Processes and rates of ice loss at the terminus of Tasman Glacier, New Zealand

Abstract

The Tasman Glacier is the largest glacier in the Southern Alps of New Zealand. Despite a century of climate warming, the glacier has until recently remained at its “Little Ice Age” terminus position, although there has been substantial downwasting. The lower glacier is covered by an extensive debris layer, which has redistributed ice losses in both space and time compared to the conventional glacier response to climate change. Ice is lost at the terminus through melt of bare ice slopes, melt under the debris layer, and calving of ice into a newly established pro-glacial lake. Over the past ten years, the glacier has evolved from an ablation regime of melt under the debris and around sinkholes, to one where calving into the lake is of greater importance. This study investigates the processes and rates of ice loss at the terminus during the summer of 1995. Melt rates of ice are greatest on bare ice slopes, averaging 96 mm day −1 of ice depth over the summer period. Melt under the debris averaged 7 mm day −1, and calving accounted for a specific ice loss of 125 mm day −1. Overall, calving is the dominant form of ice loss at the terminus, accounting for 73% of the total. Melt under the debris layer accounted for 26% of terminus ablation, and melt on bare ice slopes just 1%, due to the relatively small area covered by these. Ice loss at the terminus is therefore largely de-coupled from climatic influences, and due mainly to the effect of the pro-glacial lake. However, when water loss from the whole ablation zone of the glacier is considered, calving accounts for just 4% of ice loss, with the largest portion of ice loss (80%) coming from melt on bare ice upstream of the debris cover. A total amount of 21 million m 3 a −1 of water is estimated to be supplied to Tasman River due to ice loss from the lowest 4 km 2 of glacier. 135 million m 3 a −1 of water is estimated to be lost below Ball Hut (approximately 10 km up-glacier), with 20% of this being calved from the terminus. This proportion will increase as the glacier terminus retreats up the valley, but at present rates, it will take over a century for the glacier to retreat to Ball Hut. Water stored as ice and then released from Tasman Glacier is resulting in a mean annual flow into Lake Pukaki of 4.3 m 3 s −1. The water released from all glaciers in the region due to glacier ice loss is 6% of the annual inflow to Lake Pukaki.