Abstract:
The world-wide recession of mountain glaciers during the 20th, Century is one of the most obvious manifestations of climate warming, and has been a major contributor to recent sea level rise. Glaciers respond to short (interannual) and medium-scale (decades to centuries) climate trends, and they can teach us about local, regional and global climate change. Understanding glaciers in New Zealand is of importance for agriculture, power generation and hazards. The glaciers of Mt. Ruapehu are situated on an active volcano at the climatic limit for the formation of permanent ice in this country. Recession of Mt. Ruapehu's glaciers is well known, but our understanding of their behaviour is fairly limited.
I present an end-of-summer snowline study of Mt. Ruapehu glaciers from 1988-2007, direct measurements of mass balance on the Whangaehu Glacier for 2006 and 2007, and a climatic interpretation of glacier mass balance trends over this period. The end-of-summer snowline survey reveals a ~5 year pattern, with high snowlines evident in the late 1980s and again in the mid 1990s to early 2000s. Lower snowlines were recorded during the early to mid 1990s and between 2003 and 2007. The similar number of high (9) and low (9) snowline years indicates that the glacier may be close to balance during this period. Direct measurements on the Whangaehu Glacier showed the mass balance to be slightly negative during 2006, and positive in 2007. Velocity measurements and aerial photographic interpretation indicate that ice flow no longer connects the Summit Plateau ice to the Whangaehu Glacier, and that the summit area is effectively an ablation zone. Interestingly, although no longer dynamically connected, the Summit Plateau remains an important source of wind-drifted snow for the Whangaehu Glacier.
Snowline variations, mass balance records (and snow depth measurements at Whakapapa ski resort) show a strong relationship with local temperature trends but little relationship to precipitation. An exception occurred during 2007 when high precipitation contributed to a positive balance year. Whangaehu Glacier mass balance and snowline trends correlate with the Southern Oscillation, but show little relationship to the Southern Annular Mode or New Zealand (southerly or westerly) atmospheric circulation indices. The sensitivity of Mt. Ruapehu glaciers to temperature trends and the increasing number of positive degree days demonstrated since the early 1990s does not bode well for the future of its glaciers, although the Whangaehu may do better than the north and west-facing glaciers.
