Impacts of Climate Variability on Glaciers in Peru

Most glaciers around the world - including those in the tropics - have been retreating over most of the 20th century. Generally, their shrinkage is attributed to global warming. While the global climate conditions certainly play an important role in shaping the environment in which glaciers exist, the mass and energy balance of each individual glacier is dictated by local conditions. Given the generally complex mountain topography around glaciers, natural climate variability, and regional differences in the climate system`s evolution, it is not trivial to find a direct link between global climate change and the behavior of an individual glacier. In the Cordillera Blanca, Peru, glaciers act as important regulators and providers of water supply to the densely populated Rio Santa valley. Within the framework of the proposed project, steps will be taken towards a better understanding of the processes linking the large scales of climate variability and change to impacts on single glaciers of the Cordillera Blanca. An existing network of automatic weather stations and glaciological observations will be maintained and extended in order to create a complete observational data basis for the project. Using a limited area model, very high resolution simulations of the atmospheric conditions around the study sites will be performed for selected time slices, using reanalysis data as lateral boundary conditions. The skill of the model will be quantified, and the model dynamics will be analysed in order to identify the most important processes that govern the translation of the large to the local scale in the mountainous terrain. Parameterizations of these processes will be developed and verified through the observational data. Finally, a distributed, energy balance-based glacier mass balance model will first be optimized with the help of the field measurements. The mass balance model will then be run over the entire period of reanalysis data availability, using the parameterizations to create time series of the forcing variables. The produced long-term mass balance series will provide a detailed picture of high-altitude climate change in this region. The meteorological and glaciological observations obtained through this project will be immensely valuable also for research outside the scope of this project, since the tropical mid-troposphere, where the study site is located, is characterized through extremely scarce data. The dynamical downscaling of the reanalysis data with a numerical model, verified through on-site observations, will increase the insight into the abilities and limitations of high resolution modeling over glacierized and rough terrain. The development of parameterizations for the variables required to force a glacier mass balance model is to be seen as a first step towards the inclusion of tropical mountain glaciers into general circulation models, and thereby a necessary prerequisite for studying impacts of climate change on glaciers, water resources, and local communities in Peru.

Glaciers in the Cordillera Blanca (Peru) are of a particular socioeconomic and scientific interest. Firstly, the glaciers are important actors in the seasonal water provision of the densely populated Rio Santa valley, and at the same time represent threats because of the occurrence and ongoing potential of deadly ice avalanches and glacier lake outburst floods. Secondly, the glaciers are located in the tropical mid-troposphere, an area historically characterized by extremely sparse meteorological observations. Due to their situation in highly complex topography, it is not trivial to relate climate variability and change as projected (and reconstructed) by global climate models (GCMs) to the behavior of the glaciers. The work of the research project can be summarized in four parts as follows. (1) A glaciological and meteorological measurement network near and on glaciers in the Cordillera Blanca has been set up and serviced. High-resolution data series from several remote sites were obtained and are being made available to the scientific community for free. Despite some unavoidable instrument breakdowns these data represent a valuable basis for atmospheric and glacier mass modelling in the Cordillera Blanca. (2) A dynamic option of inferring the relevant local-scale information from large-scale GCMs has been tested and validated using the measurements available from (1) with promising results, but this option is limited to short simulation periods due to high computational demands. (3) Novel statistical models have been set up and implemented in order to establish links between GCMs and the local weather relevant for the glaciers mass balances. The skill estimation of the statistical models puts particular emphasis on the pitfalls of scarce measurement bases, which are affecting glaciated mountain environments all over the globe. We found that the choice of the GCM is of considerable importance, and that the optimum reanalysis-type GCM for the Cordillera Blanca significantly outperformed so-called first generation reanalysis data sets. (4) A generalized model of the surface mass balance of glaciers was developed and applied to the entire globe. The model is based on novel and objective calibration methods, combined with independent validation procedures. Results show that the model is skillful also at modeling the mass balance of glaciers for which no mass balance observations exist. The impact of glacier melt water runoff on water availability on a global scale, but in particular including the Rio Santa watershed with its sources in the Cordillera Blanca glaciers was estimated.