The Landsat Program - Images
Lake Chad
Climate Change and Lake Chad
Source: Earth Gauge™, a partnership
between the National Environmental Education Foundation and the American
Meteorological Society. (external link)
Lake Chad sits on the borders of Chad, Niger, Nigeria, and Cameroon, as well as on a natural border, the Sahel: a grassland which divides the Sahara Desert to the north and the more humid savannah to the south. The southern part of Lake Chad’s one million square mile watershed provides most of the Lake’s inflow. Because Lake Chad is a shallow lake, with average depths of between 16 and 26 feet, its surface area fluctuates markedly with changes in climate. During the late Pleistocene and early to mid-Holocene (14,500 – 5,500 years ago), the Lake covered an area as large as 154,000 square miles and had a maximum depth of 568 feet; during the Ice Age, it was even larger. For comparison, the Caspian Sea, which is the largest enclosed body of water on the Planet today, has an area of 143,000 square miles. Paleontologists refer to ancient Lake Chad as Lake Megachad.
Then, about 5,500 years ago, North Africa experienced a rapid transition from a relatively humid land with lakes, marshes, extensive vegetation cover, and numerous human settlements, to the much drier and more barren state that we see today. This transition has been referred to as a “regime shift.” When this happened, Lake Chad began to shrink in size and by 2,500 years ago, its area had fallen to about 8,800 square miles, which has been the Lake’s average size since then. Because it sits under a powerful northeasterly low-level jet stream, the area that used to be submerged under Lake Megachad is now the Planet’s single largest source of atmospheric dust.
In 1963, after the wet decade of the 1950’s, Lake Chad covered about 9,500 square miles. Back then, fisherman would take about 230,000 tons of fish out of the Lake each day. Fish larger than the fishermen themselves were common catches. Then, in the late 1960’s, the Region entered a period of drought from which it has yet to recover. This change has also been called a regime shift. Rainfall in the Sahel over the last three decades of the 20th Century was 25-40 percent lower than it was between 1931 and 1960. Today, the Lake covers between 115 and 200 square miles and fishermen are only catching about 50,000 tons a day. Areas that were submerged as recently as a few years ago are now being farmed, with many of the farmers being former fishermen.
This most recent regime shift illustrates the impact that global climatic changes can have on a specific region, as well as how those regional impacts can stimulate feedback cycles that reinforce the original impact. A shift in the distribution of and an overall increase in sea-surface temperatures (SST’s) since the late 1960’s appear to be the initial triggers for the last three decades of dry conditions in the Sahel Region. Specifically, the SST shift has altered the monsoon pattern that brings seasonal rainfall. Several other factors, however, have contributed to the duration and severity of these dry conditions:
- The initial drought resulted in changes in vegetation cover. This vegetation shift changed the Region’s albedo (or how reflective the surface is), and the proportions of rainfall that become surface runoff, groundwater, and water vapor. These changes worked to reinforce the dry conditions. Plant cover works to lower the frequency of wet and dry cycles. Wet conditions favor plants that promote wet conditions and dry conditions favor plants that promote dry conditions. Plant cover helps to keep a region in either a wet or a dry state in spite of small or even moderate year to year differences in rainfall. Only when the external forcing is strong enough, like the SST shift in the late 1960’s, is there a shift from one state to another.
- A boom in the Sahel Region’s population has resulted in land cover changes, primarily due to grazing activities. This has helped to quicken the pace at which the Region’s vegetation changed from a composition favoring wet conditions to one favoring dry conditions. Most of these land cover changes, however, did not happen until after the drought conditions were established.
- The Region’s population boom, and the accompanying boom in agricultural activities, have resulted in increases in the withdrawal of water that would otherwise be flowing into the Lake. Indeed, human water use accounts for an estimated 50 percent of the decline in Lake Chad’s water supply. Like the land cover changes due to grazing, however, most of the hydrologic changes did not happen until the drought conditions were established; the drought conditions stimulated much of the need for irrigation.
- In
large lakes, lake generated precipitation due to evaporation
forms a significant portion of a lake’s water
budget. Smaller lakes generally do not put enough water
vapor into the atmosphere to induce local rainfall.
A decrease in water levels in a large lake like Lake
Chad, for whatever reason, is likely to change the
Regional climate in a way that promotes further loss
of lake water.
As the lake water retreats, the exposed rich and moist soil (which is moist due to a high water table) becomes covered by green grasses and crops (which appear as red in the satellite photos). Note how as the Lake shrinks ancient dune features, which were formed hundreds of thousands of years ago, become exposed. Also note the differences between the 1987 and 2000 photographs: the dune features are enhanced, the amount of wetland vegetation in the water has increased, the amount of wetland vegetation on the outside edges has decreased, and there is more irrigated land along the Chari River, which is in the southeastern section of the photographs.
Sources:
Wang GL and Eltahir EAB (2000) Role of vegetation dynamics in enhancing the low-frequency variability of the Sahel rainfall. Water Resources Research 36, 1013-1021.
Coe, MT and JA Foley (2001). Human and natural impacts on the water resources of the Lake Chad basin. Journal of Geophysical Research 106, 3349–3356.
Foley, J.A., M.T. Coe, M. Scheffer, and G. Wang (2003). Regime Shifts in the Sahara and Sahel interactions between ecological and climatic systems in Northern Africa. Ecosystems 6, 524-539.
Wang GL, Eltahir EAB, Foley JA, Pollard D, Levis S (2004). Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model. Climate Dynamics 22, 625-637.
“Africa’s Lake Chad Shrinks by 20 Times Due to Irrigation Demands, Climate Change.” Science Daily, 1 March 2001. Accessed Online 22 February 2008 < http://www.sciencedaily.com/releases/2001/02/010228080245.htm>
Drake N and Bristow C (2006). Shorelines in the Sahara: geomorphological evidence for an enhanced monsoon from palaeolake Megachad. The Holocene 16, 901-911.
Li, K.Y., M.T. Coe, and N. Ramankutty (2005). Investigation of hydrological variability in West Africa using land surface models. Journal of Climate 18, 3173-3188.
Odada EO, Oyebande, L, and Oguntola, JA (2006). Lake Chad: Experience and Lessons Learned Brief. Lake Basin Management Initiative. Accessed Online 22 February 2008 <http://www.ilec.or.jp/eg/lbmi/reports/06_Lake_Chad_27February2006.pdf>
University of Pennsylvania: African Studies Center (2006). Excerpted from Africa’s Lakes: An Atlas of Environmental Change. Accessed Online 22 February 2008 <http://www.africa.upenn.edu/afrfocus/afrifocus091006.html>
United States Geological Survey and NASA: Landsat Project. Lake Chad, Africa. Accessed Online 22 February 2008 < http://landsat.usgs.gov/gallery/detail/386/>
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