Another region that is sensitive to rising temperature is the Andes, where the last tropical glaciers are melting. Relatively unimportant on the scale of the South American continent, these glaciers nonetheless form a natural reservoir that is essential for the regional economy. The rivers they feed irrigate the farmland in areas where the population depends on agriculture, provide drinking water to large cities, and produce hydroelectric energy. Currently, the melting of the glaciers is increasing river flow, but their eventual disappearance would mean the end of naturally regulated water supply, bringing with it serious economic and human consequences.
For surface water, the presence of water and its variability depends almost exclusively on precipitation and temperature (disregarding human activity), and this is also true for groundwater. This is the case for aquifers recharged directly by local precipitation, rivers or lakes. They are very sensitive to variations in precipitation that can lead to signifi cant increases or decreases in stored water. Some aquifers that outcrop along flood plains can also be subject to major evaporation, and are therefore, like surface waters, affected by climate change, with stores dwindling or building up again according to the region. They play a vital role as the main water resource in arid or semi-arid rural regions.
Lake Chad suffered from intense, longstanding drought in the 70's, following a strong drop in precipitation. Its low water levels lead to great fluctuations in the lake's surface area and the appearance of several hundred islands, as shown on this image taken by the Korean satellite KOMPSAT-2 in 2007. Courtesy KARI
Long-term trends
The warming of the planet has an indirect effect on groundwater reservoirs in coastal regions or on islands, potentially leading to the increasingly rapid intrusion of sea water, making the fresh water salty and rendering the aquifers unsuitable for consumption. A rise of global sea level would have a similar impact on the deltas of major rivers. How can long-term trends in hydrological variables be assessed? It's a difficult job, for several reasons. In situ data are fairly recent. Most data come from areas with a long history of land-use and resource-use planning, which leads to non- 'natural' variability. In addition, many observation networks (for instance of river flow) have been abandoned or banned. And to make things even more complicated, climatic cycles with long periods overlap with current climate change.
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