Effect of Ocean Warming on West Antarctic Ice Streams and Ice Shelves By Bryan Riel GEO 387H Physical Climatology Dr. Zong-Liang Yang November 18, 20082 Abstract The Intergovernmental Panel on Climate Change (IPCC) has deemed the Western Antarctic Ice Sheet (WAIS) as very likely to have contributed to sea level rice over the past two decades, estimating a loss of 44 ±13 Gt/yr. Focus has shifted to the behavior and dynamics of the WAIS ice streams since ~90 % of the outward ice flux flows through the ice streams. The behavior and movement of the ice streams is directly coupled with the condition of the ice shelves they flow onto. Recent satellite radar altimetry observations have shown that the three main ice shelves in the Amundsen Sea have simultaneously decreased in elevation, pointing to warming oceans as the common cause. Warming oceans have increased basal melting at the bottom of the ice shelves, causing them to thin. Models have shown that the increased basal melting can produce perturbations at the grounding line that will cause upstream ice to thin at distances ~200 km from the grounding line. This additional coupling of the ice stream to inland ice has signified a potentially dangerous instability of the WAIS that could cause it to eventually enter a rapid “collapse” phase where it would contribute 60-120 cm of sea level rise for 5-7 centuries. In terms of anthropogenic forcing, increased precipitation accompanying rising global temperatures may have decreased the salinity of the surrounding oceans, decoupling the warm ocean surface with the colder deep waters. The result is the possible warming of the surface and subsurface waters, leading to the basal melting of the ice shelves. So far, computer models have not been able to fully implement all of the dynamic forces involved with the flow of the ice streams, so the stability issue of the WAIS is not fully resolved. Nevertheless, the combination of observations of ice shelf thinning and model predictions of inland thinning demand that more attention should be spent to modeling ice streams and monitoring ocean conditions.3 Introduction With the advent of global warming, the behavior and response of the cryosphere to the increasing global temperature has become an issue of great interest. Of particular importance is the contribution of the melting ice sheets at Greenland and Antarctica to global sea level rise. The IPCC (Intergovernmental Panel on Climate Change) 2007 report states that those ice sheets “very likely” have been causing a rise of sea level over the time span of 1993 to 2003. Quantitatively, the IPCC estimates a mass balance of -50 to -100 Gt/yr for Greenland and +50 to -200 Gt/yr for the entire Antarctic Ice Sheet for the years 1993 to 2003 (IPCC, 2007). This report will focus on the current mass balance and dynamics of the West Antarctic Ice Sheet (WAIS) since there is a general consensus that it poses the most immediate threat to sea level rise due to its potential instability (Oppenheimer, 1998). It has been calculated that if the entire ice sheet were to melt, the global mean sea level would rise by approximately 4-6 meters, which would prove to be devastating to coastal areas, cities, and habitats (Oppenheimer, 1998). Even though such a complete release of ice is very unlikely, the potential danger posed by the ice sheet in conjunction with rising global temperatures has drawn an increased amount of interest. There are roughly two questions of main concern on the WAIS: 1) What are the forcing functions causing melting and are they a result of human actions, and 2) What are the dynamics and response of the ice sheet to the forcing functions and how will the subsequent response affect sea level rise. This report will evaluate several current theories and models as well as examine the current known facts about the behavior of the WAIS. General Mass Balance of WAIS Overall, the mass balance of an ice sheet is a balance between the amount of snow accumulation (positive mass balance) versus surface melting, runoff, ice discharge, etc. (negative4 mass balance). To measure these quantities, a wide variety of measurement methods have been developed. In situ measurements are direct measurements of quantities such as ice velocity, surface characteristics, ice sheet depth, et. al. (IPCC, 2007). These measurements are generally fairly accurate yet cannot efficiently cover wide areas of the ice sheet. Remote sensing methods are used to measure mass balance over the entire ice sheet and can be performed by airborne/satellite altimetry, Interferometric Synthetic Aperture Radar (InSAR), gravity field measurements, and other methods. With these methods, the drawback is a higher degree of uncertainty and bias between the measured quantities and the actual quantities. Nevertheless, both in situ and remote sensing measurements have yielded very useful information about ice sheet mass balances. Figure 1 displays the cross-section of an ice sheet with its different components. The terminology in the picture will be used throughout the report. Figure 1. Cross-section of an Ice Sheet. (Oppenheimer, 1998)5 The IPCC 2007 report estimates a mass growth for East Antarctica of 20 ± 21 Gt/yr and a mass loss for West Antarctica of 44 ±13 Gt/yr with the balance of the Antarctic Peninsula not assessed (IPCC, 2007). These figures were obtained as an average of the estimates obtained through different measurement methods. Most figures tend to agree upon a growth in East Antarctica and shrinkage in West Antarctica. Unlike the Greenland ice sheet, the IPCC estimates that surface melting plays a fairly small role in the overall mass balance of Antarctica, so more effort has been placed on studying the movement of the ice streams and melting of the glaciers and ice shelves (IPCC, 2007). WAIS is classified as a marine ice sheet, meaning that part of the ice sheet is grounded on land below sea level, where the grounded part is then joined with the floating ice shelves at the grounding line. Contrarily, Greenland is mostly grounded above sea level and less effected by ablation due to moderate warming (Oppenheimer, 1998). This fundamental difference is one of the leading reasons why it is believed that the WAIS is potentially unstable and poses the most immediate threat. Overall WAIS Dynamics While the mass balance provides a good overall picture of what is happening at the WAIS, the