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Drought by definition is “a function of rainfall” (Agnew and Chappell 1999, p.299). However, this definition is insufficient, only considering precipitation results in anomalies that cannot be explained. Unless it is a purely meteorological drought, even then local and regional human impact increases the stress (Thomas and Goudie, 2009). Factoring in local variabilities, and human influence, such as agriculture and overgrazing can solve the uncertainties in the causation and severity of a drought in a region, such as the Sahel. Therefore, place-specificity, in addition to the natural phenomenon, for a clear definition and understanding is vital.
The Sahel Region, shown in Figure 1, is a semi-arid transitional zone that encompasses the West African countries from Senegal to Chad (Agnew and Chappell, 1999). The region is infamous for its aridity, suffering from many severe droughts in its history. Consequently, known as one of the harshest, due to its variable climate (Sivakumar et al., 1991), it receives between 600-700mm rainfall in the south and 100-200mm in the north (Nicholson, 1978). As a result of the uniqueness of its location, the boundaries of the drought are often blurred with the Sahel’s precipitation variability (Brooks, 2004). This makes the region highly sensitive in terms of climate, an important factor for drought (Zeng, 2003).
Due to its unique location and climate variability, the Sahel is one of the most vulnerable regions on Earth (Agnew and Chappell, 1999). Demonstrated in its historic severe drought, drawing from Figure 2, there were wet periods in the 1930s followed by small droughts in the 40s. However, it was in the second half of the 20th Century where the severity of drought became prevalent. The expansion of agriculture in the 1950s and extreme wet periods (Figure 2) increased the severity of the droughts later. From 1955 there was a decline in rainfall, which led to prolonged drought in the late 20th Century.
The precipitation decline became significant in the early 1960s (Zheng and Eltahir, 1998), with acute rainfall deficit in 1968/9 which defined the start of the severe drought (Tickell, 1986). Agnew (1990) notes that the drought from 1969 was severe, but the spatial distribution of the drought was very sporadic when comparing yearly, representing the variability in precipitation and by areas within the Sahel. These influences will be discussed further. The peak of the Sahel drought was in 1973 (Courel et al., 1984), however the rainfall wasn’t abnormally low which suggests other pressures. The Sahel drought was widespread and persistent with continuing downward precipitation levels throughout the 1970s and 80s, with annual rain being far below the mean for the century (Brooks, 2004).
However, argued the observations are part of the Sahel’s natural pattern (Glantz, 1994). There had been plentiful rains in particular countries (Toulmin, 1988), seen in 1970s in Figure 2. Representing our lack of understanding, particularly if we treat the Sahel as a “homogenous entity” (Agnew and Chappell 1999, p.309). The precipitation variability varies from country to country (Figure 2), therefore, cannot fully comprehend the causes if locality is so influencing. Vital to represent causes by area not as a whole; country-specific stresses can either pressurise or alleviate drought.
Drought is linked to a relationship between a specific area and its volume of rainfall (Thomas and Goudie, 2009). Charney (1975) argues the hypothesis that due to Hadley cell circulation, a lack of precipitation leads to decreased vegetation, leading to increasing albedo. An increased albedo is typical of a brighter surface, such as sand and bare rock, rather than vegetated land (Charney, 1975). This land type will mean a higher reflectivity, emitting more solar radiation out than the regions around the Sahel. This leads to a sinking motion, where the region acts as a radiative sink. During and after a desiccation event Charney (1975) terms this as a self-inductive effect in arid regions. It is a self-feedback shown in Figure 3, as humidity changes, increases the arid conditions. Instabilities in the mechanism could lead to intensification of the drought in the Sahel.
To compare, Zeng (2003) discusses the importance of circulation changes on a global scale, in relation to changes in the sea surface temperature (SST) of the planet. This occurs on a cyclic pattern, both interannually and decadally (Giannini et al., 2003). Changes in SST catalyse a positive feedback response. It weakens the monsoon circulation, resulting in barer landscapes as vegetation is decreased, the exposed land leads to a higher albedo effect, meaning less moisture in the air, shown in Figure 3. The resulting decline in precipitation weakens the monsoon circulation further, reproducing the cycle. From General Circulation Models (GCM) Giannini et al. (2003) found that when running the observation figures of global SST it produced the precipitation variability shown in the Sahel. They concluded that global SST is an important forcing in the drought conditions experienced in the Sahel. Furthermore, Giannini et al. (2003) identify that there is a strong correlation between the rainfall patterns observed in the Sahel and anomalous trends in the tropical SST. Therefore, drought conditions in the Sahel can be linked to unusual occurrences in the SST in the Atlantic, Indian and Pacific oceans.
Human processes, such as overgrazing and agricultural expansion, change the landscape. Overgrazing as a mechanism specifically has been found to bare soils, resulting in high albedo and affecting ground surface temperature (Otterman, 1974). Aridity results from baring land, revealing soils with a high albedo and cooling the temperature of the surface as it is exposed. Humidity is increased, as there is less moisture in the atmosphere through the high amount of solar radiation reflected back into space from the brighter surface, identified in Figure 3. This drop of surface temperature enables instability and decreases the ability, the energy, of the lifting force for air to rise, therefore precipitation is less able to form (Otterman, 1974). A self-deprecating mechanism develops; lower moisture, rainfall is decreased), stress on vegetation increases (Zeng, 2003). In contrast, to land covered by vegetation which would be expected to have more cloud cover, as the surface temperatures are warmer, air can rise and water molecules can condense and precipitate.
