Annual ice volume changes 1976-2008 for the New Zealand Southern Alps

摘要

New Zealand has a long, continuous record of annual end-of-summer-snowline measurements for a set of Southern Alps 'index glaciers' from 1977 to present. These index glaciers are used to estimate annual mass balance and volume water equivalent changes for the over 3000 glaciers on the Southern Alps. Two methods are employed to monitor ice volume changes. Method I deals with the rapid to normal response time glaciers, which tend to be small to medium in size. It uses mass balance gradients and glacier areas to convert changes in snowlines to changes in ice volume water equivalent. Ice volume changes for the period 1976-2008 are calculated for each index glacier, and then extrapolated to most other glaciers of the Southern Alps, using the New Zealand glacier inventory. Method II deals with 12 protracted response glaciers, which tend to be large in size. These have been slow in reacting to a long-term regional warming trend. Instead they still largely retain the ablation areas of a century ago and are in a state of disequilibrium with the present climate. These valley glaciers have recently sustained substantial ice losses that are not able to be detected using Method I. Mass balance deficits and ablation from the 12 large protracted response glaciers are estimated using a geodetic approach based on topographic and lake changes determined from repeated surveys. Results show that estimated ice volume (in water equivalents) for the Southern Alps has decreased from 54.5 km3 in 1976 to 46.1 km3 by 2008. This equates to a rate of - 0.3 km~3a~(-1) over the last three decades, but this is considerably less than the rate of ice volume loss estimated for the previous 100 years. More than 3000 small and medium-size glaciers account for just 29% of the overall ice volume loss from the Southern Alps, while 71% of the loss occurs from the 12 large protracted response glaciers. Terminus calving contributes 0.8 km3 and down-wasting of ice tongues in the ablation zone contributes 5.2 km~3. Some preliminary results show that ice volume changes are related to changes in circulation over the New Zealand region.