IndexPositionHistory and understanding of drought in the Sahel during the 20th centuryCausesNaturalAnthropogenicDustDiscussionEvaluationConclusionDrought by definition is "a function of precipitation" (Agnew and Chappell 1999, p.299). However, this definition is insufficient, as only by considering precipitation we obtain anomalies that cannot be explained. Unless it is a purely meteorological drought, even then local and regional human impacts increase stress (Thomas and Goudie, 2009). Accounting for local variability and human influence, such as agriculture and overgrazing, can resolve uncertainties about the cause and severity of a drought in a region, such as the Sahel. For a clear definition and understanding, in addition to the natural phenomenon, the specificity of the place is therefore fundamental. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an Original EssayLocationThe Sahel region, shown in Figure 1, is a semi-arid transition zone encompassing West African countries from Senegal to Chad (Agnew and Chappell, 1999). The region is infamous for its aridity, suffering many severe droughts throughout its history. Consequently, known as one of the harshest, due to its variable climate (Sivakumar et al., 1991), it receives between 600 and 700 mm of rainfall in the south and 100-200 mm in the north (Nicholson, 1978). As a result of its unique location, drought boundaries are often confused with Sahel rainfall variability (Brooks, 2004). This makes the region highly sensitive in terms of climate, an important factor for drought (Zeng, 2003). History and understanding of drought in the Sahel during the 20th century Due to its unique location and climate variability, the Sahel is one of the most vulnerable regions on Earth (Agnew and Chappell, 1999). As demonstrated in its historic severe drought, as illustrated in Figure 2, there were wet periods in the 1930s followed by minor droughts in the 1940s. However, it was in the second half of the 20th century that the severity of the drought became prevalent. The expansion of agriculture in the 1950s and extremely wet periods (Figure 2) increased the severity of subsequent droughts. There has been a decline in precipitation since 1955, leading to a prolonged drought in the late 20th century. The decline in precipitation became significant in the early 1960s (Zheng and Eltahir, 1998), with an acute precipitation deficit in 1968/9 defining the onset of the severe drought (Tickell, 1986). Agnew (1990) notes that the drought since 1969 has been severe, but the spatial distribution of drought was very sporadic when compared annually, accounting for variability in precipitation and by areas within the Sahel. These influences will be discussed further. The peak drought in the Sahel occurred in 1973 (Courel et al., 1984), however precipitation was not abnormally low, suggesting other pressures. Drought in the Sahel has been widespread and persistent, with precipitation levels consistently falling throughout the 1970s and 1980s, with annual precipitation well below the century average (Brooks, 2004). However, the observations are part of the natural pattern of the Sahel (Glantz, 1994). There had been heavy rainfall in particular countries (Toulmin, 1988), as shown in the 1970s in Figure 2. This represents our lack of understanding, particularly if we treat the Sahel as a “homogeneous entity” (Agnew and Chappell 1999 , p.309). Precipitation variability varies from country to country (Figure 2), therefore, it is not possible to fully understand the causes if location is so influencing. It's fundamentalrepresent the causes by area and not as a whole; Country-specific stresses can pressurize or alleviate drought. Causes Natural drought is linked to a relationship between a specific area and its rainfall volume (Thomas and Goudie, 2009). Charney (1975) supports the hypothesis that, due to the circulation of Hadley cells, the lack of precipitation leads to a decrease in vegetation, leading to an increase in albedo. An increased albedo is typical of a brighter surface, such as sand and bare rock, rather than vegetated terrain (Charney, 1975). This type of terrain will mean greater reflectivity, emitting more solar radiation than regions around the Sahel. This leads to a sinking motion, in which the region acts as a radiative sink. During and after a desiccation event Charney (1975) calls this effect self-inductive in arid regions. This is a self-feedback shown in Figure 3, as the humidity changes, the arid conditions increase. Instabilities in the mechanism could lead to intensification of drought in the Sahel. For comparison, Zeng (2003) discusses the importance of circulation changes on a global scale, in relation to changes in the planet's sea surface temperature (SST). This occurs according to a cyclical pattern, both interannual and decadal (Giannini et al., 2003). Changes in SST catalyze a positive feedback response. It weakens the monsoon circulation, resulting in barren landscapes as vegetation declines, exposed soil leads to a higher albedo effect, meaning less moisture in the air, shown in Figure 3. The resulting drop in precipitation further weakens the monsoon circulation, reproducing the monsoon circulation. cycle. From general circulation models (GCMs) Giannini et