Modelled response of debris-covered and lake-calving glaciers to climate change, Kā Tiritiri o te Moana/Southern Alps, New Zealand

Glaciers will lose mass during the next decades as the climate warms. While the largest uncertainty in this mass loss is the global greenhouse gas emissions pathway, on a local or regional scale there are large uncertainties in some processes that influence glacier response. We explore the competing processes of reduced ablation under debris cover and increased ablation from calving by applying a coupled mass-balance/ice-flow model in the central Southern Alps/Kā Tiritiri o te Moana of New Zealand. Glacier volume loss is assessed over the period from AD 1880 to AD 2099, driven by observed climate data before 2005, and perturbing the modern climate state and imposing representative concentration pathway (RCP) scenarios 2.6–8.5 as expressed by six global circulation models (GCMs) which leads to a regional warming of between 1–4°C (2006–2099). Key findings are (1) modelled ice volume reduced from 47km³ to 29km³ between AD 1880 and 2005; (2) over the period 2006–2099 further volume loss to 24km³ (19% reduction) is committed under present-day climate; (3) modelled ice volume at AD 2099 is estimated at 2km³±6km³ (RCP8.5) to 15km³±6km³ (RCP2.6), a reduction of 50–92% relative to present day. The wide range of projected ice volumes reflects the large range of temperature projections between RCP2.6 and RCP8.5. The mode and timing of ice loss provides insight into processes that will drive future glacier behavior. Under RCP2.6 at 2099, the glaciers retain a similar configuration to present, although clean-ice glaciers will retreat significantly, and some debris-covered tongues will disconnect from their accumulation areas. For RCP4.5, RCP6.0 and RCP8.5 the clean-ice glaciers will retreat to become small remnants in the high mountains. Experiments where the debris cover is removed shows a much faster loss of ice, whereas experiments with no lake calving slows ice loss. However, under all but the most moderate warming scenarios, by 2099 the strong climatic forcing overwhelms these processes as there is little ice left at low elevations where debris cover and lake calving occur.