Changes in the morphology, mass balance, and dynamics of Brown Glacier, Heard Island, with comparison to the surrounding sub-Antarctic islands

Heard Island is located in one of the more isolated regions of the southern Indian Ocean at 53¬¨‚àû06' S 73¬¨‚àû30 E. There are 29 glacerised basins on Heard Island, which cover 70% of this 367 km2 island. Changes in the positions of Heard Island's glaciers, and by inference, changes in the region's climate, have been recorded only intermittently since the establishment of the first Australian base in 1947. In the 1950s the majority of these glaciers were stable or slightly thinning until a marked retreat began in the early 1960s. The retreat from the 1960s has continued through the last recorded observation in 2004. As of 1997, the glaciers on Heard Island had a total area of 257 km2 and an estimated volume of 14.2 km3. The limited climate records for the southern Indian Ocean have shown that there has been a +0.9¬¨‚àû C (or +1.7¬¨‚àû C 100a-1) change in temperature on Heard Island between 1947 and 2006, with similar increases in temperatures on the other sub-Antarctic islands (e. g., +2.7¬¨‚àû C 100a-1 at Marion, +1.0¬¨‚àû C 100a-1 at Kerguelen and +1.4¬¨‚àû C 100a-1 at New Amsterdam islands). Additionally there has been a decrease in precipitation at Marion Island and Kerguelen since the late 1960s. The changes in air temperature and precipitation in the southern Indian Ocean over the last 60 years has had a significant effect of the glacier coverage on the region's islands. There have been dramatic changes in the extent of several glaciers observed on Marion, Crozet, Bouvetoya, and Kerguelen. The glaciers and semi-permanent snow cover on the more northern Marion and Crozet islands have disappeared completely since these islands were first discovered. Kerguelen Island glaciers have decreased in extent, with one of the original five regions disappearing completely. The limited observations on Bouvetoya indicates there has been only minor changes in it's glacier fronts. There are few observations of glacier front changes and even fewer glacier mass balance measurements on Heard Island. The changes in glacier front and climate for Brown Glacier, located on the northeastern coast of Heard Island, was determined by combining the climatic and physical characteristics that are measured on Heard Island, from 1950 to present. Ice core and crevasse samples were collected on Brown Glacier, on the northeast coast, in 2004 to determine the net balance from stratigraphic data and glaciochemical analysis. Oxygen isotope and trace ion were measured from ice cores and crevasses to observe any seasonal signals present in the ice and thereby determining the net balance. Although meltwater affected some of the sample sites a comparison with a low melt location, at 2450 m asl, reveals that melt effects are very site specific. Two non-melt affected sites, at elevations of 756 m and 920 m, provide an estimated net balance of 1.5 m w.e. Ablation was estimated from a combination of stake networks, downward looking sonars and a degree day model. A twenty stake central flowline network and downward looking sonars were deployed on Brown Glacier during the summers of 2000/01 and 2003/04 to measure the changes in the surface height. These measurements were compared to a degree day model for the ablation over the same period. The results indicated that in remote areas, degree day models can provide good estimates of the ablation when limited energy balance variables are available. The mass balance models for Brown Glacier indicate that an increase in temperature of 0.9¬¨‚àû C between 1950 and 2001, and possibly a decrease in precipitation, has resulted in the retreat of the glacier. The steady state mass flux for 2001 indicates that if all of the current parameters remained constant then the terminus of Brown Glacier will retreat to an elevation of 350m. IPCC projections for 2090 were used to predict the changes that would occur on Brown Glacier if there was a further increase of temperature of 1.8 and 3.4¬¨‚àû C. In both of these models Brown Glacier would cease to exist or possibly retreat to a small, semi-permanent snow field. This has similar implications for the other glaciers on Heard Island. Further increases in temperature and a continuation of the decreasing precipitation trends observed on the neighbouring sub-Antarctic islands implies that the smaller glaciers on Heard Island would also begin to retreat at a more rapid rate over the next 90 years. Glacier such as the Mary Powell, Nares, and Deacock that do not originate from high on the Big Ben Plateau are likely to retreat to elevation above ‚ÄövÑvÆ1000 m asl and the glaciers on the Laurens Peninsula will have disappeared completely.