The glaciers in the tropical Andes shrunk between 30 and 50% in 30 years, which represents the highest rate observed over the last three centuries. IRD researchers and their partners(1) recently published a summary which chronicles the history of these glaciers since their maximum extension, reached between 1650 and 1730 of our era, in the middle of the Little Ice Age*. The faster melting is due to the rapid climate change which has occurred in the tropics since the 1950s, and in particular since the end of the 1970s, leading to an average temperature rise of 0.7°C in this part of the Andes. At the current pace of their retreat, small glaciers could disappear within the next 10 to 15 years, affecting water supply for the populations.
For the first time, a study conducted by IRD researchers and their partners(1), recently published in the journal The Cryosphere, provides a retrospective of more than three centuries on glacier evolution in the entire tropical Andean region (3).
300 years of glacier history
Since their maximum extension, reached between 1650 and 1750, during the Little Ice Age*, the tropical Andean glaciers have gradually retreated. Over the last 30 years, however, their decline has taken dramatic proportions. This summary clearly shows the peculiarity of these last decades, with melting speeds that had never been reached before in 300 years: the surface areas of glaciers in Colombia, Ecuador, Peru and Bolivia were reduced from 30-50% since the end of the 1970s and up to 80-100% in extreme cases. This new study confirms the acceleration of climate change in this part of the world at the end of the 20th century.
Based on a study of moraines(4) deposited along the glacier sides, scientists were able to map and date the former positions of glaciers throughout their retreat from the 1730s. Furthermore, aerial photographs and satellite images reveal changes in glacial surfaces after 1950. At the same time, researchers modelled glacier responses to current fluctuations in temperature and rainfall, in order to determine the relationship between climatic conditions and ice retreat. Thus, they have been able to reconstruct shifts in the climate that might have led to fluctuations in the glaciers observed.
A common cause
All Andean glaciers respond to the same climatic variability mechanisms. Whereas rainfall has remained fairly stable, the atmospheric temperature in the tropical Andes has increased by 0.7°C, in conjunction with the warming of the tropical Pacific since the 1970s. At the same altitude, the temperature is not directly responsible for the melting, which is due mainly to the balance between the radiation absorbed and reflected by the glacier's surface. However, the temperature affects the type of rainfall (solid or liquid form), and therefore the conditions for maintaining (or not) the snow pack, which helps reflect the majority of the solar energy. A lack of snow pack leads to a considerable increase in the melting of the glacier. This occurrence of bare glaciers -- during summer in the tropics or during the equinoxes on the equator -- has become more frequent over the last few decades.
Small glaciers -- less than 1km2 in size -- located within 5,400m of altitude appear to be the most affected. Should the rises in temperature projected for the end of the century(5) by climatic models be confirmed, most glaciers in this part of the Andes (whether large or small) could disappear, as was the fate of the Chacaltaya glacier above the city of La Paz in Bolivia in 2010.
Today, it is vital to improve forecasts in order to better anticipate future impacts on Andean populations. Mountain glaciers help regulate water resources throughout the year. Millions of people living in the Andes depend on it during the dry season for purposes of agriculture, hydroelectric energy and urban consumption.
(1) UJF-Grenoble 1, CNRS, Grenoble-INP, UMSA in Bolivia, ANA and SENAMHI in Peru, INAMHI and EPN in Ecuador, IDEAM in Colombia, Albany University in the United States, Zurich University in Switzerland and Savoie University, Météo-France, UPS-Paris 1, UVM-Paris 12, Ifsttar, UPMC, EPHE.
(2) The 4th report of the Intergovernmental Panel on Climate Change was published in 2007.
(3) The glaciers in the tropics cover an area of approximately 1,900km², according to Francou and Vincent, 2007: Les glaciers à l'épreuve du climat, IRD Editions & Editions Belin.
(4) Rock deposits left behind by the glacier reveal its previous positions.
(5) In the amount of 4°C to 5°C by the end of the century in the central Andes, scenario (A2) constructed based on maximum greenhouse gas emissions, comparable to the current emissions.
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