperatures and deep bathymetric troughs that spanned continental shelves in West Antarctica and ice shelves that did not show significant levels of thinning in West Antarctica were removed from these areas.
The warmer temperatures observed by Pritchard et al. were driven by fluctuating incursions of Circumpolar Deep Water (CDW). This change in ocean current is wind driven, saline, relatively warm, and dense, and, as this study showed, causes basal melting in ice shelves. The fluctuating CDW is channeled at depths below 300 m, and thus areas where bathymetric troughs approach the continental shelf are particularly susceptible to fluctuations in CDW. Many major glaciers that reside above these troughs are already in retreated positions and displayed greater rates of thinning.
Wind forcing patterns are not yet well understood, but it is clear that they have helped drive CDW shifts and are influenced by tropical Pacific sea surface temperature changes, ozone loss, and increased greenhouse gases. These shifting wind patterns also influence atmospheric temperatures, which drive surface ice loss in other ice shelves that are removed from the CDW on the Antarctic Peninsula. Thus, atmospheric changes influence melting above and below Antarctica’s ice shelves. Pritchard et al. conclude that these changing oceanic conditions have already driven profound changes in the ice sheets, and may have already triggered unstable glacial retreat and subsequently a substantial increase in global sea levels.