This publication represents the most recent research output of an ongoing international collaboration between researchers from New Zealand, USA and Norway, which has been using the records of past glacier fluctuations in the Southern Alps to understand climate change from the Last Glacial Maximum to present (c.30ka-present). The first step of this project was to produce detailed maps of the local glacial geomorphology in order to document the past size and extent of glaciers in the Southern Alps (e.g. Barrell et al. 2011). The second step was to date these landforms in order to correlate deposits between glacier catchments and also to be able to compare results to other well-dated climate records of similar age (e.g. Schaefer et al., 2006; 2009; Kaplan et al., 2010; Putnam et al., 2010). This paper by Golledge et al. represents the third phase of this project, which utilises computer models to numerically simulate the past glaciers, from which, quantified estimates of past climate can be derived.
To do this, Golledge et al. use a complex numerical glacier model (called ‘The Parallel Ice Sheet Model’ or ‘PISM’, for short), which has been jointly developed by researchers at the University of Alaska, Fairbanks and Potsdam Institute for Climate Impact Research. This model uses physical equations that mathematically describe the different components of ice flow. Golledge et al. perform a series of experiments, whereby they manipulate the different glaciological parameters of the model (e.g. ice deformation and flow, internal and basal stresses) against the geomorphological evidence, in order to determine the most appropriate values. Once this is completed, the climatic variables (temperature and precipitation) are manipulated and compared to the geomorphological evidence, in order to provide an envelope of the likely climate change that caused glacier advance to the mapped and dated limits.
Figure 1: Modelled ice extent using temperature depression of 6degC (top) and 9degC (bottom) from the present day (Source: Golledge et al., 2012).
Figure 1 shows that the initial, low-resolution (2km grid square) model runs showed that a temperature decline of 6-7 degrees produced the best match between modelled ice extent (grey) and ice extent identified through geomorphological mapping (black line). The mis-matches at the margins of the various outlet glaciers are likely to be due to the coarse resolution of the model, where the outlet valleys are narrower than the resolution of the model run (2km). Narrowing down the likely temperature change at the time of this glaciation (e.g. 6-7degrees lower than at present), allowed Golledge et al. to maximise their computational efficiency in simulating the past icefield at much higher spatial resolution (500m). At this scale, they find that the optimum climatic conditions (i.e. those where the modelled ice best fits the geomorphological evidence) are a 6 – 6.5degC cooling, accompanied by a precipitation change of 0-25%, relative to the present day.
Comparison of these values to those derived from other palaeoclimatic proxies allows Golledge et al. to make inferences about the past climatic regime of the region. The temperature change compares favourably with those from other proxies. For example, temperature estimates from bacteria accumulations in LGM lake deposits suggested LGM cooling of 5.6degC in the Southern Alps (Zink et al., 2010). Meanwhile, regional numerical climate models suggest a cooling of 7.5degC (Drost et al., 2007). In comparing their results to past sea surface temperatures Golledge et al. find that their results are more aligned with those from sub-Antarctic influenced water masses (6.8degC) than sub-tropical (4deg C). They suggest that the local climate at this time was therefore still significantly influenced by southerly winds. Regarding past precipitation, Golledge et al.’s finding of potentially drier conditions at the LGM in New Zealand are consistent with other studies (e.g. Whittaker et al., 2011) that have suggested this was caused by a reduction in strength of the prevailing westerly winds across New Zealand, during the LGM.
In some ways this paper represents the culmination of many years work, from the initial mapping stages, through the development of high-resolution chronologies and now to a palaeoclimatic context. The significance of the terrific geomorphological records of past glaciations in New Zealand’s Southern Alps has long been recognised – as evident in the numerous and influential studies (e.g. Porter, 1975). Understanding regional climatic response to global events such as the Last Glacial Maximum allows us to better understand processes driving past climatic change and New Zealand represent one of the few landmasses in the Southern Hemisphere where such records exist. Future work will focus on improving the chronological constraint of glacier fluctuations in the region and using high-resolution, catchment-scale glacier models to better understand the responses of different glacier types to climatic forcing.
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