Influence of lake calving and debris cover on glacier runoff changes in the Southern Alps, New Zealand

Abstract:

Glacier retreat and climate change influences runoff patterns and water resources on a regional scale, and sea-level on a global scale. To assess hydrological changes in glacial systems that include debris-covered and lake-calving glaciers, we couple a energy balance/ice flow model to a semi-distributed hydrological model TopNet. To assess the overall ice volume loss since the late 1800s, and estimate future glacier volume loss given various climate change scenarios, we generate an equilibrium state at the late 1800s position by imposing a cooling of 0.8 K from the 1980-2010 mean temperature. A 'dynamic calibration', is carried out where the temperature forcing is progressively warmed from late-1800s to present-day values. During the 100 years between the '1990s' (1980-2000) and the '2090s' (2080-2100) warming in the central Southern Alps, based on the A1B, A2 and A1FI emissions scenarios and a downscaled 12-model GCM ensemble average, is estimated to be between 2.0 and 2.8 K, with a concurrent increase in precipitation of 10-15%. In the central part of the Southern Alps, ice volume decreased from 54 km³ in the late 1800s to 38 km³ by 1980. Projected warming and precipitation increases result in a modelled ice volume of between 11 km³ (A1FI) and 15 km³ (A1B) by 2100, a loss of 60-71%. Loss of 24% of year 2000 glacier volume is already comitted under present-day climate, largely due to the imbalance of slow-response glaciers and lake-calving dynamics. The combination of debris-covered glacier tongues which persist under a warming climate for decades, and a projected increase in precipitation, means there is an increase in mean annual river flow of 12-15% in the Hooker catchment by the 2040s and 25-34% by the 2090s, and a large increase in winter flows. High rainfall rates mean that overall glacier volume loss does not make a large relative contribution to river flow, except during phases of rapid calving retreat.