al. (2003) found that when running global SST observation data, it produced the precipitation variability shown in the Sahel. They concluded that global SST is an important factor in the drought conditions experienced in the Sahel. Furthermore, Giannini et al. (2003) identify that there is a strong correlation between precipitation patterns observed in the Sahel and anomalous trends in tropical SSTs. Therefore, drought conditions in the Sahel may be linked to unusual events in SST in the Atlantic, Indian and Pacific oceans. Anthropogenic Human processes, such as overgrazing and agricultural expansion, modify the landscape. Overgrazing has been found to be a mechanism specific to bare soils, resulting in high albedo and influencing soil surface temperature (Otterman, 1974). Aridity results from the stripping of the land, revealing soils with a high albedo and cooling the surface temperature when exposed. Humidity increases, as there is less moisture in the atmosphere due to the high amount of solar radiation reflected back into space from the brighter surface, identified in Figure 3. This drop in surface temperature allows for instability and decreases capacitance, energy, of the lifting force that causes the air to rise, so precipitation is less likely to form (Otterman, 1974). A self-deprecating mechanism develops; lower humidity, precipitation decreases), stress on vegetation increases (Zeng, 2003). In contrast, over vegetated land you would expect to have greater cloud cover, as surface temperatures are warmer, air can rise and water molecules can condense and precipitate. DustA possible link to the drought observed in the Sahel over the last century is atmospheric dust. Prospero and Lamb (2003) observed that dust concentrations from the region to Barbadosshow a negative relationship with Sahel precipitation. Further interaction evidence reports that reduced dust transport occurred with the period of high precipitation in the 1930s and 1950s. Dust in the atmosphere affects drought through the reflection of solar radiation, directly affecting the energy balance of the world, in addition, indirectly affecting the formation of clouds, affecting precipitation, which can lead to changes in vegetation, triggering the process of land exposed, higher albedo effect as discussed above. This leads to an increase in dust, therefore another positive feedback, whereby the drought is self-deprecating on itself. For example, drought around the millennium increased the global dust aerosol load by a third (Zheng, 2003). Fine powder cools, while coarse powder is expected to heat up (Kok et al., 2017). Therefore, important if increase in load, caused by winds lifting it from the exposed surface, due for example to overgrazing (Otterman, 1974). Raking causes a diabatic cooling process of the troposphere, increasing the rate of subsidence (Otterman, 1974), increasing arid-type conditions. Important for global climate, as there is uncertainty about the sensitivity of dust aerosols and, whether it is hot or cold, a large change in dust supply will alter the climate and carbon cycle, Kok et al. (2017) believe that this will result in global warming. Discussion From examination of land use data, Taylor (2002) concluded that the anthropogenic disturbance of land use change was not significant enough to cause the recorded drought. However, Wikman and Timberlake (1985) argue that human activity, not precipitation reduction, is the primary cause, illustrating a lack of clarity in understanding the exact factors responsible and their extent. On the other hand, it is not possible to fully state that oceanic forcing is the strongest and only contributing factor. Since some precipitation data are missing, comparing them with models it is not possible to say for sure (Zeng, 2003). On the other hand, Otterman (1974) concludes that precipitation volumes in desert regions, such as the Sahel, depend on circulation patterns that change as anticyclone positions vary and the ITCZ ​​shifts downward (Zeng , 2003). Zeng (2003) expresses if the responsibility is mainly anthropogenic, the result is disastrous as continued landscape change, partly due to rapid population growth in the region, will further intensify the processes involved. In comparison, a natural change in ocean circulation is hoped to be cyclical and repeat naturally. However, the natural cannot be controlled, but only adapted, the anthropogenic can be changed or modified in our favor, although it may be too changed to be able to mitigate the effects or change the course of the road on which it leads. On the other hand, Otterman (1974) states that the earth's surface, the vegetation, recovers rapidly once the disturbing forces are removed. It is difficult to quantify the significance of land use change and anthropogenic forcing, compared to oceanic reasoning ( Zeng, 2003 ) that can be simulated by GCMs. Making it more difficult to prioritize and understand the causes. There are many complexities in the cases and uncertainty increases, as do doubts about their guarantees. Many factors contribute at different scales in space and time. Evaluation1. Importance of Vegetation In all the causes discussed, the role of vegetation cannot be underestimated. Any natural or anthropic forcing, which for example.