It was not long ago, in 1916, that the explorer Padre De Agostini surveyed part of the topography of the Escondidos glaciers (“hidden glaciers”, in English) in Patagonia (De Agostini, 1949). Today, space technology such as NASA’s ICESat Missions and Shuttle Radar Topography Mission (SRTM) data, allow to monitor changes in glaciers over time.
The Escondidos glaciers are located in the Southern Patagonian Ice Field (Fig. 1), a place where the climate becomes indomitable and the fog conspires with the snow to hide everything. Even today, there are parts of unexplored territory.
The Escondidos are formed by three mountain glaciers known as Frías, Gorra and Dickson. These glaciers have been contributing melting water to Argentino Lake, the biggest lake in the country and a supply of fresh water twice the size of New York City. It provides drinking water to the entire Santa Cruz river basin.
Every glacier has a high mass gain sector, where it receives the contribution of snow or upper glacial flows, called the accumulation sector, and a lower sector called ablation, where it loses mass (Chinni, 2004). A moraine is any glacially formed accumulation of unconsolidated glacial debris that occurs in both, current and formerly glaciated regions (Bertone, 1972).
A short history of the Frias glacier
Because its ablation area is covered in sediment and moraines, Frías glacier has undergone quite a different behaviour during the 20th century compared to other glaciers of the national park. In 1945, its front was not separated from its moraines (Ortone, 2020). However, between 1945 and 1986, the lower part of the glacier-covered with debris was progressively transformed into an ice-based moraine surrounded by large stones and small deposits. Between 1986 and 1995, two proglacial lakes appeared on the eastern front of the glacier in areas previously occupied by debris and moraines. These lakes have been growing since 1995 due to the glacier retreat. In 1998, field observations allowed to detect many watercourses that flowed on the surface of the sediment area, as well as sinks and small lagoons that are typical indicators of great fusion activity (Martinic, 2010). Figures 3 and 4 show superficial and elevation variations between the acquisition dates.
Glacier retreat and changes in meltwater flow direction
An interesting aspect is that the continental watershed divide between the Atlantic and Pacific oceans is currently located within these glaciers (Rivera, 2004). In fact, the Escondidos glaciers originally fed on a single ice stream. The meltwater travelled more than 250 km to cross the arid plain of Patagonia, to finally end at the Atlantic Ocean. These glaciers were a natural dam. They used to form a colossal wall with the neighbouring coast, as well as another famous Patagonian, the Perito Moreno glacier.
From the late 20th century, glacial retreat intensified considerably, and large amounts of ice melted. The waters found an escape route through Dickson's contact area with the rocky shore. This flow, in turn, accelerated the glacial melt and flow rate, which resulted in the vanishing of a deposit formed in the northern zone between Dickson and Frías glaciers (Martinic, 2010). Due to this melting, the water that used run to one ocean now flows into the other. The front ice mass that supported the water flow, surrendered to the thaw. However, the role of the front ice mass here was even more significant. The water that now flows to the west feeds Paine Lake, generates a continuous freshwater flow of 5000 million litres per year. After a long journey, it mixes with the salty fjords of the Pacific Ocean. This also implies that the Argentino Lake no longer receives contributions from this group of glaciers.
The record of an aerial image over this area from October 1965, shows that at the time no lake existed between Frías and Dickson glaciers, but they formed a single mass of ice instead. Between 1965 and 1984, the Escondidos continued to gradually move backwards, reducing the area covered by glacial ice covering by 5.76 km2 (Fig. 5). This analysis was performed by the combination of the 1965 aerial image taken by a telescopic camera system and a Landsat 5 satellite image from 1984 into one single multi-temporal colour composite image, which gives us an idea of an area’s land cover and use.
We combined different images taken at different times using an image visualisation software. In colour satellite images, every pixel has a value in multiple colour channels (typically red, green, and blue). The colours represent bands of different wavelengths in the light spectrum. A natural colour composite image displays a combination of visible red, green and blue bands with the corresponding red, green and blue channels.
However, assigning the 1965 red image band to the red channel and 1984 blue and green bands to the blue and green channels, results in an image made up of two different dates. This process allows to analyse multi-temporal remote sensing images acquired on the same geographical area for identifying changes between the considered acquisition dates (Chuvieco, 2008). In this example, red represents the glaciers situation in 1965.
The areal changes were evaluated mainly for the ablation areas, changes in the accumulation area of the glaciers are much more complicated to determine, due to the presence of snow (Rivera, 2004). According to the above described method to estimate glacier retreat by means of Landsat and Sentinel satellite imagery, the Escondidos Glaciers have retreated almost 15 km2 in the last 35 years, which is equivalent to losing approximately a quarter of their ice surface most exposed to melting (Fig. 6). Watch an animation of the retreat here.
