ReviewChanges of glaciers in the Andes of Chile and priorities for future work
Introduction
Glaciers worldwide are receding at a strong pace, with possible limited exceptions (e.g. Karakoram glaciers). The ongoing retreat has been documented by both local, ground observations e.g. Kaser et al., 2006, Bauder et al., 2007, Huss and Bauder, 2009, Zemp et al., 2009), remote sensing (e.g. Paul et al., 2004, Bolch et al., 2010, Bolch et al., 2011, Kääb et al., 2012) and modelling studies (e.g. Radić and Hock, 2006, Huss et al., 2008, Immerzeel et al., 2012, Immerzeel et al., 2013). Some regions of the Earth however have been documented much more extensively than others, because of well established data gathering efforts and abundance of long-term observations (Switzerland, Scandinavian countries, selected regions of North-America), because of their global relevance (Arctic and Antarctic) or because of growing interest due to their importance for water resources (the Himalaya–Hindu Kush–Karakoram region). In this context, the Andes of Chile have received relatively little attention. Chile's mountain system is made of a heterogeneous succession of cryospheric systems, extending from 18°S to 56°S and spanning a unique variety of climates on the globe, from the desertic North to the very wet Patagonian ice fields. In the middle, the central semi-arid Andes of Chile are one of the most interesting of such systems as they combine relatively large glaciers with densely populated valleys rich in agriculture and feeding large populated centres, including the capital city of Santiago. As their name indicates, the semi-arid Andes feature dry summers with almost zero precipitation and wet, cold winters during which snow accumulates at high elevations. Given its dependence on melt water during summer, when water needs are stronger, and its large population and increasing water demands, the central region is more markedly threatened by changes in climate and the projected warming of the atmosphere. For these reasons, a surge of interest has prompted studies of changes in glaciers and their effect on water resources in the region (Pellicciotti et al., 2008, Ragettli and Pellicciotti, 2012, Ohlanders et al., 2013, Ragettli et al., 2013a). Another focus of current research has been on the Pascua-Lama region of the Norte Chico (29°S, a very dry environment with few small glaciers), where studies supported by mining companies have focused on mass balance monitoring and understanding of glacier–climate interactions at selected locations (Rabatel et al., 2011, MacDonell et al., 2012), together with a more recent attempt to understand processes at the catchment scale (Gascoin et al., 2013). In the scarcely populated South, where precipitation is abundant, the Northern and Southern Patagonian Ice Fields have been investigated mainly through satellite remote sensing studies used to reconstruct ice volume changes (e.g. Rignot et al., 2003, Rivera et al., 2007, Willis et al., 2012).
While on one side the number of investigations has been increasing recently, on the other side studies of glacier changes, and modelling studies in particular, are still limited, hindered by the scarcity of data, and have not yet reached a level such as to provide a country-wide picture of changes. In this paper, we first provide a review of changes in glaciers and related water resources as documented by recent works. We then discuss the types of investigations conducted, and identify a missing component in distributed, continuous mass balance and runoff glacier modelling work. The scarcity of these studies is due to a number of challenges associated with this type of work, which requires grid-based modelling over large domains and accurate knowledge of the climatic forcing over such domains, as well as a representation of processes at the glacier and catchment scale. We identify some of these issues and show through a modelling study how important some of these challenges are and how they could be addressed. We provide in particular four main examples of major scientific challenges. We then conclude this paper with a discussion of possible future work.
Section snippets
Review of glacier changes
Four main regions can be identified in Chile in terms of climate and glacier characteristics: 1) the arid north, from the Chile-Peru border (18°S) to 32°S, including the relatively well studied Norte Chico region (from 26 to 32°S); 2) the central Andes (32 to 38°S), characterised by a semi-arid climate with strong winter precipitation and very few summer precipitation events; 3) the southern Chilean lake district (38–41°S), where many of the glaciers are located on active volcanoes; and 4) the
Distributed mass balance and runoff modelling at the catchment scale
As should be clear from the review of glacier changes above, most of the studies conducted in the Andes of Chile are based on remote sensing approaches used to reconstruct frontal variations (e.g. Rivera et al., 2002, Bown et al., 2008), areal changes (e.g. Rivera et al., 2002, Bown et al., 2008, Nicholson et al., 2009, Rabatel et al., 2011) and ice volumetric changes (e.g. Rignot et al., 2003, Rivera et al., 2007, Willis et al., 2012). From volumetric changes, geodetic mass balances can be
Physically-based versus empirical models
Among glaciologists, two main approaches have been adopted for calculation of melt rates at the glacier–atmosphere interface. On one side, physically-based energy balance (EB) models represent the physics of the energy exchange at the glacier–atmosphere interface using physically-based equations, while temperature index models calculate melt as a function of air temperature alone (e.g. Pellicciotti et al., 2005, Reid and Brock, 2010). A number of intermediate models have been more recently
Importance of correct characterisation of atmosphere–cryosphere–land surface interaction in high elevation catchments
Obviously, inaccurate input meteorological data translate into inaccurate model outputs or, through parameter calibration that leads to internal inconsistencies, to unwanted compensation of internal model errors. This problem has by now been recognised for the input climatic forcing of future simulations driven by general circulation models (GCMs) or regional climate models (RCMs), which can diverge substantially (Prein et al., 2011) and for which increasingly an ensemble of models is deemed
Importance of spatial processes of snow redistribution
Snow in high mountain, glacierised basins is redistributed mainly through two effects: wind and gravitational redistribution. A growing number of recent works have suggested that both processes are important and able to move considerable amounts of mass (MacDonald et al., 2009, Bernhardt and Schulz, 2010, Bernhardt et al., 2010, Gascoin et al., 2013). The effect of wind is complex and difficult to represent in models, since wind induces preferential deposition of precipitation (Lehning et al.,
Future simulations and additional uncertainties
In this section, we consider a fourth and key source of uncertainty, namely the choice of climate forcing scenarios. Several impact studies of glacier changes have been conducted with well calibrated glacio-hydrological models forced only by one climate scenario, be this one GCM or one RCM output (e.g. Machguth et al., 2009, Schaefer et al., 2013). There is growing awareness, however, that this will capture at most one of the many trajectories of possible climate change (Horton et al., 2006,
Concluding remarks
In this paper, we have provided a review of the current status of knowledge about the cryosphere in the Andes of Chile, with emphasis on methods used for these assessments. The most commonly used methods have been either remote sensing studies or detailed physically-based studies at a small number of well equipped and selected locations, which are not representative however of processes at the catchment scale. We have shown that a knowledge gap exists between these two approaches at the glacier
Acknowledgements
The authors would like to thank very much all the people that contributed to the data collection on Juncal Norte Glacier in the two field campaigns of 2005–06 and 2008–09. Thank you to Roger Bordoy for his useful comments and support on GCMs downscaling. Alvaro Ayala helped with the setup of the mass balance model on Juncal Norte Glacier. The authors are grateful to an anonymous reviewer and Graham Cogley for providing thoughtful and useful reviews.
This work was carried out in the framework of
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