Shrinking tropical glaciers hold the key to water resources in the Andes
As mountain glaciers melt in a warming climate, the people of the South American Andes face a reduced supply of the glacial melt water they rely on for drinking, hydroelectricity and agriculture. French glaciologist Kristof Sturm outlines the processes that will shape future water resources in the tropical latitudes of Bolivia, Ecuador and Peru.
Water of glacial and snow origin is crucial for a number of tropical and sub-tropical regions, all the more so because of a pronounced seasonality of precipitation affecting these regions - the strongly marked alternation of dry and wet seasons. The combination of climate change, which has particularly strong effects at low latitudes, and a constantly growing demographic pressure makes the subject of glacial water resources a priority for development-oriented research.
As researchers, we must approach this problem in a multidisciplinary fashion. One element is the study of mass balance of Andean glaciers, that is the net amount by which they grow or shrink due to the combined effects of new ice building up and melt water flowing out from beneath the glacier, which can be related to regional climate. Another factor is the hydrology of glacial water basins. Together, the information enables a direct estimate of available water resources to be made, which in turn allows for better management of resources for drinking water and hydroelectricity in the capitals of Andean countries - Quito, La Paz and Lima. Bolivia, for example, is very strongly dependent on water for hydroelectricity generation.
The great majority of tropical glaciers are located in South America, in the Andean Cordillera, principally in Peru, Bolivia and Ecuador. The Himalayas (especially in India) also hold some sub-tropical glaciers. These glaciers are found at very high altitudes, their line of equilibrium [the line marking a division between the glacier 'growing' more ice from snowfall at the top, and losing it through melting lower down] being situated typically around the altitude of 5000m.
In a tropical context, with its strong seasonality of precipitation, glaciers constitute a natural regulator of water resources, functioning somewhat like a reservoir, with melt water that had accumulated in ice during the rainy season flowing down into the valleys during the dry season. The populations of the Andes depend on the glacial melt for drinking water, hydroelectricity production and agriculture. Understanding the hydrological mechanisms of glacial water basins is therefore crucial, both from a scientific and developmental viewpoint, to enable the optimal management of this natural resource.
But the scientific interest of glaciers is not limited to their capacity to stock drinking water and release it during the dry season. Variations in glaciers' mass balance are faithful indicators of climatic changes, as successive layers of ice register information on the temperature, frequency and intensity of precipitations and atmospheric humidity as they accumulate.
To better understand the relationship between climatic parameters and the evolution of glaciers, scientists analyse the energy balance of glaciers (i.e. the balance between solar energy received by glacier, energy lost through radiation, etc.) in tandem with their mass balance. Taking meteorological measurements as the starting point, researchers use computer models to establish the relation between this energy balance and the local climate. Variations in volume of ice (current or past) can then be interpreted in terms of climatic variations.
Measuring mass and energy balance requires costly equipment and field measurements, limiting the number of glaciers that can be studied. This is why other methods of estimating the mass balance have emerged. "Tele-detection" is one such method, and comprises the determination of a glacier's state using aerial photos and satellite images. Although the accuracy of this recent technique leaves something to be desired compared to scientists actually climbing onto the glacier to physically measure its properties, it allows for more glaciers to be studied, thus improving coverage and the extent to which regional climate can be mapped out. Tele-detection is also used to identify ancient glacial moraines, giving further information on the past form of glaciers that have now shrank, some of them beyond recognition.
Information about past and present climatic changes is not only contained in the global mass balance of glaciers. The analysis of the chemical composition of ice cores - long vertical cylinders of ice extracted from glaciers by drilling down at high altitude - reveals a continuous history of climate many thousands of years into the past.
As snow fell and was compressed into ice, it encoded information about the ambient atmosphere at that time, including chemical composition and other characteristics. Tiny bubbles of air trapped in the glacier are analysed along with the ice itself, giving scientists a glimpse into past atmospheres. An ice core thus contains a record of past climate. The deeper the ice, the older it is, going back as far as the last glacial maximum - that is 20,000 years.
This information can be consulted by analysing horizontal slices of ice cores chemically - particularly through the interpretation of their composition in stable water isotopes, the natural heavier water molecules. The relative amount of stable water isotopes is sensitive both to modifications in temperature and the water cycle, so the analysis of water isotopes contained in ice layers gives information on variations in temperature, hydrological cycle, but also on the origin and trajectory of the precipitations that eventually formed the ice. In this way, chemical analysis of water isotopes contained in an ice core allows a history of regional climate to be reconstituted.
Knowledge about past climate forms the basis for predicting future climatic changes, and in the case of tropical glaciers particularly their repercussions on water resources. The general retreat of glaciers worldwide demonstrates that global warming is taking place. But to discriminate between natural climate variations and perturbation of planet Earth induced by human activity, we need to understand past changes and the numerous factors that produce them. Computer models that simulate the behaviour of the earth's climate help improve this understanding, but they need measurements as input - which is where glacial mass balance, aerial and satellite measurements, and ice cores come in.
Once climate models accurately reproduce the climate settings currently observed, they can be applied to past climate conditions in order to translate isotopic measurements, e.g. in ice-core records, into variations in temperature and water cycle. If the same model accurately reproduces observations of current and past climate, we gain more confidence in its predictions for future climatic changes.
Studied in their role as climatic markers, tropical glaciers not only help us better understand regional and global climate change, but to anticipate the consequences of their alarming retreat for South American populations in the 21st century.