Monitoring elevation changes from space
Based on NASA’s ICESat Missions and Shuttle Radar Topography Mission (SRTM) data, it is possible to monitor the elevation change through time on many points of the glaciers. The SRTM was flown aboard the space shuttle Endeavour in February 2000. Endeavour orbited Earth 16 times each day during the 11-day mission, completing 176 orbits. SRTM successfully collected radar data of over 80% of the Earth's land surface between 60° north and 56° south latitude with data points posted every 1 arc-second (approximately 30 meters). Each point contains a vertical elevation value measured in geographic latitude and longitude units. NASA’s ICESat mission consists of two generations of laser altimeter satellites called ICESat and ICESat-2, which provide multi-year elevation data needed to determine ice sheet mass balance as well as topography and vegetation knowledge around the globe.
We obtained 358 points from the GLAS (Geoscience Laser Altimeter System) and ATLAS (Advanced Topographic Laser Altimeter System) instruments onboard the ICESat and ICESat-2 satellite respectively, downloaded from the National Ice and Snow Data Center (NSIDC). The products available for the period 2003-2009 and from 2018 to the present time are called Global Land Surface Altimetry Data and Advanced Topographic Laser. The data obtained from these instruments are distributed in binary format. They were converted to ASCII with the Altimetry elevation extractor Tool (NGAT), provided by the NSIDC GLAS. By doing so, we could compare the individual altitudes of these georeferenced points using QGIS software. QGIS is an Open Source Geographic Information System (GIS) under the GNU - General Public License, based on an official Open Source Geospatial Foundation (OSGeo) project.
The differences in altitude between datasets were obtained arithmetically. We used masks that delimit the glacier area (obtained from the official GLIMS website) for data selection. Using ICESat data, in addition to the SRTM digital elevation model, we could calculate the elevation differences between 2000 and the present. Measurements made at some points over the ablation area of Dickson glacier show a reduction of more than 60 meters high over the last 20 years (Fig. 7).
From native explorers to modern space-based maps
Patagonia was originally inhabited by the Aonikenk, or "Patagones". They were the first native explorers to meet Francisco de Magallanes when he landed at San Julián port, in 1520. This place is only 120 kilometres from where Darwin started his journey up the Santa Cruz River, surveying the topography. Darwin’s survey was later used by Pascasio Moreno as a guide on his journey through southern Patagonia; both foreigners were guided by the Aonikenks. Their valuable work is still suitable today, because their stories illustrate contemporary studies and open the doors to decipher the evolution of these glaciers.
The best way to measure ablation is by observing the descent of the glacier surface (snow or ice) at numerous points on the glacier (Geoestudios, 2008). Traditionally, glacier mass balance was measured with the “direct” glaciologic method, which consists of placing a network of stakes and pits at representative points on the glacier surface and measuring the distance between the top and the bottom of the stakes. Measurements using the traditional method are conducted either between two fixed dates or at the end of the accumulation and ablation seasons. Due to intense manual labour, this method has limited applicability in rugged or remote glacierized areas because of logistic difficulties involved in maintaining a monitoring network, lack of logistical support and political, economic or cultural conflicts. In such areas, spaceborne remote sensing may offer complementary information on glacier parameters (Racoviteanu, 2008).
Los Glaciares National Park was declared “World Heritage” by UNESCO in 1981 (UNESCO, 2020). Even today, this enchanted and remote landscape is still difficult to access and impresses even the most experienced hikers.
The increased availability of imagery from remote sensing platforms with an adequate spatial and temporal resolution, near global coverage and low financial costs, allows extending the measurements of glacier parameters over larger areas and longer periods (Racoviteanu, 2008). Nevertheless, traditional glacier monitoring and field measurements are very valuable, and it is crucial to maintain such monitoring networks in the long term. Remote sensing methods sometimes present challenges, such as limited data, insufficient of accurate elevation and satellite data, lack of standardized image analysis methods for delineation of debris-covered ice (Racoviteanu, 2008), finding suitable imagery, cloud-free and accurate pixel resolution. The precision of remote sensing methods is closely linked to the errors that satellite imagery may contain. Even so, they are complementary techniques and are very useful when combined for a better understanding of the processes and changes (Gari, 2015).
In light of global warming, these massive glaciations have suffered a large retreat (IPCC, 2019). This study’s finding shows that ice area lost represents more than 35% percent of the total glacier area of 1965. And this is an underestimation of the total area lost due to the exclusion of the accumulation area in the analysis. Thus, it is very important to monitor recent variations. The variations and related glacier behaviour is regarded to be indicative of climate changes. In such a difficult access area, the importance of spatial technologies and especially remote sensing is highlighted as an invaluable aid for change detection studies.