A possible link to the droughts observed in the Sahel over the last century is atmospheric dust. Prospero and Lamb (2003) observed that dust concentrations from the region to Barbados show a negative relationship with the Sahel’s precipitation. Further interaction evidence reports there was reduced dust transportation with the high rainfall period in the 1930s and 50s. The dust in the atmosphere affects drought through its reflection of solar radiation, directly affecting the energy balance of the world, also, indirectly through affecting the formation of clouds, influencing the precipitation, which can lead to variance in the vegetation, setting off the process of exposed soil, higher albedo effect as discussed earlier. This leads back to an increase in dust, therefore another positive feedback, whereby the drought is self-deprecating on itself. For example the drought around the millennium increased the global dust aerosol loading by a third (Zheng, 2003). Fine dust cools, predicted that coarse dust will warm (Kok et al., 2017). Therefore, important if increased loading, caused by winds raising it from the exposed surface, due to e.g. overgrazing (Otterman, 1974). The raking up, causes a diabatic process of cooling the troposphere, increasing the speed of subsidence (Otterman, 1974), increasing the arid like conditions. Important for the global climate, as there is uncertainty of the sensitivity of dust aerosols, and if it is warm or cool, a large modification of the dust supply will alter the climate and carbon cycle, Kok et al. (2017) believe it will result in a warming of the planet.
From looking at land use data, Taylor (2002) concluded that anthropogenic disturbance of land use change was not significant enough to cause the drought that has been recorded. However, Wikman and Timberlake (1985) argue human activity is the major cause, not rainfall reduction, illustrating a lack of clarity of understanding the exact responsible factors and their extent of the causation.
On the other side, cannot fully say the oceanic forcing is the strongest and only contributing factor. As some rainfall data is missing, therefore when running against models, cannot say for certain (Zeng, 2003). On the other hand, Otterman (1974) concludes that volumes of precipitation in desert regions, such as the Sahel, are reliant on circulation patterns which change with varying anticyclone positions and shifting the ITCZ downwards (Zeng, 2003).
Zeng (2003) expresses if the responsibility is mainly anthropogenic, it is disastrous as continued change of the landscape, in part due to the rapid population growth in the region, will further intensify the processes involved. In comparison, a natural oceanic circulation change would hopefully be cyclic and naturally return. However, the natural cannot be controlled, only adapted to, the anthropogenic can be changed or modified into our favour, although it may be too far changed to be able to mitigate the effects or change the course of the road its leading down on. On the other hand, Otterman (1974) states that the land surface, the vegetation, recovers rapidly once the disturbing forces have been removed.
It is hard to quantify the significance of land use change and anthropogenic forcing, in comparison to the oceanic reasoning (Zeng, 2003) which can be simulated by GCMs. Making it more difficult to hierarchize and understand the causes. There are many complexities within the causes and uncertainty increases, and doubt in their sureties. Many factors contribute on different scales in space and time.
1. Importance of Vegetation
In all the causations discussed, the role of vegetation cannot be undervalued. Any forcing natural or anthropogenic, for example that leads to a decrease in vegetation sets off in motion events that culminant in relentless drought in the Sahel (Figure 2). Zeng (2003) agree and stress the importance of natural vegetation in the formation and progression of drought and arid conditions.
2. Importance of change
It is the strength of the change that can dictate the severity of the drought, for example, the strength of the variability in SST, the extent to which the land has been modified by humans. Also, these changes can affect the long term, increasing the severity of later droughts and the effects become fully realized. The uncertainty of the extent to cause an impact on aridity is also prevalent in the idea of ground surface temperature, the importance of the strength of the gradient between areas, to cause a shift, such as uplift of dust, rising air for cloud formation, leading to rainfall etc.
3. Importance of Local variations
For example, drought in the Sahel has a cyclic pattern of every 30 years (Charney, 1975), a different pattern to many other regions. In comparison, Nicholson and Palao (1993) say every 20, illustrating uncertainty.
4. Importance of feedbacks
Positive feedback is an amplification of a change in response to a forcing (IPCC, 2013). Positive feedbacks may have a strong causation on the drought in the Sahel. The link and interaction between the climatic factors of land and atmosphere result in a positive feedback. Vegetation is reduced by lack of rain, leading to increase in albedo (reflectivity of the surface), reduction in evaporation and moisture in the surrounding air (Zeng, 1999). This means the dryness of the Sahel continues to dry in a cycle as it is triggered by global SST change. It is the uncertainties contribution in feedbacks that are often unappreciated.
In our understanding, it is the combination of natural forcing and human pressure that led to the Sahel Drought in the late 20th Century. Brooks (2004) concludes it was propelled by changes in the surface temperature and warming of the oceans that changed the atmosphere circulation. The interaction between the surface and the atmosphere changed through vegetation and dust production. The land-atmosphere feedbacks exacerbate the mechanism, leading to more severe drought with human stress combined through land use change. The contribution of these all, although uncertainties of extent, increased the severity of the drought occurred.
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