Climate trends and glacier retreat in the Cordillera Blanca, Peru, revisited
Introduction
The tropical Andes – and especially the Cordillera Blanca (CB) – have been recognized as a region highly vulnerable to climate change and the related glacier recession (e.g. Bury et al., 2010, Mark et al., 2010, Deutsch, 2012). Glaciers in this region act as a temporal water storage for precipitation falling as snow at high elevations in the wet season from about October to April. The stored water is partly released during the dry season, compensating for the lack of water due to scarce precipitation events between May and September (Kaser et al., 2003). The discharge from the glaciated catchments is used in the downstream settlements particularly for mining, agriculture, domestic consumption and hydropower (Vuille et al., 2008a). The disappearance of these natural reservoirs has a dominant impact on the water availability in the Rio Santa valley particularly during the dry season (Juen et al., 2007, Baraer et al., 2012). As outlined by Deutsch (2012), rural communities and poor urban neighbourhoods in the Santa watershed, which drains the western part of the CB, face a threat of losing access to clean water, adequate to meet their basic domestic and livelihood needs. It is therefore indispensable to understand the response of glaciers to a changing climate in order to develop and implement related adaptation measures.
This study focuses on climatic trends and related glacier changes in the CB in the Peruvian Andes. Glaciers in the tropical Andes have witnessed a strong retreat during the last decades (e.g. Kaser et al., 1990, Hastenrath and Ames, 1995, Kaser and Georges, 1997, Georges, 2004, Mark and Seltzer, 2005, Silverio and Jaquet, 2005, Raup et al., 2007, Vuille et al., 2008a, Rabatel et al., 2013, Salzmann et al., 2013). Small glaciers in the tropical Andes at low altitudes show a more pronounced retreat, as the current equilibrium line altitude (ELA) climbed up towards the upper reaches causing a reduction or even loss of the accumulation area (Rabatel et al., 2013).
Several studies focusing on climate trends in the tropical Andes and the CB have been published. Based on a large number of stations along the tropical Andes between 1°N and 23°S, Vuille and Bradley (2000) and later Vuille et al. (2008a) observed a significant warming of approximately 0.1 °C per decade between 1939 and 2006. They included station data from the network maintained by SENAMHI, however, they did not analyse temperature and precipitation trends for the region of the CB specifically. For the area of the CB, Mark and Seltzer (2005) reported a temperature increase of 0.39 °C per decade between 1951 and 1999 and 0.26 °C per decade between 1962 and 1999. They used data from the SENAMHI network from 29 and 45 stations for temperature and precipitation respectively, until 1998. They used temperature data to compute a trend for two time periods (1951–1999 and 1962–1999) and did not consider 30-year running trends as in the present work.
Precipitation changes are more difficult to document than temperature trends because of missing station records (Rabatel et al., 2013). In southern Peru and the Bolivian Altiplano, precipitation has decreased in the period 1950 to 1994, while station data indicate a slight increase for northern Peru for the same period (Vuille et al., 2003). Since precipitation is characterised by a large spatial variability, no clear pattern of increasing or decreasing precipitation can be found on a regional scale for the tropical Andes (Vuille et al., 2003). The understanding of local trends in meteorological variables is crucial to examine the glacier retreat in the CB. Therefore, trends of precipitation and air temperature in the CB are identified based on an extensive and unique in-situ data base. It is assessed how these local trends differ from general trends along the tropical Andes as published in e.g. Vuille et al. (2003) or Rabatel et al. (2013). The results are related to existing studies about linear temperature change in the CB such as from e.g. Mark and Seltzer (2005) and it is assessed how running 30-year trends varied in time.
The main objectives of this study can be summarized as follows: (i) Assessing recent trends in precipitation and near-surface as well as 500 hPa air temperature in the CB based on extensive in-situ measurements and reanalysis data with a focus on differences to the general trends in the tropical Andes. Additionally, it is examined how the running 30-year linear trends have changed in time since the 1960s and meteorological variables are compared to the upper-air zonal wind component during the austral summer and the Pacific Decadal Oscillation (PDO). (ii) Applying a novel approach to assess the increase in the freezing line altitude during precipitation days and to estimate the amount of precipitation needed to balance such an increase. (iii) Analysing the relation of precipitation and air temperature trends to observed glacier change using available mass balance measurements.
Section snippets
Study area
The CB is located between approximately 8°S and 10°S in the Ancash Region of Peru (Fig. 1), spanning roughly 180 km in length and 20 km in width. The highest peak in this mountain range is the southern summit of the glaciated Nevado Huascarán with an elevation of 6768 masl. Although the distance to the Pacific Ocean is only about 100 km and more than 4000 km to the Atlantic, this range marks the continental divide. The Río Santa drains the western part of the CB, flows to the northwest into the
Meteorological station data
Station data were provided by the National Meteorological and Hydrological Service of Peru (SENAMHI), which maintains a national network of climate stations. The network consists of over 100 stations in the Ancash and the surrounding regions of which several are located in the CB. Additional daily time series are available from a network of six stations maintained by the Glaciology and Water Resources Unit (UGRH) of the National Water Authority (ANA) in Huaraz. The latter time series are
Quality check and homogenisation of climate data for trend analyses
In order to characterise spatial patterns of running 30-year temperature and precipitation trends, we analysed a large data set from stations along the Ancash coast and the mountainous region of the CB. Some of the stations from SENAMHI are operating since the early 1960s, but due to political and economic reasons, observations have been frequently interrupted or even shut down at times. This is why most records have gaps of different duration and some do not operate all the way to the present.
Temperature and precipitation changes
Temperature changes are analysed by calculating 30-year running mean changes for maximum, minimum and mean air temperature (Fig. 4). Significant trends are highlighted in Fig. 4 according to the Mann–Kendall test at the 0.05 level based on annual mean air temperature. Our results show that there is a notable difference between air temperature trends for the CB and the coast. In the Cordillera region, the running 30-year mean annual air temperature increase has slowed down during the recent
Climatic trend
Temperature records from numerous stations in the CB show a reduced warming in the last 30 years as compared to earlier decades. The trends computed for the 30-year period before 1999 are consistent with the results by Mark and Seltzer (2005). They observed a slightly reduced warming for 29 stations in the CB in an analysis of temperature trends in the period 1962 to 1999 as compared to the earlier period 1951 to 1999, which agrees with the reduced warming until 2012 observed here. Despite the
Conclusions
Here we presented air temperature and precipitation trends since the 1960s based on reanalysis and station data of a relatively dense station network in the region of the CB. The main aim was to identify changes in temperature and precipitation patterns and to relate these changes to the glacier retreat during the last 30 years until 2012. We summarize as follows:
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Air temperature trends are characterized by large regional differences. A slowdown in temperature increase was identified for the CB,
Acknowledgments
This research was developed in the framework of Proyecto Glaciares, a programme in collaboration with CARE Peru financed by the Swiss Agency for Development and Cooperation SDC. We acknowledge also the use of data from the SENAMHI and the UGRH. NCEP/NCAR reanalysis data are provided by the NOAA/OAR/ESRL PSD Boulder, Colorado, USA. ERA-40 and ERA-interim data are obtained from the ECMWF. GLIMS data are provided by the National Snow and Ice Data Center and the ASTER DEM is obtained through the
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