Coincident glacier and lake evolution across New Zealand: Past, present, and future

Mountain glaciers are rapidly diminishing and causing widespread environmental and socio-economic concern. The stability of mountain glaciers is influenced by the expansion of proglacial landscapes and meltwater impounded as lakes within natural topographic depressions or ‘overdeepenings’. In particular, the relative sensitivity of mid-latitude glaciers to modern climate change makes them especially important to consider. One of the most striking features of South Island, New Zealand, is the sequence of glacial lakes that occupy mountain valleys along the Southern Alps. Our previous work has highlighted that the presence of these lakes is likely to have had an impact on ice-marginal dynamics of their adjacent glaciers, thereby influencing the rate of deglaciation on sub-millennial timescales. This emphasizes the need to incorporate proglacial lakes into palaeoglacier reconstructions and into analyses of future glacier evolution. In this new study we (i) document contemporary loss of glacier ice across the Southern Alps, (ii) analyse ice-marginal lake development since the 1980s, (iii) utilise modelled glacier ice thickness to suggest the position and size of future lakes, and (iv) employ a large-scale glacier evolution model to suggest the timing of future lake formation and future lake expansion rate. In recent decades, Southern Alps glaciers have fragmented both by separation of tributaries and by detachment of ablation zones. Glacier margins in contact with a proglacial lake have experienced the greatest terminus retreat. Our analysis indicates a positive relationship between mean glacier mass balance and rate of lake growth and with length of an ice-contact lake boundary. We project sustained and relatively homogenous glacier volume loss for east-draining basins but in contrast a heterogenous pattern of volume loss for west-draining basins. Our model results show that ice-marginal lakes will increase in number and combined size towards 2050 and then decrease to 2100 as glaciers disconnect from them. Overall, our findings should inform (i) glacier evolution models into which ice-marginal lake effects need incorporating, (ii) studies of rapid landscape evolution and especially of meltwater and sediment delivery, and (iii) considerations of future meltwater supply and water quality.